Publications
2024
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(2024) Advanced Materials. 36, 38, 2408060. Abstract
Organic crystals are widely used by animals to manipulate light for producing structural colors and for improving vision. To date only seven crystal types are known to be used, and among them β-guanine crystals are by far the most widespread. The fact that almost all these crystals have unusually high refractive indices (RIs) is consistent with their light manipulation function. Here, the physical, structural, and optical principles of how light interacts with the polarizable free-electron-rich environment of these quasiaromatic molecules are addressed. How the organization of these molecules into crystalline arrays introduces optical anisotropy and finally how organisms control crystal morphology and superstructural organization to optimize functions in light reflection and scattering are also discussed. Many open questions remain in this fascinating field, some of which arise out of this in-depth analysis of the interaction of light with crystal arrays. More types of organic crystals will probably be discovered, as well as other organisms that use these crystals to manipulate light. The insights gained from biological systems can also be harnessed for improving synthetic light-manipulating materials.
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(2024) ACS Central Science. 10, 8, p. 1504-1514 Abstract
Detoxification of heme in Plasmodium depends on its crystallization into hemozoin. This pathway is a major target of antimalarial drugs. The crystalline structure of hemozoin was established by X-ray powder diffraction using a synthetic analog, β-hematin. Here, we apply emerging methods of in situ cryo-electron tomography and 3D electron diffraction to obtain a definitive structure of hemozoin directly from ruptured parasite cells. Biogenic hemozoin crystals take a striking polar morphology. Like β-hematin, the unit cell contains a heme dimer, which may form four distinct stereoisomers: two centrosymmetric and two chiral enantiomers. Diffraction analysis, supported by density functional theory analysis, reveals a selective mixture in the hemozoin lattice of one centrosymmetric and one chiral dimer. Absolute configuration has been determined by morphological analysis and confirmed by a novel method of exit-wave reconstruction from a focal series. Atomic disorder appears on specific facets asymmetrically, and the polar morphology can be understood in light of water binding. Structural modeling of the heme detoxification protein suggests a function as a chiral agent to bias the dimer formation in favor of rapid growth of a single crystalline phase. The refined structure of hemozoin should serve as a guide to new drug development.
2023
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(2023) Helvetica Chimica Acta. 106, 5, e202200172. Abstract
Here we describe the story behind the link between molecular chirality and macroscopic phenomena, the latter being a probe for the direct assignment of absolute configuration of chiral molecules. First, a brief tour of the history of molecular stereochemistry, starting with the classic experiment reported by Pasteur in 1848 on the separation of enantiomorphous crystals of a salt of tartaric acid, and his conclusion that the molecules of life are chiral of single-handedness. With time, this study raised, inter alia, two fundamental questions: the absolute configuration of chiral molecules and how a molecule of given configuration shapes the enantiomorphous morphology of its crystal. As for the first question, following the beginning of crystal structure determination by X-ray diffraction in 1912, it took almost 40 years before Bijvoet assigned molecular chirality through the esoteric method involving anomalous X-ray scattering. We have been able to address and link both questions through everyday concepts of left and right (in the words of Jack Dunitz) by the use of tailor-made auxiliaries. By such means, it proved possible to reveal, through morphology, etch patterns, epitaxy and symmetry reduction of both chiral and, paradoxically, centrosymmetric crystals, the basic chiral symmetry of the molecules of life, the α-amino acids and sugars.
2022
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(2022) Faraday Discussions. 240, p. 127-141 Abstract
Malaria is a potentially fatal infectious disease caused by the obligate intracellular parasite Plasmodium falciparum. The parasite infects human red blood cells (RBC) and derives nutrition by catabolism of hemoglobin. As amino acids are assimilated from the protein component, the toxic heme is released. Molecular heme is detoxified by rapid sequestration to physiologically insoluble hemozoin crystals within the parasite's digestive vacuole (DV). Common antimalarial drugs interfere with this crystallization process, leaving the parasites vulnerable to the by-product of their own metabolism. A fundamental debate with important implications on drug mechanism regards the chemical environment of crystallization in situ, whether aqueous or lipid. This issue had been addressed previously by cryogenic soft X-ray tomography. We employ cryo-scanning transmission electron tomography (CSTET) to probe parasite cells throughout the life cycle in a fully hydrated, vitrified state at higher resolution. During the acquisition of CSTET data, Bragg diffraction from the hemozoin provides a uniquely clear view of the crystal boundary at nanometer resolution. No intermediate medium, such as a lipid coating or shroud, could be detected surrounding the crystals. The present study describes a unique application of CSTET in the study of malaria. The findings can be extended to evaluate new drug candidates affecting hemozoin crystal growth.
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(2022) Journal of the American Chemical Society. 144, 12, p. 5304-5314 Abstract
We revisit the important issues of polymorphism, structure, and nucleation of cholesterol·H2O using first-principles calculations based on dispersion-augmented density functional theory. For the lesser known monoclinic polymorph, we obtain a fully extended H-bonded network in a structure akin to that of hexagonal ice. We show that the energy of the monoclinic and triclinic polymorphs is similar, strongly suggesting that kinetic and environmental effects play a significant role in determining polymorph nucleation. Furthermore, we find evidence in support of various O-H···O bonding motifs in both polymorphs that may result in hydroxyl disorder. We have been able to explain, via computation, why a single cholesterol bilayer in hydrated membranes always crystallizes in the monoclinic polymorph. We rationalize what we believe is a single-crystal to single-crystal transformation of the monoclinic form on increased interlayer growth beyond that of a single cholesterol bilayer, interleaved by a water bilayer. We show that the ice-like structure is also relevant to the related cholestanol·2H2O and stigmasterol·H2O crystals. The structure of stigmasterol hydrate both as a trilayer film at the air-water interface and as a macroscopic crystal further assists us in understanding the polymorphic and thermal behavior of cholesterol·H2O. Finally, we posit a possible role for one of the sterol esters in the crystallization of cholesterol·H2O in pathological environments, based on a composite of a crystalline bilayer of cholesteryl palmitate bound epitaxially as a nucleating agent to the monoclinic cholesterol·H2O form.
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(2022) Annual Review of Materials Research. 52, p. 57-78 Abstract
Cholesterol is an essential component of animal cell membranes because it influences and controls cell membrane fluidity. Cholesterol is also responsible for the most frequent lethal pathologies in developed countries because of its intimate association with atherosclerotic plaques, the rupture of which may cause heart attacks or strokes. The question is under which conditions cholesterol activity manifests itself, whether in physiology or in pathology. The answer is complex, and there is probably not one certain answer. This review article has its foundations in abundant published knowledge and evidence, but it cannot possibly be comprehensive, because the extent of cholesterol's involvement in chemistry, biology, biophysics, and medicine is so vast that we cannot embrace it all. We review cholesterol as a molecule and in its various crystalline polymorphs. We then examine cholesterol assembly pathways and, finally, cholesterol in biology and in pathology. We propose that cholesterol activity depends on its assembly states in cholesterol crystals or with other lipids in the form of more-or-less organized crystalline domains. In other words, we analyze cholesterol material properties because the assembly state of the cholesterol molecules profoundly affects the properties of the environment in which they reside.
2021
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(2021) ChemMedChem. 16, 10, p. 1515-1532 Abstract
Biogenic formation of hemozoin crystals, a crucial metabolic process of heme detoxification by the malaria parasite, is reviewed as an antimalarial drug target. Starting from a historical description of malaria pigment, the original term for hemozoin, this Perspective first focuses on hemozoins invivo formation. A model is presented, based on nativecontrast threedimensional imaging achieved by Xray and electron microscopy, that hemozoin nucleates at the inner membrane leaflet of the digestive vacuole, growing in the adjacent aqueous medium. We present observation of quantities of hemozoin found in the digestive vacuole and our model whereby heme liberation from hemoglobin and hemozoin formation is an assemblyline process. The crystallization is preceded by reaction between heme monomers yielding hematin dimers, which constitute hemozoin. Biogenic hemozoin embodies fewer types of dimeric hematin isomers than synthetic hemozoin, indicative of proteinactivated heme dimerization. This assemblyline process has implications on the drug types, which inhibit hemozoin formation. We review antimalarial drugs and models of their adsorption onto hemozoin surfaces. Finally, we present our observation of bromoquine, a chloroquine drug analogue, capping a significant fraction of hemozoin crystal surfaces within Plasmodium infected red blood cells and accumulation of the drug, presumably a bromoquinehematin dimer complex, at the digestive vacuole membrane.
2020
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Evidence for laser-induced homogeneous oriented ice nucleation revealed via pulsed x-ray diffraction(2020) Journal of Chemical Physics. 153, 2, 024504. Abstract
The induction of homogeneous and oriented ice nucleation has to date not been achieved. Here, we report induced nucleation of ice from millimeter sized supercooled water drops illuminated by ns-optical laser pulses well below the ionization threshold making use of particular laser beam configurations and polarizations. Employing a 100 ps synchrotron x-ray pulse 100 ns after each laser pulse, an unambiguous correlation was observed between the directions and the symmetry of the laser fields and that of the H-bonding arrays of the induced ice crystals. Moreover, an analysis of the x-ray diffraction data indicates that, in the main, the induced nucleation of ice is homogeneous at temperatures well above the observed and predicted values for supercooled water.
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(2020) Crystal Growth & Design. 20, 2, p. 858-865 Abstract
Calcium oxalate minerals are broadly present in nature. They form through biogenic, geogenic, and pathogenic processes that lead to different pseudopolymorphs. Being the most common solid phase in human nephrolithiasis, calcium oxalate monohydrate (COM) in particular has been the focus of much investigation. It exists in several crystalline forms, two of which appear to be of biological and medical relevance: the low- and high-temperature forms (COM-LT and COM-HT, respectively). While there is broad consensus on the ordered structure of COM-LT, which possesses the P2(1)/n space group symmetry, for COM-HT controversy remains. Experimental results suggest that there is a certain degree of structural disorder in the high-temperature form. However, the exact character of disorder in COM-HT is yet an open question. Here, we examine the effect of the disorder of water molecules on the structure of COM using first-principles calculations based on dispersion-augmented density functional theory. Such calculations allow for controlled examination of specific disorder features and their effect on crystal structure and stability. On the basis of our first-principles analysis, we suggest that in COM-HT each water dimer site is randomly occupied by any of four water dimer arrangements present in COM-LT, leading to statistical 2/m point symmetry at each site and a statistical I2/m space group symmetry.
2019
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(2019) Journal of the American Chemical Society. 141, 50, p. 19736-19745 Abstract
The eyes of many fish contain a reflecting layer of organic crystals partially surrounding the photoreceptors of the retina, which are commonly believed to be composed of guanine. Here we study an unusual fish eye from Stizostedion lucioperca that contains two layers of organic crystals. The crystals in the outer layer are thin plates, whereas the crystals in the inner tapetum layer are block-shaped. We show that the outer layer indeed contains guanine crystals. Analyses of solutions of crystals from the inner layer indicated that the block-shaped crystals are composed of xanthopterin. A model of the structure of the block-shaped crystals was produced using symmetry arguments based on electron diffraction data followed by dispersion-augmented DFT calculations. The resulting crystal structure of xanthopterin included, however, a problematic repulsive interaction between C=O and N of two adjacent molecules. Knowing that dissolved 7,8-dihydroxanthopterin can oxidize to xanthopterin, we replaced xanthopterin with 7,8-dihydroxanthopterin in the model. An excellent fit was obtained with the powder X-ray diffraction pattern of the biogenic crystals. We then analyzed the biogenic block-shaped crystals in their solid state, using MALDI-TOF and Raman spectroscopy. All three methods unequivocally prove that the block-shaped crystals in the eye of S. lucioperca are crystals of 7,8-dihydroxanthopterin. On the basis of the eye anatomy, we deduce that the guanine crystals form a reflective layer producing the silvery color present on part of the eye surface, whereas the block-shaped crystals backscatter light into the retina in order to increase the light sensitivity of the eye.
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(2019) Proceedings of the National Academy of Sciences of the United States of America. 116, 46, p. 22946-22952 Abstract
The most widely used antimalarial drugs belong to the quinoline family. Their mode of action has not been characterized at the molecular level in vivo. We report the in vivo mode of action of a bromo analog of the drug chloroquine in rapidly frozen Plasmodium falciparum-infected red blood cells. The Plasmodium parasite digests hemoglobin, liberating the heme as a byproduct, toxic to the parasite. It is detoxified by crystallization into inert hemozoin within the parasitic digestive vacuole. By mapping such infected red blood cells with nondestructive X-ray microscopy, we observe that bromoquine caps hemozoin crystals. The measured crystal surface coverage is sufficient to inhibit further hemozoin crystal growth, thereby sabotaging heme detoxification. Moreover, we find that bromoquine accumulates in the digestive vacuole, reaching submillimolar concentration, 1,000-fold more than that of the drug in the culture medium. Such a dramatic increase in bromoquine concentration enhances the drug's efficiency in depriving heme from docking onto the hemozoin crystal surface. Based on direct observation of bromoquine distribution in the digestive vacuole and at its membrane surface, we deduce that the excess bromoquine forms a complex with the remaining heme deprived from crystallization. This complex is driven toward the digestive vacuole membrane, increasing the chances of membrane puncture and spillage of heme into the interior of the parasite.
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(2019) Chemistry of Materials. 31, 12, p. 4479-4489 Abstract
Until recently it was thought that the only optical function of pteridines in biology was to act as light-absorbing pigments, but a recent report by some of us revealed that crystalline isoxanthopterin is a reflector in the eyes of decapod crustaceans. Here, we report the formation of crystalline isoxanthopterin synthetically from the polar dimethyl sulfoxide solvent, with X-ray diffraction analysis revealing a crystal structure different from that of biogenic isoxanthopterin. The structure of the new polymorph was determined in two independent ways. In one approach, it was generated and optimized using first-principles calculations, followed by comparison of simulation and experiment for high-resolution powder X-ray diffraction (PXRD) and electron diffraction. In the other approach, the structure was obtained definitively from PXRD data using a direct-space genetic algorithm for structure solution followed by Rietveld refinement. The synthetic structure is different from its biogenic counterpart, especially in having a nonplanar criss-cross H-bonded arrangement. We also rationalized the morphology of the crystals and the effect of the DMSO thereon, via a comparison between observed and theoretical growth morphologies. In addition, we calculated the optical properties of the synthetic structure and found its two dominant refractive indices to be somewhat lower than those of its biogenic counterpart, but still as high as those of reflecting guanine crystals. Synthetic isoxanthopterin therefore emerges as a promising candidate for incorporation in artificial optical systems.
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(2019) ChemPlusChem. 84, 4, p. 338-344 Abstract
Cholesterol crystallization from mixtures of unesterified cholesterol with phospholipids and cholesterol esters is believed to be a key event in atherosclerosis progression. Not much is understood, however, about the influence of the lipid environment on cholesterol crystallization. Here we study cholesterol monohydrate crystal formation from mixed bilayers with palmitoyl-oleoyl-phosphatidylcholine (POPC), dipalmitoyl-phosphatidylcholine (DPPC) and sphingomyelin. We show that disordered phospholipids and sphingomyelin stabilize the formation of crystal plates of the triclinic cholesterol monohydrate polymorph, whereas saturated glycerolipids stabilize helical and tubular crystals of the metastable monoclinic polymorph. We followed the subsequent transformation of these helical crystals into the stable triclinic plates. Discovering the relations between membrane lipid composition and cholesterol crystal polymorphism may provide important clues to the understanding of cholesterol crystal formation in atherosclerosis.
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(2019) ChemPlusChem. 84, 4, p. 317-317 Abstract
Invited for this month's cover are the group of Prof. Lia Addadi at the Weizmann Institute of Science, Israel and collaborators at the Università Degli Studi di Milano, Italy, and the ALBA Synchrotron Light Source, Spain. The front cover shows how cholesterol crystals form in macrophage cells and in lipid bilayers of different compositions. Cholesterol monohydrate stable triclinic crystals form in vitro as rhomb-shaped plates, whereas the monoclinic crystals fold into tubular or helical shapes. Read the full text of the article at 10.1002/cplu.201800632.
2018
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(2018) Proceedings Of The National Academy Of Sciences Of The United States Of America-Biological Sciences. 115, 30, p. 7662-7669 Abstract
The formation of atherosclerotic plaques in the blood vessel walls is the result of LDL particle uptake, and consequently of cholesterol accumulation in macrophage cells. Excess cholesterol accumulation eventually results in cholesterol crystal deposition, the hallmark of mature atheromas. We followed the formation of cholesterol crystals in J774A.1 macrophage cells with time, during accumulation of LDL particles, using a previously developed correlative cryosoft X-ray tomography (cryo-SXT) and stochastic optical reconstruction microscopy (STORM) technique. We show, in the initial accumulation stages, formation of small quadrilateral crystal plates associated with the cell plasma membrane, which may subsequently assemble into large aggregates. These plates match crystals of the commonly observed cholesterol monohydrate triclinic structure. Large rod-like cholesterol crystals form at a later stage in intracellular locations. Using cryotransmission electron microscopy (cryo-TEM) and cryoelectron diffraction (cryo-ED), we show that the structure of the large elongated rods corresponds to that of monoclinic cholesterol monohydrate, a recently determined polymorph of the triclinic crystal structure. These monoclinic crystals form with an unusual hollow cylinder or helical architecture, which is preserved in the mature rodlike crystals. The rod-like morphology is akin to that observed in crystals isolated from atheromas. We suggest that the crystals in the atherosclerotic plaques preserve in their morphology the memory of the structure in which they were formed. The identification of the polymorph structure, besides explaining the different crystal morphologies, may serve to elucidate mechanisms of cholesterol segregation and precipitation in atherosclerotic plaques.
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(2018) Proceedings of the National Academy of Sciences of the United States of America. 115, 10, p. 2299-2304 Abstract
The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.
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(2018) Journal of the Royal Society Interface. 15, 139, 20170930. Abstract
This study investigates the structural basis for the red, silver and black coloration of the theridiid spider, Phoroncidia rubroargentea (Berland, 1913) from Madagascar. Specimens of this species can retain their colour after storage in ethanol for decades, whereas most other brightly pigmented spider specimens fade under identical preservation conditions. Using correlative optical, structural and chemical analysis, we identify the colour-generating structural elements and characterize their optical properties. The prominent silvery appearance of the spider's abdomen results from regularly arranged guanine microplatelets, similar to those found in other spiders and fish. The microplatelets are composed of a doublet structure twinned about the [02(1) over bar] axis, as suggested by electron diffraction. The red coloration originates from chambered microspheres (approx. 1 mu m in diameter), which contain structured fluorescent material. Co-localization of the red microparticles on top of the reflective guanine microplatelets appears to enhance the red coloration. The spider's thick cuticular layer, which encases its abdomen, varies in its optical properties, being transparent in regions where only guanine reflectors are present, and tanned, exhibiting light absorption where the red microspheres are found. Moreover, colour degradation in some preserved spider specimens that had suffered damage to the cuticular layer suggests that this region of the exoskeleton may play an important role in the stabilization of the red coloration.
2017
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(2017) Scientific Reports. 7, 1, 7610. Abstract
A key drug target for malaria has been the detoxification pathway of the iron-containing molecule heme, which is the toxic byproduct of hemoglobin digestion. The cornerstone of heme detoxification is its sequestration into hemozoin crystals, but how this occurs remains uncertain. We report new results of in vivo rate of heme crystallization in the malaria parasite, based on a new technique to measure element-specific concentrations at defined locations in cell ultrastructure. Specifically, a high resolution correlative combination of cryo soft X-ray tomography has been developed to obtain 3D parasite ultrastructure with cryo X-ray fluorescence microscopy to measure heme concentrations. Our results are consistent with a model for crystallization via the heme detoxification protein. Our measurements also demonstrate the presence of considerable amounts of non-crystalline heme in the digestive vacuole, which we show is most likely contained in hemoglobin. These results suggest a tight coupling between hemoglobin digestion and heme crystallization, highlighting a new link in the crystallization pathway for drug development.
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(2017) Scientific Reports. 7, 1, 802. Abstract
Red blood cells infected by the malaria parasite Plasmodium falciparum are correlatively imaged by tomography using soft X-rays as well as by scanning hard nano-X-ray beam to obtain fluorescence maps of various elements such as S and Fe. In this way one can deduce the amount of Fe bound either in hemoglobin or in hemozoin crystals in the digestive vacuole of the malaria parasite as well as determine the hemoglobin concentrations in the cytosols of the red blood cell and of the parasite. Fluorescence map of K shows that in the parasite's schizont stage the K concentration in the red blood cell cytosol is diminished by a factor of seven relative to a pristine red blood cell but the total amount of K in the infected red blood cell is the same as in the pristine red blood cell.
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(2017) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 56, 32, p. 9420-9424 Abstract
Guanine crystals are widely used in nature as components of multilayer reflectors. Guanine-based reflective systems found in the copepod cuticle and in the mirror of the scallop eye are unique in that the multilayered reflectors are tiled to form a contiguous packed array. In the copepod cuticle, hexagonal crystals are closely packed to produce brilliant colors. In the scallop eye, square crystals are tiled to obtain an image-forming reflecting mirror. The tiles are about 1 mm in size and 70 nm thick. According to analysis of their electron diffraction patterns, the hexagon and square tiles are not single crystals. Rather, each tile type is a composite of what appears to be three crystalline domains differently oriented and stacked onto one another, achieved through a twice-repeated twinning about their and crystal axes, respectively. By these means, the monoclinic guanine crystal mimics higher symmetry hexagonal and tetragonal structures to achieve unique morphologies.
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(2017) Israel Journal of Chemistry. 57, 1, p. 9-12 Abstract
The starting point was an overlap with Jack Dunitz in studies on the preferred conformation of the carboxyl group in saturated and unsaturated acids, esters and amides, orientational disorder of the hydrogen-bonded carboxylic acid dimer in benzoic acid crystal structures, and intramolecular lone pair-lone pair repulsion between oxygen atoms. The journey described here outlines a search to establish the existence of C−H⋅⋅⋅O hydrogen bonds and engineering cocrystals that incorporate intermolecular lone pair-lone pair repulsion between oxygen atoms.
2016
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(2016) Journal of the American Chemical Society. 138, 45, p. 14931-14940 Abstract
We have developed a high resolution correlative method involving cryo-soft X-ray tomography (cryo-SXT) and stochastic optical reconstruction microscopy (STORM), which provides information in three dimensions on large cellular volumes at 70 nm resolution. Cryo-SXT morphologically identified and localized aggregations of carbon-rich materials. STORM identified specific markers on the desired epitopes, enabling colocalization between the identified objects, in this case cholesterol crystals, and the cellular environment. The samples were studied under ambient and cryogenic conditions without dehydration or heavy metal staining. The early events of cholesterol crystal development were investigated in relation to atherosclerosis, using as model macrophage cell cultures enriched with LDL particles. Atherosclerotic plaques build up in arteries in a slow process involving cholesterol crystal accumulation. Cholesterol crystal deposition is a crucial stage in the pathological cascade. Our results show that cholesterol crystals can be identified and imaged at a very early stage on the cell plasma membrane and in intracellular locations. This technique can in principle be applied to other biological samples where specific molecular identification is required in conjunction with high resolution 3D-imaging.
2015
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(2015) Chemistry of Materials. 27, 24, p. 8289-8297 Abstract
Living organisms display a spectrum of wondrous colors, which can be produced by pigmentation, structural coloration, or a combination of the two. A relatively well-studied system, which produces colors via an array of alternating anhydrous guanine crystals and cytoplasm, is responsible for the metallic luster of many fish. The structure of biogenic anhydrous guanine was so far believed to be the same as that of the synthetic one, a monoclinic polymorph (denoted as α). Here we re-examine the structure of biogenic guanine, using detailed experimental X-ray and electron diffraction data, exposing troublesome inconsistencies, namely, a "guanigma". To address this, we sought alternative candidate polymorphs using symmetry and packing considerations and then utilized first-principles calculations to determine whether the selected candidates could be energetically stable. We identified theoretically a different monoclinic polymorph (denoted as β), were able to synthesize it, and confirmed using X-ray diffraction that it is this polymorph that occurs in biogenic samples. However, the electron diffraction data were still not consistent with this polymorph but rather with a theoretically generated orthorhombic polymorph (denoted as γ). This apparent inconsistency was resolved by showing how the electron diffraction pattern could be affected by crystal structural faults composed of offset molecular layers.
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(2015) PHYSICA SCRIPTA. 90, 11, 118003. Abstract
This cooperative endeavour first describes early studies in chemical crystallography, encompassing molecular packing modes, characterization of weak hydrogen bonds, the engineering of functional crystals and monitoring of reaction pathways in molecular crystals by x-ray and neutron diffraction. With the design of 'tailor-made' auxiliary molecules, it became possible to correlate molecular enantiomerism and crystal enantiomorphism, to control the early stages of crystal nucleation, to resolve enantiomers by crystallization, induce the precipitation of metastable polymorphs, and shed light on the role played by solvent on crystal growth. With such auxiliaries, the structure of mixed crystals was revised and the ability to perform 'absolute' asymmetric synthesis in host centrosymmetric crystals demonstrated. With the introduction of grazing incidence synchrotron x-ray diffraction from liquid surfaces it also became possible to design and characterize crystalline thin film architectures at the air-water interface providing a general insight on the mechanism of crystal nucleation at the molecular level, in particular that of ice and cholesterol. Finally the collective knowhow from these studies were crucial for obtaining homochiral peptides prepared from the polymerization of racemates of amphiphilic amino acids dissolved in aqueous solution, and for experiments towards elucidating the pathological crystallization of cholesterol and the malaria pigment in Plasmodium-infected red blood cells.
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(2015) Journal of the American Chemical Society. 137, 4, p. 1601-1607 Abstract
Atherosclerosis is the major precursor of cardiovascular disease. The formation of cholesterol crystals in atherosclerotic plaques is associated with the onset of acute pathology. The cholesterol crystals induce physical injury in the plaque core, promoting cell apoptosis and triggering an increased inflammatory response. Herein we address the question of how cholesterol crystal formation occurs in atherosclerosis. We demonstrate that three-dimensional (3D) cholesterol crystals can undergo directed nucleation from bilayer membranes containing two-dimensional (2D) cholesterol crystalline domains. We studied crystal formation on supported lipid bilayers loaded with exogenous cholesterol and labeled using a monoclonal antibody that specifically recognizes ordered cholesterol arrays. Our findings show that 3D crystals are formed exclusively on the bilayer regions where there are segregated 2D cholesterol crystalline domains and that they form on the domains. This study has potentially significant implications for our understanding of the crucial step in the mechanism by which atherosclerotic lesions form.
2014
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(2014) Crystal Growth & Design. 14, 4, p. 1543-1554 Abstract
Hemozoin is a crystalline byproduct formed upon hemoglobin digestion in Plasmodium-infected blood cells. Based on X-ray powder diffraction (XRPD), hemozoin and its synthetic analogue β-hematin are very similar in structure, consisting of cyclic dimers (cd) of ferriprotoporphyrin IX [Fe(3+)-PPIX] molecules coordinated via Fe-O(propionate) bonds. Enantiofacial symmetry of Fe(3+)-PPIX implies formation of four different stereoisomeric dimers, two centrosymmetric (1̄), labeled cd1̄1 and cd1̄2, and two enantiomeric, cd2(+) and cd2(-), in which the Fe(3+)PPIX moieties are related by pseudo-2-fold symmetry. Only the cd1̄1 stereoisomer was reported as the repeat unit in the initial structural elucidation of β-hematin and refinement of hemozoin. Our recent study of β-hematin, employing a combination of XRPD and density functional theory (DFT), revealed besides the published phase, characterized in terms of a disordered cd1̄1/cd2(±) mixture, which is diffractionally equivalent to a cd1̄1/cd1̄2 mixture, a minor phase considered to comprise mainly cd1̄2 dimers. As a consequence single-phase β-hematin powders were recently reanalyzed in terms of a cd1̄1/cd1̄2 mixture, yielding an average occupancy ≅75:25. Here, we present evidence enhancing the biphase model of β-hematin. The primary focus is on a reexamination of the hemozoin structure in light of a biochemically based dimerization mechanism that we recently hypothesized. We suggest that upon hemoglobin degradation, the heme byproduct retains the O2 molecule bound to Fe on the Re side of the heme until Fe-O(propionate) coordination between such heme molecules occurs across their unbound Si sides yielding the cd2(+) dimer. We report Rietveld refinement of the hemozoin structure using data measured on an all-in-vacuum powder diffractometer assuming the following models: cd1̄1, cd1̄2, cd2(+), and the two mixtures cd1̄1/ cd1̄2 and cd1̄1/cd2(+). The best figures of merit were obtained for the mixture cd1̄1/cd2(+) with a 50:50 occupancy, followed by the cd1̄1/cd1̄2 mixture with an occupancy ≅75:25, which we interpret as a structure that comprises the cd1̄1, cd2(+), cd2(-), and cd1̄2 isomers with occupancies ≅58:17:17:8. In this model system, the cd1̄1 "host" molecule is uniformly distributed throughout the crystal, whereas the enantiomeric molecules cd2(+) and cd2(-) are preferentially occluded in different crystalline sectors, which are thus enantiomorphous, related by overall centrosymmetric symmetry. Various arguments appear to favor the 50:50 cd1̄1/cd2(+) mixture, namely, a hemozoin crystal of overall chiral symmetry, consistent with our hypothesis. However, we cannot overrule the alternative model.
2013
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(2013) Langmuir. 29, 47, p. 14595-14602 Abstract
Crystallization of the malaria pigment hemozoin sequesters the toxic heme byproduct of hemoglobin digestion in Plasmodium-infected red blood cells (RBCs). Recently, we applied electron and X-ray imaging and diffraction methods to elucidate this process. We observed crystals oriented with their {100} faces at the inner membrane surface of the digestive vacuole (DV) of Plasmodium falciparum in parasitized RBCs. Modeling of the soft X-ray tomographic (SXT) images of a trophozoite-stage parasite indicated a 4-16 nm DV membrane thickness, suggesting a possible role for lipid multilayers. Here, we reanalyzed the trophozoite SXT images quantitatively via X-ray absorption to map the DV membrane thickness. Making use of the chemical structure and crystal density of the lipid, we found, predominantly, a bilayer 4.2 nm thick, and the remainder was interpreted as patches ∼8 nm thick. Image analysis of electron micrographs also yielded a 4-5 nm DV membrane thickness. The DV lipid membrane is thus mainly a bilayer, so induced hemozoin nucleation occurs primarily via the inner of the membrane's two leaflets. We argue that such a leaflet embodying mono- and di-acyl lipids with appropriate OH or NH bearing head groups may catalyse hemozoin nucleation by stereochemical and lattice match to the {100} crystal face, involving a two-dimensional nucleation aggregate of ∼100 molecules.
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(2013) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 52, 26, p. 6629-6632 Abstract
Molecular crystals: The structures and relative energies of glycine polymorphs are determined using dispersion corrections to PBE and PBEh density functionals. The picture shows a potential-energy surface for the a-b plane of γ-glycine obtained with density functional theory including many-body dispersion interactions.
2012
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(2012) Proceedings of the National Academy of Sciences of the United States of America. 109, 28, p. 11184-11187 Abstract
The human malaria parasite Plasmodium falciparum detoxifies the heme byproduct of hemoglobin digestion in infected red blood cells by sequestration into submicron-sized hemozoin crystals. The crystal is composed of heme units interlinked to form cyclic dimers via reciprocal Fe - O (propionate) bonds. Templated hemozoin nucleation was envisaged to explain a classic observation by electron microscopy of a cluster of aligned hemozoin crystals within the parasite digestive vacuole. This dovetails with evidence that acylglycerol lipids are involved in hemozoin nucleation in vivo, and nucleation of β-hematin, the synthetic analogue of hemozoin, was consistently induced at an acylglycerol-water interface via their {100} crystal faces. In order to ascertain the nature of hemozoin nucleation in vivo, we probed the mutual orientations of hemozoin crystals in situ within RBCs using synchrotron-based X-ray nanoprobe Fe fluorescence and diffraction. The X-ray patterns indicated the presence of hemozoin clusters, each comprising several crystals aligned along their needle c axes and exposing {100} side faces to an approximately cylindrical surface, suggestive of nucleation via a common lipid layer. This experimental finding, and the associated nucleation model, are difficult to reconcile with recent reports of hemozoin formation within lipid droplets in the digestive vacuole. The diffraction results are verified by a study of the nucleation process using emerging tools of three-dimensional cellular microscopy, described in the companion paper.
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(2012) Proceedings of the National Academy of Sciences of the United States of America. 109, 28, p. 11188-11193 Abstract
Heme detoxification is a critical step in the life cycle of malariaca-using parasites, achieved by crystallization into physiologically insoluble hemozoin. The mode of nucleation has profound implications for understanding the mechanism of action of antimalarial drugs that inhibit hemozoin growth. Several lines of evidence point to involvement of acylglycerol lipids in the nucleation process. Hemozoin crystals have been reported to form within lipid nanospheres; alternatively, it has been found in vitro that they are nucleated at an acylglycerol lipid-water interface.We have applied cryogenic soft X-ray tomography and three-dimensional electron microscopy to address the location and orientation of hemozoin crystals within the digestive vacuole (DV), as a signature of their nucleation and growth processes. Cryogenic soft X-ray tomography in the "water window" is particularly advantageous because contrast generation is based inherently on atomic absorption. We find that hemozoin nucleation occurs at the DV inner membrane, with crystallization occurring in the aqueous rather than lipid phase. The crystal morphology indicates a common {100} orientation facing the membrane as expected of templated nucleation. This is consistent with conclusions reached by X-ray fluorescence and diffraction in a companion work. Uniform dark spheres observed in the parasite were identified as hemoglobin transport vesicles. Their analysis supports a model of hemozoin nucleation primarily in the DV. Modeling of the contrast at the DV membrane indicates a 4-nm thickness with patches about three times thicker, possibly implicated in the nucleation.
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(2012) Biophysical Journal. 103, 2, p. 255-264 Abstract
Grazing incidence x-ray diffraction measurements were performed on single hydrated bilayers and monolayers of Ceramide/Cholesterol/1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocyholine at varying concentrations. There are substantial differences in the phase and structure behavior of the crystalline domains formed within the bilayers relative to the corresponding monolayers, due to interactions between the opposing lipid leaflets. Depending on the lipid composition, these interactions lead to phase separation and formation of cholesterol crystals. The cholesterol and ceramide/cholesterol mixed phases were further characterized at 37°C by immunolabeling with specific antibodies recognizing ordered molecular arrays of cholesterol. Previous studies have shown that cholesterol may nucleate in artificial membranes to form thick two-dimensional bilayer crystals. The study herein demonstrates further growth of cholesterol into three-dimensional crystals. We believe that these results may provide further insight into the formation of cholesterol crystals in early stages of atherosclerosis inflammation.
2011
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(2011) Israel Journal of Chemistry. 51, 10, p. 1017-1033 Abstract
The use of achiral crystalline architectures as intermediate auxiliaries for the performance of "absolute" asymmetric transformations is reviewed. Such architectures are delineated, in some cases, by pairs of homochiral surfaces of opposite handedness. This phenomenon is more common among organic crystals that frequently appear in triclinic, monoclinic, orthorhombic, or tetragonal space groups. Consequently, the chiral surfaces of such crystals have been shown to display enantiomeric recognition for molecules of the environment, a process that has been instrumental in the conversion of achiral host crystals into enantiomorphous solid-solutions, for a successful performance of "absolute" asymmetric transformations and for the control of crystal polymorphism. Mixed crystals of reduced symmetry display properties such as second harmonic generation or pyroelectricity. On the other hand, achiral faces delineate metals, which crystallize in cubic space-groups of high symmetry. However, by slicing such crystals in particular directions, they might express homochiral high Miller index faces that contain homochiral kink sites, which have been successfully exploited in electrochemical separations of sugars and for the resolution of enantiomers by enantioselective desorption. Representative examples of each class of materials are described.
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(2011) Crystal Growth & Design. 11, 8, p. 3342-3350 Abstract
Hemozoin is a crystalline byproduct formed upon host hemoglobin digestion in malaria-infected blood cells, crucial for parasitic survival. On the basis of published spectroscopic and X-ray powder diffraction (XRPD) data, hemozoin is believed to be very similar to the synthetic compound β-hematin, which consists of cyclic centrosymmetric dimers of ferriprotoporphyrin IX [Fe(3+) PPIX] molecules coordinated via Fe-O bonds. The enantio-facial symmetry of Fe(3+) PPIX implies, however, that four different Fe-O cyclic stereoisomers, two centrosymmetric and two chiral, of opposite handedness, should be formed in the crystallizing solution of β-hematin. A low-temperature XRPD study of β-hematin, i.e. synthetic hemozoin, revealed the presence, not only of the published phase (Pagola, S.; Stephens, P. W.; Bohle, D. S.; Kosar, A. D.; Madsen, S. K.Nature 2000, 404, 307) but also of a minor phase. We propose, based on Rietveld refinement and DFT+vdW computations (companion manuscript, DOI: 10.1021/cg200409d), that the minor phase consists mainly of the second centrosymmetric isomeric type in a crystal structure similar to that of the major phase. The enantiomeric chiral isomers may, on symmetry grounds, be enantioselectively occluded into the growing crystals, introducing disorder. The chiral dimers, on being first adsorbed on the crystal faces, would act as tailor-made additives, retarding crystal growth, which also explains the crystalline micrometer size. The existence of two phases in β-hematin may be crucial for a fuller understanding and more complete determination of the crystal structure of hemozoin, of which only one phase has crystallized according to published data.
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(2011) Crystal Growth & Design. 11, 8, p. 3332-3341 Abstract
Malaria, an infectious disease once considered eradicated, has reemerged in recent years, primarily due to parasite resistance to commonly used synthetic antimalarial drugs. These drugs act by inhibiting crystallization of the malaria pigment, hemozoin (HZ). Thus, there is a vital need for understanding the process of HZ nucleation. In a companion paper, the pseudopolymorphic behavior of β-hematin, the synthetic form of HZ, has been characterized by X-ray diffraction (XRD) (Straasø, T.; Kapishnikov, S.; Kato, K.; Takata, M.; Als-Nielsen, J.; Leiserowitz, L.Cryst. Growth Des. 2011, 11, DOI: 10.1021/cg200410b). Here, we employ van der Waals (vdW)-corrected density functional theory (DFT) to study the two β-hematin crystal structures and their repeat unit, a heme dimer. We find that vdW interactions play a major role in the binding of the heme dimer and the β-hematin crystal. In addition, accounting for the periodic nature of the system is essential to obtaining the correct geometry of the heme dimer, which is affected by vdW interactions with adjacent dimers in the β-hematin crystal. The different stereoisomers of the heme dimer and their molecular crystals are close in energy, which is consistent with pseudopolymorphism in β-hematin, in agreement with recent XRD experiments. Finally, we use our results to comment on β-hematin crystallization mechanisms. This work demonstrates the viability of vdW-corrected DFT as a tool for gaining valuable insight into pertinent problems involving biological systems.
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(2011) Angewandte Chemie - International Edition. 50, 16, p. 3620-3629 Abstract
Biological membranes comprise thousands of different lipids, differing in their alkyl chains, headgroups, and degree of saturation. It is estimated that 5%of the genes in the human genome are responsible for regulating the lipid composition of cell membranes. Conceivably, the functional explanation for this diversity is found, at least in part, in the propensity of lipids to segregate into distinct domains, which are important for cell function. X-ray diffraction has been used increasingly to characterize the packing and phase behavior of lipids in membranes. Crystalline domains have been studied in synthetic membranes using wide- and small-angle X-ray scattering, and grazing incidence X-ray diffraction. Herein we summarize recent results obtained using the various X-ray methods, discuss the correlation between crystalline domains and liquid ordered domains studied with other techniques, and the relevance of crystalline domains to functional lipid domains in biological membranes.
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(2011) ACS Chemical Biology. 6, 3, p. 275-287 Abstract
Inhibition of hemozoin biocrystallization is considered the main mechanism of action of 4-aminoquinoline antimalarials including chloroquine (CQ) but cannot fully explain the activity of ferroquine (FQ) which has been related to redox properties and intramolecular hydrogen bonding. Analogues of FQ, methylferroquine (Me-FQ), ruthenoquine (RQ), and methylruthenoquine (Me-RQ), were prepared. Combination of physicochemical and molecular modeling methods showed that FQ and RQ favor intramolecular hydrogen bonding between the 4-aminoquinoline NH group and the terminal amino group in the absence of water, suggesting that this structure may enhance its passage through the membrane. This was further supported by the use of Me-FQ and Me-RQ where the intramolecular hydrogen bond cannot be formed. Docking studies suggest that FQ can interact specifically with the {0,0,1} and {1,0,0} faces of hemozoin, blocking crystal growth. With respect to the structure-activity relationship, the antimalarial activity on 15 different P. falciparum strains showed that the activity of FQ and RQ were correlated with each other but not with CQ, confirming lack of cross resistance. Conversely, Me-FQ and Me-RQ showed significant cross-resistance with CQ. Mutations or copy number of pfcrt, pfmrp, pfmdr1, pfmdr2, or pfnhe-1 did not exhibit significant correlations with the IC50 of FQ or RQ. We next showed that FQ and Me-FQ were able to generate hydroxyl radicals, whereas RQ and me-RQ did not. Ultrastructural studies revealed that FQ and Me-FQ but not RQ or Me-RQ break down the parasite digestive vacuole membrane, which could be related to the ability of the former to generate hydroxyl radicals.
2010
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(2010) Journal of the American Chemical Society. 132, 28, p. 9920-9927 Abstract
Grazing incidence X-ray diffraction measurements were performed on single hydrated bilayers and monolayers of DPPC:Cholesterol:POPC at varying concentrations. There are substantial differences in the phase and structure behavior of the crystalline domains formed within the bilayers relative to the corresponding monolayers, due to interactions between the opposing leaflets. Depending on the lipid composition, these interactions led to phase separation, changes in molecular tilt angle, or formation of cholesterol crystals. In monolayers, DPPC and cholesterol form a single crystalline phase at all compositions studied. In bilayers, a second crystalline phase appears when cholesterol levels are increased: domains of cholesterol and DPPC form monolayer thick crystals where each of the lipid leaflets diffracts independently, whereas excess cholesterol forms cholesterol bilayer thick crystals at a DPPC:Chol ratio
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(2010) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 49, 13, p. 2354-2357 Abstract
A pulsed IR laser beam was used to align a peptide at the air-water interface. This peptide was designed to form a cyclic ß-strand dimer through Glu-Lys interactions in solution, which, when spread onto water, yielded a self-assembled ß-sheet bilayer (see picture) following solvent evaporation. During this process illumination with linearly polarized laser light induced formation of an aligned crystalline film, whereas circular polarization did not. (Figure Presented).
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2009
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(2009) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 48, 47, p. 8958-8961 Abstract
Interacting leaflets: Grazing-incidence X-ray diffraction measurements are performed on single hydrated lipid bilayers. Comparison between the structure of a monolayer composed of sphingomyelin, cholesterol, and phosphocholine (36:54:10) with that of a bilayer of the same composition (see picture) shows differences in structure and phase behavior, because of the formation of bilayerthick cholesterol crystals.
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(2009) European Physical Journal E. 30, 2, p. 215-221 215. Abstract
X-ray scattering experiments on mixed films of cholesterol and phospholipids at air-water and Si solid-water interfaces were undertaken to glean information on pathological crystallization of cholesterol bilayers. Grazing-incidence X-ray diffraction patterns at the air-water interface of various cholesterol:dipalmitoyl-phosphatidylcholine (Ch:DPPC) monolayer mixtures compressed beyond monolayer collapse yielded the established 10×7.5 Å2 Ch bilayer motif, for Ch:DPPC molar ratios higher than 2.5:1. Attempts to obtain a diffraction signal from various Ch:phospholipid film mixtures at the Si solid-water interface, indicative of the presence of the Ch bilayer motif, were unsuccessful. Only after removal of sufficient water from the cell was a weak diffraction signal obtained suggestive of a cholesterol film two bilayers thick. Off-specular X-ray reflectivity measurements made on a 1.75:1 mixture of Ch and bovine cardiac phosphatidylcholine (BCPC) deposited as a bilayer on a Si wafer and placed in a cell filled with water yielded positive results. The derived electron density profile showed the presence of a bilayer mixture consistent with a phase separation of cholesterol and BCPC, and possible formation of a crystalline cholesterol bilayer within the hydrated mixed bilayer, but not a proof thereof.
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(2009) Journal of Chemical Physics. 130, 14, 144704. Abstract
The key to functionalize of engineered molecularly nanometer thick films lies in the ability to reproducibly control their structure. A number of factors influence the film morphology of self-assembled films on solid or liquid surfaces, such as the structure of the molecules/particles, wetting, solvent hydrodynamics, and evaporation. An important example is the deposition of amphiphilic molecules from a volatile solution, self-assembled onto a water surface at monolayer coverage. Upon evaporation, a myriad of microscopic two-dimensional (2D) crystallites forms a ruptured film lying in random orientation on the surface, resulting in "2D powders." Here we present a general technique, employing linearly polarized laser pulses and varying solvent composition to influence the assembly of molecules such as poly-benzyl-L-glutamate and alamethicin on water surfaces, resulting in ultrathin molecular films with aligned regions that point in the same direction, though macroscopically separated. The experimental results are tentatively explained by a mechanism that is based on excluded volume forces and "kick model" for the effect of laser pulses to induce molecular rotation that eventually results in an aligned pattern when the system is at a collective state.
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(2009) Scanning Microscopy 2009. Vol. 7378. Abstract
New developments in X-ray instrumentation and analysis have facilitated the development and improvement of various scanning X-ray microscopy techniques. In this contribution, we offer an overview of recent scanning hard X-ray microscopy measurements performed at the Swiss Light Source. We discuss scanning transmission X-ray microscopy in its transmission, phase contrast, and dark-field imaging modalities. We demonstrate how small-angle X-ray scattering analysis techniques can be used to yield additional information. If the illumination is coherent, coherent diffraction imaging techniques can be brought to bear. We discuss how, from scanning microscopy measurements, detailed measurements of the X-ray scattering distributions can be used to extract high-resolution images. These microscopy techniques with their respective imaging power can easily be combined to multimodal X-ray microscopy.
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(2009) Crystal Growth & Design. 9, 1, p. 626-632 Abstract
The nucleation of malaria pigment (hemozoin) and β-hematin crystals (synthetic hemozoin) can be promoted by lipid molecules. To determine the orientation of β-hematin crystals nucleated by 1-myristoyl-glycerol (MMG) on the water surface, we undertook a grazing incidence synchrotron X-ray diffraction and X-ray reflectivity study. Our results indicate that premixed α-hematin with MMG yielded β-hematin nanocrystals oriented with the {100} face parallel to water surface, which we explain insofar that the spreading solution allows MMG molecular aggregation into clusters exposing OH groups and oxygen lone-pair electrons that interact with hematin molecules. Hematin molecules, which did not interact with MMG clusters, do not yield β-hematin, but rather an unknown crystalline phase. Independently, evidence is presented that self-assembled functionalized alkanethiol monolayers (SAMs) exposing OH groups induce β-hematin nucleation primarily via its {100} face, whereas those exposing CH3 induce nucleation of either the {100} or {010} face, preferentially {010}. The results can be explained in terms of a difference in affinity of the SAMs to these two crystal faces. The two sets of β-hematin nucleation experiments, conducted under different conditions, strengthen the notion that MMG molecules, mimicking a digestive vacuole lipid surface, induce, via stereospecific interactions, oriented nucleation of β-hematin at the {100} face.
2008
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(2008) Chemical Reviews. 108, 11, p. 4899-4914 Abstract
We review the interplay between malaria, the formation of synthetic β-hematin and natural crystalline hemozoin, and antimalarial drug action and design. The crystal morphology of β-hematin and natural hemozoin is described, including a simple analysis of why only very few types of stable crystal faces may be expected in keeping with observation. Drug action of various types of antimalarials, such as quinolines, phenanthrene derivatives, and diethylamino-alkoxyxanthones is postulated as a result of stereospecific binding thereof to specific hemozoin crystal faces and subsequent inhibition of crystal growth. The experimental evidence indicating adsorption of the quinoline type drugs to particular crystal faces is reviewed in terms of possible binding sites on those faces, the functional groups of the drug, change in crystal (β-hematin) morphology, and various methods, suggesting that the quinoline drugs were occluded within the growing crystals. The drugs may be described as hemozoin crystal capping agents (CMOLs) in view of their proposed ability to bind stereospecifically to particular faces of the crystal, despite being completely different in molecular structure from that of the β-hematin cyclic molecular dimer, and thus inhibit growth along two principal directions. In concert with pure drug, halofantrine or quinoline methanol-Fe3+-PPIX complexes would act as TMAs binding stereo-selctively to the third principal face and inhibiting its growth. On the other hand, artemisinin drug action is described as a result of binding of artemisinin-β-hematin cyclic dimer adducts to all of the principal faces of hemozoin, resulting in effective crystal growth inhibition. Such proposed adducts may be considered as tailor-made crystal growth inhibitors since their molecular structures are the same as that of the β-hematin cyclic dimer but for an added (artemisinin) moiety bound to various sites on the periphery of the porphyrin rings and thus tailored for adsorption to specific faces. A detailed description of the various hemozoin surface binding sites with a focus on their spatial arrangement is given, to best design effective inhibitors of hemozoin nucleation and growth. Finally, nucleation of hemozoin crystals is reviewed in view of very recent findings that acyl-glycerols within lipid bodies promote the- nucleation process. Future work would have to provide evidence whether there is any crystal alignment within the lipid bodies including the crystallographic nature thereof and develop methods to map the distribution of antimalarial drugs occluded within hemozoin crystals.
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(2008) Langmuir. 24, 16, p. 8458-8464 Abstract
We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by α-quaterthiophene on gold (α-4TC12H24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The α-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the α-4T on the gold surface adopts a 2D crystal structure.
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(2008) Angewandte Chemie : international edition. 47, 20, p. 3680-3682 Abstract
Symmetry violation: Recent results by Tremel and co-workers on the phase selection of calcium carbonate through the chirality of adsorbed amino acids appear to provide a deterministic route to "mirror-symmetry breaking", which may have ramifications for the emergence of homochirality on Earth. In our view, however, the crystallization experiments violate basic rules of symmetry and we suspect the presence of chemical, biological, or other homochiral or achiral contaminants in the system.
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(2008) Chirality. 20, 5, p. 736-748 Abstract
A method for direct assignment of the absolute configuration of molecules and the absolute structures of polar crystals, independent to that of Bijvoet, is described. The method correlates between the two-dimensional packing arrangement of specific faces, that delineate crystals during their growth and dissolution, with molecules present in the environment. The structural information stored in these faces is transferred to "tailor-made" molecules added to the solvent by controlled morphological changes induced to the growing crystals and by the creation of etch pits at specific crystal faces during their dissolution. In addition, the "tailor-made" molecules are occluded enantioselectively as guests within specific sectors of the host crystals. The method is illustrated for a variety of molecules and crystals including the assignment of the absolute configuration of several α-amino acids as "tailor-made" additives in centrosymmetric crystals of glycine and serine, for the absolute structure of polar crystals of sugars and α-amino acids and consequently the absolute configuration of molecules packed in such crystals.
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(2008) Crystal Growth & Design. 8, 2, p. 507-511 Abstract
The metallic luster from the skin of fish is due to a photonic crystal system composed of multilayer stacks of cytoplasm and crystals. The crystals are described as thin (50-100 nm) plates of guanine, with no reference to their hydration state. We established through X-ray diffraction that their crystal structure is that of anhydrous guanine. We noted that their crystal structurefunction relationship is exceptional compared to other purines with similar molecular stacking of the crystal structure. These elongate in the direction of molecular stacking, in contrast to the biogenic anhydrous guanine crystals whose smallest dimension is in the stacking direction. On the basis of the known crystal structure of anhydrous guanine, theoretical growth morphology was calculated. These calculations predict crystals elongated in the direction of the molecular stacking. The exposed molecular plane of the biogenic crystals is the (102) plane, which is composed of densely packed H-bonded guanine molecules. It is known that the in-plane polarizability of guanine molecules is significantly higher than the direction perpendicular to the molecular plane, most likely causing anisotropy of the crystals refractive index. It is therefore conceivable that the unique morphology observed in crystals from the skin of fish is designed to enhance their light reflective properties.
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(2008) Current Opinion In Colloid & Interface Science. 13, 1-2, p. 12-22 Abstract
Recent experimental and theoretical studies on functional self-assemblies formed at interfaces are presented and placed in perspective to earlier studies. The focus is on chiral and achiral systems that are at the boundary between two-dimensional and three-dimensional architectures and investigated at almost molecular level. More specifically, we present studies on: (i) self-organization of achiral molecules and separation of racemates into two-dimensional enantiomorphous domains at the air-water interface; (ii) molecular recognition between solute molecules and Langmuir films; (iii) the structural correlation between chiral morphology of mesoscopic architectures and the absolute configuration of the component molecules; (iv) polymerization of amphiphilic molecules at various interfaces and (v) the use of two-dimensional films as templates in three-dimensional crystallization, with relevance to biomineralization.
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(2008) Engineering Of Crystalline Materials Properties: State Of The Art In Modeling, Design And Applications. p. 271-290 Abstract
The lecture notes describe the design, characterization and use of crystalline thin film architectures at the air-water interface. Grazing incidence X-ray diffraction (GIXD) using synchrotron radiation has been applied as the key method for structural elucidation of the crystalline films, which are generally composed of amphiphilic molecules. The following topics are covered in these notes. (a) A brief description of GIXD, given in the Appendix; (b) the design of cc-amino acid amphiphiles, a racemic mixture of which on water spontaneously separates into two-dimensional (21)) crystalline islands of opposite handedness according to GIXD measurements; (c) the use of amphiphilic monolayers for induced nucleation of molecular 31) crystals at the air-solution interface, with a focus on the induced nucleation of ice; (d) casting light on pathological crystallization of cholesterol, which precipitates in gallstones and atherosclerotic plaques, via the use of GIXD for trapping the cholesterol nucleation process.
2007
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(2007) The Lock-and-Key Principle, The State of the Art--100 Years On. Vol. 1. p. 173-246 Abstract
This chapter contains sections titled:IntroductionGeneration and Amplification of Chirality in the Solid StateSelfAggregation of Amphiphilic Molecules at AirSolution InterfacesMolecular Recognition During Crystal NucleationChiral Amplification in The System GlycineSoluble CYAmino AcidsConclusion and Outlook
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(2007) Journal of the American Chemical Society. 129, 9, p. 2615-2627 Abstract
The morphology of micrometer-sized β-hematin crystals (synthetic malaria pigment) was determined by TEM images and diffraction, and by grazing incidence synchrotron X-ray diffraction at the air-water interface. The needle-like crystals are bounded by sharp {100} and {010} side faces, and capped by {011} and, to a lesser extent, by {001} end faces, in agreement with hemozoin (malaria pigment) crystals. The β-hematin crystals grown in the presence of 10% chloroquine or quinine took appreciably longer to precipitate and tended to be symmetrically tapered toward both ends of the needle, due to stereoselective additive binding to {001} or {011} ledges. Evidence, but marginal, is presented that additives reduce crystal mosaic domain size along the needle axis, based on X-ray powder diffraction data. Coherent grazing exit X-ray diffraction suggests that the mosaic domains are smaller and less structurally stable than in pure crystals. IR-ATR and Raman spectra indicate molecular based differences due to a modification of surface and bulk propionic acid groups, following additive binding and a molecular rearrangement in the environment of the bulk sites poisoned by occluded quinoline. These results provided incentive to examine computationally whether hemozoin may be a target of antimalarial drugs diethylamino-alkoxyxanthones and artemisinin. A variation in activity of the former as a function of the alkoxy chain length is correlated with computed binding energy to {001} and {011} faces of β-hematin. A model is proposed for artemisinin activity involving hemozoin nucleation inhibition via artemisinin-β-hematin adducts bound to the principal crystal faces. Regarding nucleation of hemozoin inside the digestive vacuole of the malaria parasite, nucleation via the vacuole's membranous surface is proposed, based on a reported hemozoin alignment. As a test, a dibehenoyl- phosphatidylcholine monolayer transferred onto OTS-Si wafer nucleated far more β-hematin crystals, albeit randomly oriented, than a reference OTS-Si.
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(2007) AIP conference proceedings. 916, p. 401-415 Abstract
In this lecture we present a stereochemical approach to crystal growth and dissolution and processes related to topochemical polymerizations in the solid phase. Understanding of such processes should take into consideration the packing arrangements of the crystals involved, the texture and kinetics of growth of the faces that delineate these crystals as well as the nature of the solvent where the crystals are grown or reacted and the structures of possible supramolecular architecures formed in the solution by the molecules undergoing crystallization. This stereochemical methodology is represented by the following three examples: (i) The role played by the addition of methanol or ethanol in the precipitation of the metastable β-form of glycine, (ii) The resolution of several α-amino acids salts that crystallize in enantiomorphous space groups, however, are composed form alternating lamellae of opposite handedness, and (iii) Polymerization reaction in crystals where the short chains formed in the course of the reaction self-assemble and induce the formation of isotactic polymers.
2006
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(2006) Journal Of Physical Chemistry B. 110, 44, p. 22178-22184 Abstract
We have analyzed grazing incidence diffration (GIXD) data from condensed phases of Langmuir films of long-chain fatty acids at the air-water interface by using a new method consisting of a careful extraction of the structure factors followed by fitting of molecular parameters. We show that, contrary to the general belief, the information contained in GIXD spectra is enough to obtain near-atomic structural information. In particular, we directly determine for the first time the orientation of the chain backbone planes and of the carboxylic headgroups and we evaluate chain conformation defects. This new method allowed us to evidence a new phase of symmetry p2gm at high pressure, corresponding to a minimum in lattice energy, but never observed.
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(2006) Crystal Growth & Design. 6, 3, p. 625-628 Abstract
In a recent paper, Srinivasan and Sherwood reported that α-resorcinol crystals grow unidirectionally at the {011} faces in the vapor phase and suggested that this phenomenon of unidirectional growth is intrinsic to polar crystals. Here, we present a molecular modeling study suggesting a plausible "self-poisoning" mechanism that may explain the unidirectional growth of α-resorcinol in the vapor phase, a process that does not exclude the inhibition of crystal growth by solvent.
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2005
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(2005) Crystal Growth & Design. 5, 6, p. 2190-2196 Abstract
The control of crystal polymorphism of the trimorphic crystals of glycine (Gly) grown in aqueous solutions in the presence of α-amino acids operating as stereospecific nucleation inhibitors is reported. The presence of enantiopure α-amino acids phenylalanine (Phe), methionine (Met), and tryptophan (Trp) in the crystallizing aqueous solutions induces changes in the morphology of α-Gly leading to the formation of pyramidal instead of bipyramidal crystals. Increased concentrations of racemic Phe and Met inhibit both the α- and β-polymorphs of glycine and induce precipitation of the thermodynamically most stable γ-polymorph. α-Amino acids that bear bulky side groups such as racemic tryptophan (Trp), N-CH3-Trp, and α-naphthylalanine induce precipitation of the least stable β-Gly polymorph. Quasi-racemic mixtures of R-Trp and S-Phe (or S-Met), for example, lead to the precipitation of one of the enantiomorphs of β-Gly. The roles played by the different α-amino acids in affecting morphology and polymorphism are discussed in terms of their interactions with and stereoselective occlusion in the various sectors of the {010} faces of the β-Gly crystals.
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(2005) Prebiotic Chemistry. p. 123-165 Abstract
Theories of prebiotic life suggest that homochirality emerged in Nature in abiotic times via deterministic or chance scenarios. This chapter deals with experiments demonstrating the feasibility of stochastic mirror symmetry breaking that occurs via autocatalytic processes involving the self-assembly of molecular clusters, 2-D and 3-D crystals, supramolecular organo-metallic catalysts, and polymeric helices and sheets. Once generated spontaneously by chance, chirality can be preserved and propagated to the environment provided that the symmetry breaking step is coupled with a sequential step of efficient amplification via self-replication reactions. Common features for the systems of relevance are that they take into consideration small fluctuations from the racemic state, and they display non-linear kinetic effects induced by diastereoisomeric supramolecular self-assemblies that exhibit different physical or chemical properties.
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(2005) Angewandte Chemie : international edition. 44, 21, p. 3226-3229 Abstract
(Graph Presented) Crystallization of the thermodynamically metastable β glycine is explained on the basis of its growth at the opposite poles of the crystal (see graph) and by considering the structure of the relevant surfaces of the three polymorphs and their interactions with solvent molecules.
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(2005) Biophysical Journal. 88, 5, p. 3381-3391 Abstract
The structure of monolayers of cholesterol/ceramide mixtures was investigated using grazing incidence x-ray diffraction, immunofluorescence, and atomic force microscopy techniques. Grazing incidence x-ray diffraction measurements showed the existence of a crystalline mixed phase of the two components within a range of compositions of cholesterol/ ceramide between 100:0 and 67:33. The mixed phase coexists with the ceramide crystalline phase in the range of compositions between 50:50 and 30:70; between 30:70 and 0:100 only the highly crystalline phase of ceramide was detected. The latter was determined and modeled. Immunolabeling was performed with an antibody specific to the cholesterol monohydrate crystalline arrangement. The antibody recognizes crystalline cholesterol monolayers, but does not interact with crystalline ceramide. Immunofluorescence and atomic force microscopy data show that in uncompressed ceramide monolayers, the highly crystalline phase coexists with a disordered loosely packed phase. In contrast, no disordered phase coexists with the new crystalline mixed phase. We conclude that the new mixed phase represents a stable homogeneous arrangement of cholesterol with ceramide. As ceramide incorporates the lipid backbone common to all sphingolipids, this arrangement may be relevant to the understanding of the molecular organization of lipid rafts.
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(2005) Biophysical Journal. 88, 3, p. 1809-1817 Abstract
Crystalline nucleation of cholesterol at the air-water interface has been studied via grazing incidence x-ray diffraction using synchrotron radiation. The various stages of cholesterol molecular assembly from monolayer to three bilayers incorporating interleaving hydrogen-bonded water layers in a monoclinic cholesterol · H2O phase, has been monitored and their structures characterized to near atomic resolution. Crystallographic evidence is presented that this multilayer phase is similar to that of a reported metastable cholesterol phase of undetermined structure obtained from bile before transformation to the triclinic phase of cholesterol · H2O, the thermodynamically stable macroscopic form. According to grazing incidence x-ray diffraction measurements and crystallographic data, a transformation from the monoclinic film structure to a multilayer of the stable monohydrate phase involves, at least initially, an intralayer cholesterol rearrangement in a single-crystal-to-single-crystal transition. The preferred nucleation of the monoclinic phase of cholesterol · H2O followed by transformation to the stable monohydrate phase may be associated with an energetically more stable cholesterol bilayer arrangement of the former and a more favorable hydrogen-bonding arrangement of the latter. The relevance of this nucleation process of cholesterol monohydrate to pathological crystallization of cholesterol from cell biomembranes is discussed.
2004
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(2004) Journal Of Physical Chemistry B. 108, 22, p. 7228-7240 Abstract
The structure and reactivity of alkyl esters of several α-amino acids self-assembled at the air/water interface have been investigated as part of our studies on mechanisms that are possibly relevant for the generation of homochiral prebiotic peptides. Grazing incidence X-ray diffraction (GIXD) studies of monolayers of racemic and enantiopure alkyl esters and thio-esters of alanine on the water surface demonstrated that these racemates self-assemble in the form of mixed solid solutions, because of disorder of the headgroups of the two enantiomers (enantiomeric disorder) within the two-dimensional (2D) crystallites. Matrix-assisted laser-desorption ionization time-of-flight Mass Spectrum (MALDI-TOF MS) analysis of the products collected from the air/water interface indicated the formation of low-molecular-weight oligopeptides (primarily dimers) and, in the case of some of the thioesters, small quantities of trimers and tetramers. Mass spectrometric studies on the diastereoisomeric distribution of the oligopeptides, starting from deuterium enantio-labeled monomers, demonstrated binomial statistics, such as that in reactions occurring in an isotropic environment. The alkyl esters of phenylalanine and tyrosine did not form 2D crystallites at the air/water interface, and, upon polycondensation, they yielded only dipeptides. The enantiomeric disorder within the 2D crystallites of the monomers of the alkyl esters and thioesters of racemic serine was absent. Polycondensation of these esters, however, yielded only dipeptides and tripeptides and they were not investigated further. In contrast to previous reports, the present studies demonstrate that this reaction does not proceed beyond the dipeptide stage and, therefore, cannot be regarded as a plausible system for the generation of prebiotic peptides.
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(2004) Origins of Life and Evolution of the Biosphere. 34, 1-2, p. 79-92 Abstract
A possible role that might have been played by ordered clusters at the air/water interface for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated α-amino acids that assembled as two-dimensional (2-D) crystallites at this interface. Three type of processes are described: (i) polymerization of racemates of activated α-amino acids that undergo spontaneous resolution into enantiomorphous 2-D crystallites to yield racemic mixtures of oligopeptides enriched with the oligomers of homochiral sequence, (ii) enhanced formation of racemic mixtures of homochiral oligopeptides via lattice-controlled polymerization within 2-D racemic compounds and (iii) generation of homochiral oligopeptides of a single handedness from chiral non-racemic mixtures of monomers that self-assemble into two different phases, racemic crystallites composed from both enantiomers and enantiomorphous crystallites of the enantiomer in excess. The structures of the 2-D crystallites have been determined by grazing incidence X-ray diffraction and the diastereoisomeric composition of the oligopeptides by matrix-assisted laser-desorption time-of-flight mass spectrometry with enantio-labeling.
2003
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(2003) Journal of the American Chemical Society. 125, 51, p. 15922-15934 Abstract
Crystalline ternary inclusion monolayers consisting of a two-dimensional hydrogen-bonded host network of guanidinium (G) ions and organosulfonate (S) amphiphiles, and biphenylalkane guests, can be generated at the air-water interface through synergistic structural enforcement by hydrogen bonding and host-guest packing. Surface pressure-area isotherms of the 4 -hexadecylbiphenyl-4-sulfonate (C16BPS) amphiphile in the presence of G, with or without guest, are characterized by lift-off molecular areas expected for the GS sheet based on single-crystal X-ray structures of homologous bulk crystals. Intercalation of biphenylalkane guests (4-CnH 2n+1-C6H4-C6H5, n = 1, 4, 6, 10, 16; denoted CnBP) between organosulfonate hydrophobes, which define pocketlike cavities in the GS monolayer host, afford ternary inclusion monolayers with a 1:1 host-guest stoichiometry. These inclusion monolayers are less compressible than the guest-free host, consistent with dense packing of the biphenylalkane moieties of the host and the biphenylalkane guests. The inclusion monolayers are distinguished from the amorphous guest-free host and from selected guanidinium-free mixed monolayers by structural characterization with grazing-angle incidence X-ray diffraction (GIXD). The GIXD data for the ternary (G)C16BPS:C16BP and (G)C16BPS:C6BP inclusion monolayers obtained upon compression are consistent with a rectangular unit cell. The dimensions of these unit cells and refinement of the GIXD data suggest a "rotated shifted ribbon" GS hydrogen-bonding motif similar to that observed in some bulk GS crystals, including (G) (ethylbiphenylsulfonate). GIXD reveals that (G)C16BPS:C16BP and (G)C16BPS:C6BP are more crystalline than the corresponding guanidinium-free mixed monolayers. The (G)C16BPS:C6BP inclusion monolayer is stable upon compression, even though the alkyl-alkyl host-guest interactions are reduced due to the shorter hexyl substituents of the guest, demonstrating an important reinforcing role for the hydrogen-bonded GS sheet. The structure of a C16BPS: tetracosane (C24) mixed monolayer is independent of G; the unit cell symmetry and dimensions suggest a structure governed by alkyl-alkane interactions that prohibit formation of a GS network. These results illustrate that the existence of ternary inclusion monolayers with an intact GS network requires guest molecules that are structurally homologous with the hydrophobes of the host, in this case biphenylalkanes. The observation of these inclusion compounds suggests an approach for introducing functional nonamphiphilic molecules to an air-water interface through inclusion in a well-defined host.
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(2003) Crystal Growth & Design. 3, 5, p. 683-690 Abstract
Cholesteryl-L-glutamate (CLG) thin films were investigated when deposited on pure water and on saturated solutions of water soluble amino acids, to test the capacity of the CLG monolayer to recognize and incorporate amino acids from the subphase. Analysis of grazing incidence X-ray diffraction measurements indicates that the hydrophobic amino acids, leucine, isoleucine, and valine, are adsorbed between the glutamate moieties, within the CLG crystalline monolayer, but also around their domains, forming, for L-Leu and D-Val, crystalline bilayers. The presence of Cu2+ ions in the aqueous subphase enhances the crystallinity of the CLG monolayer without overly affecting the lateral packing, by bridging glutamate moieties on neighbor molecules. When amino acids are injected in a Cu2+ containing solution, the domain size of the mixed monolayers formed with the CLG molecules and hydrophobic amino acids is drastically increased.
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(2003) Chemistry-A European Journal. 9, 8, p. 1782-1794 Abstract
A possible role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions has been probed by a catalyzed polymerization of amphiphilic activated α-amino acids, in racemic and chiral non-racemic forms, which had self-assembled into two-dimensional (2D) ordered crystallites at the air-aqueous solution interface. As model systems we studied Nε-stearoyl-lysine thioethyl ester (C18-TE-Lys), γ-stearyl-glutamic thioethyl ester (C18-TE-Glu), Nα-carboxyanhydride of γ-stearyl-glutamic acid (C18-Glu NCA) and γ-stearyl-glutamic thioacid (C18-thio-Glu). According to in-situ grazing incidence X-ray diffraction measurements on the water surface, (R,S)-C18-TE-Lys, (R,S)-C18-TE-Glu, and (R,S)-C18-Glu-NCA amphiphiles self-assembled into ordered racemic 2D crystallites. Oligopeptides 2-12 units long were obtained at the air-a-queous solution interface after injection of appropriate catalysts into the water subphase. The experimental relative abundance of oligopeptides with homochiral sequence generated from (R,S)-C18-TE-Lys and (R,S)-C18-TE-Glu, as determined by mass spectrometry on enantioselectively deuterium-labeled samples, was found to be significantly larger than that obtained from (R,S) C18-thio-Glu which polymerizes randomly. An efficient chiral amplification was obtained in the polymerization of non-racemic mixtures of C18-Glu-NCA since the monomer molecules in the racemic 2D crystallites are oriented such that the reaction occurs between heterochiral molecules related by glide symmetry to yield heterochiral oligopeptides whereas the enantiomer in excess, in the enantiomorphous crystallites, yield oligopeptides of a single handedness.
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(2003) Crystal Growth & Design. 3, 2, p. 125-150 Abstract
In this review, the delicate interplay between stereochemical control, monitoring at the subnanometer level, and an understanding of crystal nucleation is probed. Control of crystal nucleation may be achieved employing tailor-made auxiliaries, which are either nucleation inhibitors or promoters. The process may be monitored at an interface via grazing incidence X-ray diffraction (GIXD). By these means, we can glean experimental knowledge of crystal nucleation in various molecular systems. A hypothesis was invoked that supersaturated solutions containing molecular clusters adopt various arrangements and shapes, some of which resemble the crystals into which they develop. This hypothesis was taken advantage of for the design of tailored inhibitors in achieving kinetic resolution of enantiomers and induced precipitation of particular crystal polymorphs. The control and behavior of polymorphic crystallization may be understood at the molecular level through the interplay between inhibitor, solvent, solute, crystal and layer lattice energies, as well as surface layer structures. With respect to promotion of crystal nucleation, it may be achieved by Langmuir monolayers at the air-aqueous solution interface, acting as a templating agent. Determination of the monolayer crystal structure by GIXD yields the extent and nature of the complementary fit between nucleator and nucleant. Finally, GIXD was applied to monitor by a snapshot technique the layer-by-layer crystalline assembly of cholesterol molecules at the air-water interface, which involved changes in molecular packing as the film grew in thickness.
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(2003) Helvetica Chimica Acta. 86, 11, p. 3867-3874 Abstract
A grazing-incidence X-ray-diffraction (GIXD) study of the self-assembly, on water, of nonracemic γ-stearyl glutamic acid (pure or as a mixture with racemic or (S)-1,2 -dipalmitoyl-glycero-3-phosphoethanolamine (DPPE)) demonstrated a phase separation of the α-amino acid amphiphile into racemic and enantiomorphous two-dimensional crystallites within the phospholipid domains. The packing arrangements of the two α-amino acid crystalline phases were identical to those found in the absence of DPPE and have been determined, at almost atomic resolution, by X-ray structure-factor calculations. By contrast, racemic and nonracemic Nε -stearoyllysine spontaneously segregated into two-dimensional enantiomorphous domains within the DPPE environment that induced a change in the tilt direction of the hydrocarbon chains of the α-amino acid molecules. Phase separation of nonracemic amphiphiles, originating from preferred lateral homochiral or heterochiral intermolecular interactions, is in agreement with the formation of enantiomerically pure or enriched homochiral oligopeptides in overrepresented amounts in the polycondensation of activated nonracemic amphiphilic α-amino acids on plain water or within phospholipid monolayers.
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(2003) Advanced Materials. 15, 1, p. 38-42 Abstract
The nanoscale structure and molecular organization of an organic hydrogel in its native state are evaluated. The data collected allows to understand the molecular assembly processes that lead to the gelation of water by organic molecules. Various spectroscopic, microscopic and X ray techniques are used to study the molecular organization and structure of gels.
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(2003) Helvetica Chimica Acta. 86, 8, p. 2711-2725 Abstract
A strong Bragg peak, attributed to regular corrugation in a crystalline film, was detected in a series of self-assembled supramolecular complexes of bifunctional bolaamphiphiles of different lengths, with divalent ions of Pb and Cu, at the air - aqueous interface. This peak has a d spacing half that of the proposed corrugation-repeat length of ca. 82 Å, which is not a multiple of the long lattice spacing of the crystalline structure, and is susceptible to film compression. This corrugation is interpreted in terms of unfavorable interactions between the polar aqueous surface and the nonpolar hydrocarbon part of the bolaamphiphiles. To establish the nature of the corrugation, grazing-incidence X-ray diffraction and specular X-ray reflectivity with synchrotron radiation were applied, as well as scanning force microscopy of films deposited on mica.
2002
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(2002) Crystal Growth & Design. 2, 6, p. 553-562 Abstract
Human malaria, one of the most striking, reemerging infectious diseases, is caused by several types of Plasmodium parasites. As the parasite digests hemoglobin in human red blood cells, the heme byproduct crystallizes into micron-sized malaria-pigment (hemozoin). Making use of a recently reported powder-crystal structure determination of synthetic hemozoin (β-hematin), we describe here its theoretical growth form and show it to be similar in habit and form to that of natural hemozoin. With this information, we propose a noncovalent binding site for the quinoline drug family at the end face of the fastest-growing direction of β-hematin. This adsorption mechanism is examined in terms of crystal growth inhibition vis-à-vis published data. The surface binding site elucidates the difference in activity of various quinolines, revealing the importance of the different quinoline functionalities. The interplay between molecular chirality of quinolines and the chirality of centrosymmetric β-hematin crystal faces is analyzed in terms of crystal growth inhibition. We additionally propose a molecular isomerism of the crystalline building blocks, with implications on quinoline surface binding, as well as on nucleation and size of β-hematin crystals.
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(2002) Journal of the American Chemical Society. 124, 31, p. 9093-9104 Abstract
The feasibility of generating oligopeptides with homochiral sequence via lattice-controlled polymerization of racemic mixtures of precursor molecules that undergo spontaneous segregation into two-dimensional (2-D) enantiomorphous domains at the air-aqueous solution interface was analyzed. For model systems, we studied the polymerization reaction within 2-D crystalline domains of mixtures of (R,S)-Nεstearoyl-thio-lysine with ∼10% (R,S)-Nε-stearoyl-lysine, and (R,S)-Nα-carboxyanhydride of Nε-stearoyllysine. According to in situ grazing incidence X-ray diffraction (GIXD) measurements at the air-water interface, the molecules form 2-D crystallites packing by translation symmetry only. Oligopeptides 4-6 units long were obtained at the air-solution interface after injection of an appropriate catalyst into the subphase. The course of the chemical transformations was monitored by GIXD. The distribution of the diastereoisomeric oligopeptides was determined by matrix-assisted laser-desorption ionization time-offlight (MALDI-TOF MS) mass spectrometry on samples prepared from precursor molecules enantioselectively labeled with deuterium. The experimental relative abundance of oligopeptides with homochiral sequence was found to be larger than that calculated for a theoretical random process, yielding an excess by a factor of 2.5-3.5 for the tetra- to hexapeptides. The present studies may be relevant for probing the role that might have been played by ordered clusters at interfaces for the generation of homochiral oligopeptides under prebiotic conditions.
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(2002) Journal of the American Chemical Society. 124, 32, p. 9342-9343 Abstract
A 30-residue peptide, BS30, which incorporates two proline residues to induce reverse turns, was designed to form a triple-stranded β-sheet monolayer at the air-water interface. To discern the structural role of proline, a second peptide, BS30G, identical to BS30 but with glycine residues replacing proline, was prepared and examined in parallel fashion. Surface pressure-molecular area isotherms indicated a limiting area per molecule (ca. 460 Å2) for BS30 that corresponds well to that estimated from the known dimensions of crystalline β-sheet monolayers (492 Å2). Comparable measurements on BS30G yielded a smaller molecular area (380 Å2). Grazing incidence X-ray diffraction measurements performed on the BS30 monolayer at nominal area per molecule of 500 Å2, exhibited two Bragg peaks corresponding to 4.79 and 34.9 Å spacings, consistent with formation of triple-stranded β-sheet structures that assemble into two-dimensional crystallites at the air-water interface. Visualized by Brewster angle microscopy, BS30 monolayers displayed uniform, solidlike domains, whereas BS30G appeared to be disordered.
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(2002) Colloids And Surfaces A-Physicochemical And Engineering Aspects. 208, 1-3, p. 3-27 Abstract
The advent of intense X-rays from synchrotron sources made possible to probe, at the molecular level, the structural aspects of self-assemblies generated at interfaces. Here we present the two-dimensional (2-D) packing arrangements of two-, three- and multi-component organo-metallic self-assemblies formed via interfacial reaction at the air-aqueous solution interface, as determined by grazing incidence X-ray diffraction (GIXD) and X-ray specular reflectivity techniques. GIXD yields structural information on the crystalline part of the Langmuir film, including the ions and counterions lateral order. Specular reflectivity yields the electron density profile of the film in the direction normal to the air-solution interface and, by modeling, gives independent information on the structure of both crystalline and amorphous parts of the film. In particular, we focus on the following systems: ordering of metal ions bound to the polar head groups of amphipilic molecules; use of bolaamphiphiles to generate oriented thin films with metal ions arranged in periodic layers; delineation of differences in the lateral organization of metal ions at interfaces as induced by racemates and enantiomerically pure amphiphiles; organization of ionophores in the presence of metal ions in membrane-like environment; self-assembly of 2×2 and 3×3 silver(I) grid-type complexes generated at the air-solution interface.
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(2002) ACS Symposium Series. 810, p. 242-253 Abstract
An experimental model for the spontaneous separation of enantiomers of racemic α-amino acids, between crystals of glycine grown at the air-aqueous solution interface and the solution itself, is presented. This process involves several steps. A small enantiomeric excess of chiral α-amino acids is first achieved by oriented growth of few glycine crystals at the solution surface. The growing glycine crystals occlude only one of the α-amino acid enantiomers through the appropriate enantiotopic {010} face exposed to the solution, yielding an enrichment in the solution of the other enantiomer. This chiral bias created in the solution is preserved and amplified by virtue of two effects. First is the formation, at the air-solution interface, of monolayer clusters of partially resolved hydrophobic α-amino acids arranged in a way akin to the glycine layer structure. Such monolayer clusters serve as templates for an oriented crystallization of fresh glycine crystals. Second, the water-soluble hydrophobic and hydrophilic α-amino acids enantiomerically enriched in the aqueous solution inhibit nucleation of "wrongly" oriented glycine crystals. Recent grazing incidence diffraction studies have provided direct information on the structure and dynamics of the monolayer clusters at the solution surface. Of particular importance are the observations that certain crystallites of racemic amphiphilic α-amino acids undergo spontaneous resolution in two dimensions. This effect opens new opportunities for the generation of homochiral oligopeptides from hydrophobic α-amino acids by their polymerization at interfaces.
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(2002) Science. 295, 5558, p. 1266-1269 Abstract
Differences in the two-dimensional packing arrangements of racemic and enantiomeric crystalline self-assemblies on the water surface of amphiphilic activated analogs of lysine and glutamic acid have been used to prepare oligopeptides of homochiral sequence and oligopeptides of single handedness from chiral nonracemic mixtures. The crystalline structures on the water surface were determined by grazing incidence x-ray diffraction and the diastereomeric composition of the oligopeptides by matrix-assisted laser desorption time-of-flight mass spectrometry with enantio-labeling. These results suggest that reactivity of ordered clusters at interfaces might have played a role in the generation of early homochiral biopolymers.
2001
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(2001) Journal Of Physical Chemistry B. 105, 46, p. 11447-11455 Abstract
Five bifunctional alpha -amino acid-epsilon -carboxy bolaamphiphiles [(2-RS, or RS)-(HOOC)-(CH2)(m)-CONH-(CH2)(n)-CH(COOH)(NH2) m=20,22, n=3,4 labeled (l), (d,l)-C(22)Orn (l), (d), (d,l)-C(22)Lys, (d,l)-C(24)Lys respectively] were synthesized. These molecules were deposited on different aqueous subphases, and studied by means of grazing incidence X-ray diffraction (GIXD). On deionized water, the bolaamphiphiles (dl)-C(22)Lys yield a mixture of crystallites: both a monolayer, in which the chains are tilted from the normal, and a multilayer in which the molecules lie parallel to the water. On the other hand, when deposited on mono- or bi-metal ionic subphases, they self-assemble into crystalline multilayer films in which the molecules lie parallel to the aqueous solution surface, linked head-to-head and tail-to-tail in the form of extended chains. The latter are juxtaposed such that the metal ions form sheets separated by the organic molecules, aligned perpendicular to the plane of the aqueous solution. Deposition of either the enantiomerically pure or racemic bifunctional bolaamphiphiles on an aqueous solution of mixed Cu(Acetate)(2) and Pb(Acetate)(2), yields self-assembled crystalline films composed of the two different metal cations, arranged in alternating sheets, separated by the organic spacer. Both GIXD and X-ray-photoelectric-spectroscopy (XPS) studies (after deposition on solid support) demonstrate that the structures of these films differ form the ones formed on either pure Pb(Ac)(2) or on Cu(AC)(2) solutions, thus excluding the simultaneous formation of the two monometallic crystalline phases.
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(2001) Journal Of Physical Chemistry B. 105, 36, p. 8563-8568 Abstract
Cholesterol and some cholesterol-like derivatives such as stigma-sterol and cholesterol acetate self-assemble into semicrystalline monolayers, bilayers, or trilayers on the surface of water, as has been reported previously. Here we have extended our thin film studies toward cholesteryl esters that, in their bulk crystals, pack as interdigitated bilayers due to the mismatch between the cross-sectional area of the rigid cholesterol moiety (40 Å2) and the attached hydrocarbon chain (20 Å2). As shown by grazing incidence X-ray diffraction (GIXD), cholesteryl esters spontaneously self-assemble on the surface of water in order to form interdigitated bilayer films. The unit cell parameters of such bilayers at the water surface almost match those of their three-dimensional counterparts. Other experiments such as surface pressure vs area per molecule isotherms and ellipsometry measurements corroborate this result. To control the growth of the interdigitated films, we have used "tailor-made" additives (long-chain alcohols or acids) that fill the voids in the chain region of the cholesteryl ester layer caused by the structural mismatch, thereby inhibiting interdigitation. Using this strategy, we were able to form a mixed monolayer composed of the ester and the additive molecules in a 1:1 ratio, where the hydrocarbon chain of additive molecule intercalates between the ester chains and effectively inhibits the growth of the interdigitated bilayer.
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(2001) Advanced Materials. 13, 8, p. 580-584 Abstract
Hybrid organic/inorganic Langmuir-Blodgett (LB) films were prepared by the synthetic solid/gas reaction. Addition to the thiocarboxylic acid to the LB film stabilized the ordered phase.
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(2001) Chemical Engineering Science. 56, 7, p. 2245-2253 Abstract
Keywords: MOLECULAR-DYNAMICS SIMULATIONS; OCTYL-D-GLUCONAMIDE; ALPHA-RESORCINOL; SUPERSATURATED SOLUTIONS; SOLUTION INTERFACE; ORGANIC-CRYSTALS; SURFACE; WATER; RECOGNITION; WETTABILITY
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(2001) Biophysical Journal. 81, 5, p. 2729-2736 Abstract
The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol · H2O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.
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(2001) Advances In Crystal Growth Research. p. 381-400 Abstract
Tailor-made molecules have been designed and used as additives, in crystallization processes from solution, to control the nucleation and growth of molecular crystals. These additives can be either inhibitors or promoters of crystal growth. Tailor-made inhibitors can be used for a variety of purposes that include crystal morphology engineering, kinetic resolution of racemates, reduction of crystal symmetry, assignment of absolute configuration of chiral molecules and polar crystals and control of crystal polymorphism. As for crystal growth promoters tailor-made additives, that self-assemble at the air-aqueous solution interface due to their intrinsic hydrophobic nature, have been used to induce oriented crystallization by means of a structural match.
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(2001) Journal of the American Chemical Society. 123, 16, p. 3771-3783 Abstract
Electrostatic interactions between amidinium and carboxylates were used for the construction of interdigitated architectures at the air-solution interface. Spreading the water-insoluble amphiphile p-pentadecylbenzoic acid (A) on an aqueous solution of p-methylbenzamidinium (B) ions results in an intercalation of the water-soluble base between the acidic headgroups of the water-insoluble amphiphile to form an amorphous A-B-A-B monolayer according to grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements. Upon compression the monolayer transforms into a crystalline film composed of three bilayers with interdigitated hydrocarbon chains, and a top layer whose chains are disordered. Water-insoluble p-heptadecylbenzamidinium spread on an aqueous solution of benzoic acid displays a surface pressure-area isotherm similar to that obtained from the above system. A mechanism that accounts for the formation of these films is presented. Deposition of p-heptadecylbenzamidinium and p-pentadecylbenzoic acid amphiphiles in a 1:1 ratio on pure water led to the formation of a crystalline monolayer phase but which is partially disordered. Over an aqueous solution containing a 1:1 mixture of benzamidinium and benzoic acid no measurable binding of these solute molecules to the polar headgroups of the 1:1 mixed monolayer could be detected by X-ray reflectivity or GIXD.
2000
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(2000) Journal of the American Chemical Society. 122, 50, p. 12523-12529 Abstract
Amphiphilic peptides comprising alternating hydrophilic and hydrophobic amino acid residues were designed to form super-secondary structures composed of self-assembled β-strands as monolayers at the air-water interface. Insights provided by in situ grazing-incidence X-ray diffraction (GIXD), surface pressure vs area isotherms, and Fourier transform infrared spectroscopy allow structural characterization of the assembled nanostructures and rational correlation with the peptide sequence. Peptides seven to seventeen amino acids in length were found to form crystalline arrays with coherence lengths in the range of 100 to 1000 Å. Two-dimensional registry of the self-assembled peptides was induced by placement of proline residues at the peptide termini. The films were found to intercalate ordered arrays of ions between juxtaposed β-sheet ribbons to generate peptide-ion composite phases.
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(2000) Journal of Physical Organic Chemistry. 13, 7, p. 426-434 Abstract
Structural studies on two-dimensional films of chiral amphiphiles at the air-solution interface, as investigated in situ primarily by grazing incidence x-ray diffraction (GIXD) using synchrotron radiation, yielded the crystalline packing arrangement at almost the molecular level. Results regarding three different topics are described. (1) It has proven possible to establish whether racemic mixtures of amphiphiles spread on water self-organize into 2-D crystals in which the two enantiomers either form heterochiral domains or spontaneously separate into enantiomorphous islands composed of homochiral molecules. Diastereoisomeric acid-base interactions between two different chiral amphiphiles were also used to achieve spontaneous chiral separation in two dimensions. (2) Ordered binding of solute molecules to the chiral amphiphiles could be applied in order to study their enantioselective interactions with chiral solutes present in the aqueous subphase. (3) Crystalline multilayer films of supramolecular architecture composed of water-soluble and water-insoluble chiral components may be formed at the air-solution interface and their packing arrangement determined by GIXD.
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(2000) Journal Of Physical Chemistry B. 104, 29, p. 6843-6850 Abstract
The absolute orientations of the amphiphilic molecules α-hydroxy ω-bromo alcohols BrCnH2n OH, n= 21, 22, and the alkyl hydroxy esters CmH2m+1COO(CH2)nOH, m = 14, 15, n = 10, in crystalline monolayer forms on water have been determined, the former by grazing incidence X-ray diffraction (GIXD) and the latter by sum frequency generation (SFG). The assignment was made for the alkyl hydroxy esters by establishing the polar angle between the terminal CH3-C bond and the normal to the plane of the monolayer; for the bromo alcohols the assignment was made by a determination of the two-dimensional crystal structure via X-ray structure factor calculations. The SFG results are in agreement with reported GIXD and lattice energy analyses of the alkyl hydroxy esters m = 19, n = 9, 10. These studies have further revealed the absolute orientation of the alcohol C-OH bonds at the water surface, which in turn can be correlated with the ice-nucleating behavior of the monolayers on supercooled water drops in terms of the odd and even values of n.
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(2000) Chemistry-A European Journal. 6, 12, p. 2173-2183 Abstract
Insertion of the 1,3-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 Å, induces chain folding at the air-water interface. These folded molecules self-assemble into crystalline monolayers at this interface, with the plane of the folding unit almost perpendicular to the water surface, as determined by synchrotron grazing-incidence X-ray diffraction. Three distinct molecular shapes, of the types U, inverted U, and M, were obtained in the two-dimensional crystal-line state, depending upon the number of spacer units, and the number and position of the hydrophilic groups in the molecule. The molecules form ribbons with a higher crystal coherence in the direction of stacking between the molecular ribbons, and a lower coherence along the ribbon direction. A similar molecule, but with a spacer unit that imposes a 5 Å separation between alkyl chains, yields the conventional herring-bone arrangement.
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(2000) Journal Of Physical Chemistry B. 104, 7, p. 1399-1428 Abstract
The packing arrangements of Langmuir films on aqueous solution of simple amphiphiles, such as fatty acids, alcohols, amides, and amino acids, are now established to near atomic resolution by the method of grazing incidence X-ray diffraction (GIXD), complemented by various spectroscopic and lattice energy computational techniques. For simple aliphatic chainlike amphiphilic molecules, it is possible to correlate the extent of two-dimensional (2-D) crystallinity of the Langmuir film with molecular interactions, in terms of the nature and length of the hydrophobic chain, the type of hydrophilic headgroup, and the binding properties thereto of solute ions and molecules from the aqueous subphase. The monolayer packing arrangements of amphiphilic molecules can be engineered for the performance of photoinduced topochemical reactions, and characterized by GIXD. Racemic mixtures of amphiphiles can also be engineered, by taking advantage of intermolecular hydrogen bonding, to undergo a spontaneous separation of the left- and right-handed molecules into 2-D chiral crystals at the air-solution interface. The geometry of binding of molecules or ions from the aqueous subphase to the hydrophilic headgroups can be pinpointed by GIXD, in favorable systems. The ordered binding of solutes to the amphiphile monolayer can lead to induced nucleation of oriented organic and inorganic crystals at the solution interface. GIXD has shown that such an induction can occur via even a partial lattice match, or by structural complementarity, sometimes involving a molecular rearrangement of the amphiphiles. It is possible from monolayer-induced crystallization to glean information on the process of nucleation and on the critical size of the nuclei. A variety of different types of crystalline multilayers, composed of water-insoluble molecules such as bolaform amphiphiles, alkanes, heterosubstituted aromatics, can be formed at the air-solution interface. The number of layers formed and their polymorphic behavior can be controlled, albeit within limits, with the use of tailor-made additives. Their structures can be determined by GIXD, thus providing data on the initial stages of 3-D crystalization. Multilayers, comprising water-insoluble and water-soluble components, assembled in situ at the solution surface leading to thin film supramolecular architectures, have been engineered. These crystals have been found to be oriented vis-à-vis the solution surface and thus amenable to characterization by GIXD and other methods.
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(2000) Chemistry-A European Journal. 6, 4, p. 725-734 Abstract
Oriented crystalline monolayers, ~ 14 Å thick, of a 2 x 2 Ag+ grid complex, self-assembled at the air-solution interface starting from an waterinsoluble ligand 3,6-bis[2-(6-phenylpyridine)]pyridazine spread on silver-ion-containing solutions, were examined by grazing-incidence X-ray diffraction and specular X-ray reflectivity using synchrotron radiation. The monolayer structure was refined, including a determination of the positions of the counterions, with the SHELX-97 computer program. The monolayers were transferred from the interface onto various solid supports and visualized by scanning force microscopy, and characterized by X-ray photoelectron spectroscopy in terms of molecular structure. On surface compression, the initial selfassembled monolayer undergoes a transition to a crystalline bilayer in which the two layers, almost retaining the original arrangement, are in registry. Such a phase transition is of relevance to the understanding of crystal nucleation.
1999
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(1999) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 38, 17, p. 2533-2536 Abstract
In 1848 Louis Pasteur manually separated enantiomorphous three- dimensional crystals. A 2D analogue of this experiment involving clusters of 1-nitronaphthalene bound to a gold surface has been recently performed by the research groups of Berndt and Schneider. The general principles governing separation of enantiomers from 3D crystals to 2D clusters at interfaces, and their characterization, are discussed.
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(1999) Journal Of Physical Chemistry B. 103, 33, p. 6891-6899 Abstract
To provide more direct information on the role played by "tailor-made" auxiliary molecules in the early stages of crystal nucleation, the interplay between clusters of polar headgroups of monolayers of the copper complexes 5-Cu-S and S-Cu-R and water-soluble copper complexes S-Cu-S and R-Cu-R were investigated [where S represents enantiomerically pure (S)-palmitoyl-Nε-lysine, and S and R represent chiral resolved (S) and (R) forms of alanine, serine, or valine]. The different monolayers were formed by spreading the amphiphilic (R) or (S) α-amino acid on an aqueous solution of copper acetate followed by injection of the water-soluble (S) or (R) α-amino acid into the subphase. The surface pressure-molecular area isotherms of the Langmuir monolayers of the two type of complexes (S-Cu-S and S-Cu-R) are different, the former being substantially more expanded. The polar headgroups of the S-Cu-S and of the S-Cu-R monolayers transferred onto a solid support assume a trans and cis configuration, respectively, according to comparative X-ray photoelectron spectroscopy (XPS) studies with appropriate cis and trans three-dimensional (3-D) α-amino acid Cu complexes. A grazing incidence X-ray diffraction (GIXD) analysis demonstrated that the S-Cu-S and S-Cu-R monolayers have different 2-D crystal structures, in keeping with the XPS results. A model is presented suggesting that the water-soluble S-Cu-S copper complexes are enantioselectively bound to the periphery of the domains of the cis S-Cu-S monolayers, but not to the domains of the trans S-Cu-R monolayers. By symmetry, the same principal holds for the monolayers and water soluble copper complexes of α-amino acids of the opposite handedness.
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(1999) Advanced Materials. 11, 4, p. 328-331 Abstract
On studies of large scale optical resolution by crystallization, it was found that α-amino acid salts crystalline in the form of conglomerates. The conglomerates were composed of lamellar twins, hence are not amenable to large scale resolution. By conducting further studies, it was found that the occurrence of the lamellar twins can be prevented with the assistance of 'tailor-made' enantioselective crystal nucleation inhibitors that allow the resolution of hydrophobic racemic α-amino acids by kinetic crystallization.
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(1999) Journal of the American Chemical Society. 121, 12, p. 2657-2661 Abstract
Acid - base interactions between two different chiral amphiphilic molecules (p-pentadecylmandelic acid and p-tetradecylphenylethylamine) are used in order to achieve spontaneous chiral segregation from a racemic mixture at the air - water interface, as shown by grazing incidence X-ray diffraction. The extent of possible chiral disorder for both acid and base components is examined. We show that an oblique lattice symmetry is insufficient to guarantee spontaneous segregation of chiral molecules in two- dimensional crystals because of possible chiral disorder.
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(1999) Journal of the American Chemical Society. 121, 4, p. 632-637 Abstract
Single crystals of benzamide which form hydrogen-bonded (001) bilayers are obtained from ethanol or 1-propanol solutions. The added presence of amide cosolvent such as X-CONH2, X = CH3 or CF3, yields very thin (001) plates which are twinned. Grazing incidence X-ray diffraction (GIXD) measurements from the (001) surface of pure benzamide indicates that the surface almost exclusively exposes the phenyl substituents. The observed GIXD data from the (001) surface of the affected crystals revealed lamellar twinning. Structural modeling of the data showed that the top surface layer contains about 15% adsorbed amide cosolvent, all oriented with their -X moieties exposed to air. A simple structural model has been invoked to account for the solvent-induced twinning.
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(1999) Journal of the American Chemical Society. 121, 6, p. 1186-1191 Abstract
The assembly, orientation, and structural features of nanoscale tubes composed of cyclic peptides, formed at the air-water interface, were detected by grazing incidence X-ray diffraction (GIXD). The peptide cyclo-[(L-Phe-D- N-MeAla-)4] (1) exhibits two-dimensional crystallinity in which the plane of the peptide ring is parallel to the water interface. The peptide cyclo-[(L- Trp-D-Leu)3-L-Ser-D-Leu] (2) forms predominantly planar aggregates composed of several tubes, lying with their long axes parallel to the air-water interface. In contrast, the peptide cyclo-[(L-Trp-D-Leu)4] (3) exhibits a very low tendency to form ordered two-dimensional arrays of nanotubes. Films of peptides 2 and 3 as well as their mixtures with the phospholipid DPPA were transferred onto a solid support and visualized by scanning force microscopy (SFM).
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(1999) Current Challenges On Large Supramolecular Assemblies. Vol. 519. p. 97-114 Abstract
Self-assembly of molecules into structured clusters is an important intermediate state en route to the growth of three-dimensional crystals or the formation of supramolecular architectures. We directed our efforts towards the preparation of functional clusters at the air-aqueous solution interface for several reasons: most important, thin crystalline films can be easily prepared at the water surface; furthermore, it is possible, at the air-liquid interface, to combine water-soluble and water-insoluble components to form multicomponent films; and thirdly, the flat nature of the water surface is ideal for performing surface X-ray diffraction measurements (GIXD) using synchrotron radiation for structure elucidation of the films, an analytical method that has emerged over the last decade. The thin-film clusters generated at the air-solution interface can be deposited on solid surfaces and then analyzed by other methods such as scanning force microscopy (SFM) and cryoelectron microscopy (cryo-TEM).
1998
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(1998) Acta Polymerica. 49, 10-11, p. 626-635 Abstract
Structural studies on Langmuir films of C50H102, nylon-6,6 polymer and its oligomeric analog provide information on the crystallization behavior of these substances in thin layers. While a nylon-6,6 oligomer and C50H102 form stable crystalline monolayers with the chains aligned mainly normal to the water surface, nylon-6,6 polymer assembles into a fibrous multilayer film (see SFM micrograph).
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(1998) Journal Of Physical Chemistry B. 102, 33, p. 6313-6317 Abstract
Model molecules, designed to simulate a chain fold, self-assemble into crystalline monolayers at the air - water interface, the structure of which has been determined, in situ, by synchrotron grazing-incidence X-ray diffraction. The two-dimensional packing arrangement in such monolayers is consistent with a structure composed of ribbons containing translationally related molecules adopting an inverted U-shape. The distance between these molecules along the ribbon is optimum for creating an additional chain fold, leading to the possibility that a polymeric molecule will undulate on the water surface.
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(1998) Langmuir. 14, 14, p. 3882-3888 Abstract
We compare the packing characteristics of alkyl chains in Langmuir monolayers of nonchiral and racemic compounds as determined from available grazing incidence X-ray diffraction data. The analysis demonstrates a gradual change of the projected unit cell dimensions from those of a hexagonal packing of hydrocarbon chains, characteristic of high-temperature monolayer phases, to one of two more dense rectangular packing modes with the projected unit cell dimensions 5.0 × 7.5 Å2 and 4.4 × 8.7 Å2, characteristic of low-temperature phases. The 5.0 × 7.5 Å2 unit cell incorporates the well-known herringbone arrangement, with an ideally 90° dihedral angle between the planes of carbon backbone chains. The 4.4 × 8.7 Å2 cell, almost never observed in 3D structures, is characterized by a 40° dihedral angle. We characterize the packing modes by lattice energy calculations. The distribution of the projected unit cell dimensions for the various Langmuir monolayers reveals no discontinuity in the local molecular order between crystalline phases and mesophases. The local symmetry of the molecular packing, as determined from the X-ray data, is compared with the symmetry deduced from the Landau theory of phase transitions. The symmetry of the local order in the mesophases is not maintained on the long-range length scale. We show that two phases of the same local molecular arrangement, a herringbone-ordered two-dimensional crystal and the corresponding one-dimensional mesophase, possess mutually orthogonal directions of glide symmetry.
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(1998) Journal of the American Chemical Society. 120, 19, p. 4850-4860 Abstract
Oriented crystalline films, ~11-20 Å thick, of metal ion complexes of the grid type [Co4L4]8+·8PF6- and [Ag9L6]9+ 9·CF3SO3-, based on various ligands L, were prepared in-situ at the air-aqueous solution interface by the interaction of the free ligand molecules spread onto aqueous solutions containing Co2+ or Ag+ ions. The structure of the complex architectures composed of a 2 x 2 Co2+ grid coordinated to four ligand molecules and a 3 x 3 Ag+ grid coordinated to six ligand molecules as well as their molecular organization in thin films were characterized by grazing incidence synchrotron X-ray diffraction (GIXD) and specular X-ray reflectivity (XR) measurements performed at the air-aqueous solution interface and by UV, X-ray photoelectron spectroscopy (xPs), and scanning force microscopy (SFM) after film transfer onto various solid supports. The results open perspectives toward an implementation of the air-water interface for the self-assembly and subsequent deposition of organized arrays of complex inorganic architectures onto solid support.
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(1998) Journal Of Physical Chemistry B. 102, 9, p. 1543-1548 Abstract
The structure of mixed Langmuir films of the normal alkane C32H66 with different quantities of the normal alcohol C31H63OH or the diol HO(CH2)30OH on pure water were determined in situ by applying surface-sensitive methods that include grazing incidence X-ray diffraction (GIXD) and infrared reflection - absorption spectroscopy (IRRAS) and by induced freezing of water drops. Random solid solutions were formed between the alkane and C31H63OH. On the other hand, segregation of the two components was observed in the mixed systems of the alkane and HO(CH2)30OH. These findings suggest new ways for the design of hydrophobic surfaces bearing hydrophilic sites, which may be appropriate for epitaxial ice nucleation and cloud seeding purposes.
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(1998) Chirality. 10, 1-2, p. 60-65 Abstract
A direct relationship between the structures of two-dimensional (2-D) crystallites of 4-octadecyloxy-E-cinnamic acid and 4-octadecyloxy-E- cinnamamide amphiphiles at the air-water interface and their photochemical behavior, is presented. The detailed packing arrangements of the monolayers were determined, close to the atomic level, from the diffraction pattern measured, in situ on the water surface, by the grazing incidence X-ray diffraction (GID) technique using synchrotron radiation. The products of the photochemical reaction, performed by irradiating the monolayer films directly on the water surface, were collected and analyzed. While the acid crystallites yield a β-truxinic acid product together with the cis-isomer, the amide undergoes only trans-cis isomerisation, in full agreement with the 2-D crystalline packing arrangements of the two amphiphiles on the water surface.
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(1998) Advanced Materials. 10, 2, p. 117-121 Abstract
Oriented crystalline thin films of alpha,omega-tetracosanedioic acid metal salts form in situ at the air-liquid interface, following the simple procedure described here. It is shown that the Cd2+ and Pb2+ metal ions have a dramatic influence on the packing of the diacid molecules, which self-assemble as multilayer domains approximately 50 Angstrom thick, with the molecules and bound ions perfectly ordered laterally over distances up to 1000 Angstrom. These oriented crystallites preserve their integrity upon transfer to a mica support and can be used to prepare quantum dots (see following communication).
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Grazing incidence X-ray diffraction studies of two-dimensional packing properties of amphiphilic alcohols at air-water interface(1998) Applied Crystallography : Proceedings of the XVII International Conference. p. 43-48 Abstract
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(1998) Chirality. 10, 5, p. 415-424 Abstract
Spontaneous separation of enantiomers in two- and three-dimensional crystals is driven by the same thermodynamic and kinetic factors. However, amphiphilic crystalline monolayers at an interface cannot possess a center of inversion, the most common symmetry element in bulk crystals. This fact should, in principle, lead to better chances for spontaneous separation in the Langmuir or Langmuir-Blodgett monomolecular films. On the other hand, the monolayers of most amphiphiles studied to date incorporate long aliphatic chains that have an intrinsic tendency to pack in a herringbone motif requiring glide plane symmetry, thus creating a bias towards racemate formation. Moreover, 2-D crystals supposedly have a much higher degree of molecular and therefore enantiomeric disorder compared to bulk crystals. All these factors necessitate a careful choice of molecules to guarantee enantiomeric separation in two dimensions. Unambiguous detection of spontaneous resolution in 2-D appears to require atomic resolution of molecular packing arrangement, which can in principle be obtained by grazing incidence X-ray diffraction or atomic force microscopy, whereas in bulk solids spontaneous resolution can be easily detected by various macroscopic methods. This short review provides analogies between spontaneous separation in 3-D and recent examples in 2-D, showing that spontaneous separation generally depends upon subtle differences in molecular structure.
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(1998) Journal of Physical Chemistry B. 102, 5, p. 761-765 Abstract
We present a study of two-dimensional (2D) crystallites of cholesterol formed at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) measurements performed along the surface pressure-area isotherm revealed a transition from a monolayer to a highly crystalline rectangular phase, about two layers thick. This variation in the film thickness was confirmed by ellipsometry measurements. Films transferred onto solid support by the Langmuir-Blodgett technique were seen by atomic force microscopy (AFM) to display elongated and faceted crystallites about 10 layers thick. The effect of the phospholipid dipalmitoylphosphatidylcholine (DPPC) on the 2D crystallization of cholesterol was studied by GIXD and AFM with three cholesterol:DPPC mixtures in molar ratios 1:1, 2:1, and 5:1. The phospholipid additive reduced the crystallinity of the cholesterol in the 5:1 and 2:1 mixtures, totally suppressing it in the 1:1 mixture.
1997
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(1997) Journal of the American Chemical Society. 119, 46, p. 11211-11216 Abstract
Grazing incidence X-ray diffraction (GID) has been applied to the study of structural characteristics of two-dimensional crystallites of the ionophores valinomycin (VM) and nonactin (NA) when complexed with various cations at the air-solution interface. The VM complexes assume a bracelet shape that packs in a two-dimensional hexagonal unit cell. Other crystalline phases were formed on potassium iodide and barium perchlorate solutions. The presence of particular lipids induced ordered stacking of VM-potassium chloride complexes into three to four layers. NA packs in a pseudotetragonal unit cell on solutions of NH4SCN and KSCN. Upon compression of the NA-NH4SCN film, crystallites seven to eight layers thick were detected and the GID data enabled the determination of the structure. The tendency of these complexed ionophores to form multilayer crystallites at interfaces may have bearing on ion transport through membranes, via stacking.
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An insight into the ice nucleation process via design of crystalline ice nucleators of variable size(1997) Journal Of Physical Chemistry B. 101, 44, p. 8874-8877 Abstract
Monolayers of the long-chain alcohols on water promote nucleation of ice. In order to determine the minimum size of crystalline alcohol monolayer domains that induce ice nucleation, we reduced their size in two distinct ways. One approach encompassed embedding the pure hydrocarbon alcohol, CnH2n+1OH(n = 20, 31), into a matrix of an immiscible monolayer of perfluoro alcohol C10F21C2H4OH, which is inert as an ice nucleator. The second set of experiments involved the introduction of random defects into the monolayer crystalline domains of C29H59OH through the use of the fully miscible guest alcohol molecule, C31H63OH, as additive. By these methods we estimated that ∼450 water molecules are necessary to form a stable ice cluster at the onset of induced ice nucleation at a temperature just below 0°C.
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(1997) Surface Review and Letters. 4, 4, p. 721-732 Abstract
X-ray scattering experiments have been performed on the surfaces of organic crystals. The (010) cleavage planes of β-alanine and α-glycine were investigated, and both specular and off-specular crystal truncation rods were measured. This allowed a determination of the molecular layering at the cleavage planes. Furthermore, the surfaces were investigated during growth and dissolution. No change in surface structure was observed, suggesting that the growth and dissolution is performed via the step edges. Finally, experiments on stereospecific adsorption of methionine on α-glycine were performed, showing that the (S)-methionine molecules are orderly attached to the (010) surface of α-glycine and also attached to the (010) surface, but not in registry with the substrate.
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(1997) Chemistry-A European Journal. 3, 6, p. 930-939 Abstract
The formation of Langmuir monolayers at the air water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono- and multilayer self-assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self-aggregated at the air water interface. As model systems we have examined the de position of quaterthiophene (S4), quinquethiophene (S5), and sexithiophene (S6) from chloroform solutions on tile water surface. The structures of the films were determined by surface pressure area isotherms, by scanning force microscopy (SFM after transfer of the films onto atomically smooth mica, by cryo-transmission electron microscopy (Cryo-TEM) on vitreous ice, and by grazing incidence synchrotron X-ray diffraction (GII) directly on the water surface. S4 forms two polymorphic crystalline multilayers. In polymorph χ, of structure very similar to that of the three-dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23 from the normal to the water surface. In polymorph β the long molecular axis is perpendicular to the water surface. S5 self-aggregates at tile water surface to form mixtures of monolayers and bilayers of the β polymorph: S6, forms primarily crystalline monolayers of both χ and β forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports. The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.
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(1997) Journal of the American Chemical Society. 119, 5, p. 933-942 Abstract
Studies are presented on the two-dimensional (2-D) crystalline packing arrangements of enantiomerically pure and racemic α-amino acid RHC(NH3+)CO2- monolayers on water and on glycine aqueous solutions, as determined by synchrotron grazing incidence X-ray diffraction. The amphiphiles have been designed such that their racemic mixtures form 2-D crystals which are either heterochiral (for R = C(n)H(2n+1)-, n = 10, 12, 16) due to the tendency for herringbone chain arrangements via glide symmetry or homochiral (for R = C(n)H(2n+1)CONH(CH2)4-, n = 11, 17, 21) by virtue of hydrogen bonding by translation of the amide group in the chains leading to a spontaneous separation into islands of opposite chirality. The two different crystalline motifs led to a correlation between their packing arrangements and induced oriented nucleation of 3-D crystals of α-glycine by these monolayers. The relevance of the present results to the possibility of ordering and spontaneous segregation of racemates of the natural hydrophobic α-amino acids at the air-solution interface is discussed.
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1996
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(1996) Science. 274, 5295, p. 2046-2049 Abstract
Thin interdigitated films composed of a long-chain, water-insoluble chiral acid (p-pentadecylmandelic acid of absolute configuration R) and a water-soluble chiral base (phenylethylamine, R') were constructed at the air- solution interface. The (R, R') structure was characterized to near-atomic resolution by grazing-incidence x-ray diffraction (GIXD). The two diastereomeric systems, (R, R') and (R, S'), demonstrate similar surface pressure-molecular area isotherms, but their structures are completely different on the molecular level, as monitored by GIXD. Complementary data on these two architectures were provided by atomic force microscopy.
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(1996) Croatica Chemica Acta. 69, 2, p. 689-708 Abstract
A correlation is presented between the crystalline structure of monolayers and multilayers of α,ω-alkanediols HO-(CH2)n-OH (n = 16, 18, 19, 21, 22, 23, 24, 30) at the air-water interface and their function as ice nucleators. Structural elucidation was carried out by the following methods: grazing incidence X-ray diffraction, scanning force microscopy, cryo-transmission electron microscopy and external reflection Fourier transform-infrared spectroscopy.
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(1996) Journal of Physical Chemistry. 100, 20, p. 8356-8362 Abstract
The crystalline properties of the amphiphilic systems N-methylarachidamide (NM) and of N,N-dimethyldocosanediamide (NND) in the uncompressed state at 5°C on surfaces of water and liquid formamide were studied by grazing incidence synchrotron X-ray diffraction (GID). On water, the amphiphile NM forms a crystalline monolayer. On formamide, NM behaves differently. One h after deposition on the liquid surface, GID patterns indicated coexistence of two different crystalline monolayer structures. Three h later, a crystalline bilayer had formed. The fundamental question as to whether the interlayer growth took place in the liquid subphase or in the air was elucidated with the help of the tailor-made additive N-benzylarachidamide (NB). Addition of 10% of NB in the spreading solution yielded only a monolayer. From this result we conclude that bilayer growth of pure NM must occur from the liquid side. It also helps to establish a similar interlayer growth mechanism for the trilayer formation of pure arachidamide on liquid formamide.1 The amphiphile NND behaved differently from NM as it formed a crystalline monolayer on both water and formamide liquid surfaces.
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(1996) Langmuir. 12, 4, p. 1011-1017 Abstract
Grazing incidence synchrotron X-ray diffraction measurements were made on an uncompressed monolayer of 1:1 triacontanoic acid (C29H59COOH) and nonacosylamine (C29H59NH2), spread on pure water and on aqueous ZnCl2 solution. The measured diffraction data of the mixed monolayer on water provides clear evidence of a complete mixing of the acid and amine molecules into a two-dimensional crystal where the hydrocarbon chains pack in a herringbone motif. A model has been proposed in which the ionized acid and amine moieties are ordered and form a continuous hydrogen-bonded array.
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(1996) Israel Journal of Chemistry. 36, 1, p. 97-110 Abstract
Structure, morphology, and mechanical properties of mono- and several-layer structures of amphiphiles or pure n-alkane crystallites generated by spontaneous self-assembly on aqueous subphase have been analyzed by scanning force microscopy (SFM). Pure-component and heterogeneous mixtures of molecules were allowed to spread and self-assemble without compression on an aqueous subphase. The self-assembled films were transferred to an atomically smooth mica substrate by drainage for measurement using SFM. Results were compared with a variety of techniques including cryo-transmission electron microscopy, grazing incidence X-ray diffraction. X-ray reflectivity, and reflectance/absorption infrared spectroscopy. Whereas the collaborative techniques provide spatially-averaged information, we find that the SFM accesses both individual crystallites and amorphous material, thus providing unique information on the morphology, number of layers, and complementary structural features.
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Tailor-made auxiliaries for the control of nucleation, growth and dissolution of crystals.(1996) Crystallography Of Supramolecular Compounds. 480, p. 331-345 Abstract
Keywords: Chemistry, Inorganic & Nuclear; Chemistry, Physical; Crystallography
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Nanocrystals at the air/liquid interface as transients in crystallization(1996) Nanoparticles In Solids And Solutions. Vol. 18. p. 49-64 Abstract
Keywords: Engineering, Multidisciplinary; Materials Science, Multidisciplinary
1995
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(1995) Chemistry-A European Journal. 1, 7, p. 414-422 Abstract
Organisms can exert a remarkable degree of control over crystal growth. One way of achieving this is by the adsorption of specialized macromolecules on specific planes of the growing crystals. With continued growth of the crystal, the macromolecules are incorporated inside the crystal bulk. Their presence does not change the crystal structure, but creates discontinuities in the perfect lattice. Here we study in detail three unusual cases of reduction in symmetry at the level of crystal domain shapes, induced by this controlled intercalation. We examined sponge spicules, which are single crystals of Mgbearing calcite. They were specifically chosen for this study, because their morphologies do not reflect the hexagonal symmetry of calcite. Their crystal textures (coherence lengths and angular spreads) were characterized by highresolution Xray diffraction with wellcollimated synchrotron radiation. The results are compared to analogous studies of synthetic calcite and Mgbearing calcite. In all the selected spicules reduction in symmetry is observed in the coherence lengths among symmetryrelated crystallographic directions. The reconstructed shapes of the domains of perfect structure closely match the specific spicule morphologies. The synthetic crystals show no such reduction in symmetry. Although the manner by which such exquisite control is achieved is not known, we envisage it involving a combination of oriented nucleation with either physical or stereochemically driven adsorption.
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(1995) Advanced Materials. 7, 10, p. 857-862 Abstract
The induced formation of multilayer crystallites at liquid interfaces provides a direct means of monitoring the dynamics of crystal nucleation and of elucidating the effect of molecular inhibitors on polymorphism and crystal growth. The spontaneous formation of crystalline multilayers of nalkanes spread from solution onto the water interface are described and Xray structural investigations discussed which indicate that the full crystal symmetry is present even in layers that are only a few unit cells thick.
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(1995) Chemistry-A European Journal. 1, 5, p. 304-311 Abstract
A systematic analysis of grazing incidence synchrotron Xray diffraction data of uncompressed amphiphilic alcohols CnH2n+1 OH (n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5°C is presented. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2D) crystalline material formed fell drastically; shorter crystalline coherence lengths were also observed. For n3118 the molecules are arranged in a rectangular cell (a ≈ 5 Å, b increases from ca. 7.4 to ca. 8.2 Å) with plane symmetry p1 g1. For n 2 and in the atomic displacement parameter parallel to the water surface; it increases from 0.1 Å2 for n = 31 to 0.30 Å2 for n = 19. We explain the preference for chain tilt along the b axis, rather than a, in terms of hydrogen bonding to the water subphase. The various structural properties of the CnH2n+2OH (n = 3113) monolayer series, such as degree of crystallinity and coherence length, lattice dimensions, chain orientation, and molecular motion, may be correlated with the icenucleating efficiency of these alcohol monolayers as a function of n.
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(1995) Acta Crystallographica Section B-Structural Science. 51, 2, p. 115-148 Abstract
Tailor-made auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes, which include morphological engineering and etching, reduction of crystal symmetry, assignment of absolute structure of chiral molecules and polar crystals, elucidation of the effect of solvent on crystal growth, and crystallization of a desired polymorph. As for crystal growth promoters, monolayers of amphiphilic molecules on water have been used to induce the growth of a variety of three-dimensional crystals at the monolayer-solution interface by means of structural match, molecular complementarity or electrostatic interaction. A particular focus is made on the induced nucleation of ice by monolayers of water-insoluble aliphatic alcohols. The two-dimensional crystalline structures of such monolayers have been studied by grazing incidence X-ray diffraction. It has become possible to monitor, by this method, the growth, dissolution and structure of self-aggregated crystalline nonolayers, and indeed multilayers, affected by the interaction of solvent molecules in the aqueous. suphase with the amphiphilic headgroups, and by the use of tailor-made amphiphilic additives.
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(1995) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 34, 6, p. 649-652 Abstract
Double layers are formed by α,ω-docosanediol (C22 diol) when it is spread on a water surface. The space group (a slightly relaxed form of P21/a) and the lattice parameters of the unit cell were determined by X-ray diffraction. Furthermore, multilayer formation was inhibited by addition of 10% of C24H49OH or 5% of HO(CH2)22OCH2Ph to the spreading solution. The coverage with the double layer was reduced to less than 15%; the rest was occupied by a monolayer.
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(1995) Advanced Materials. 7, 1, p. 26-35 Abstract
The structure of selfassemblies of amphiphiles formed at the airaqueous solution interface can be determined by cryo transmission electron microscopy (CryoTEM). The method is based on fast vitrification of the thin layer of aqueous solution covered with amphiphilic monolayer by plunging the specimen into liquid ethane at its freezing point. During the process of fast cooling the aggregates maintain their twodimensional crystalline integrity and structure, as demonstrated by comparative studies involving grazing incidence Xray diffraction (GID) at the airsolution interface, and the CryoTEM measurements of the same amphiphilic systems on vitreous ice. Brightfield and darkfield images from the CryoTEM observations provide more detailed information than from epifluorescence and Brewster angle microscopy. Furthermore, the electron diffraction patterns have the additional advantage that they allow structural characterization of the crystallites almost at a molecular level, and furnish data on micro twinning and defects occurring between crystalline domains. CryoTEM has been applied to elucidate the structure of 2D and 3D selfaggregates of amphiphilic alcohols, acids and their cadmium salts, bolaamphiphiles and mixed monolayers. Epitaxial crystallization of hexagonal ice underneath the monolayer of long chain alcohol was also directly demonstrated by this method.
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(1995) FASEB Journal. 9, 2, p. 262-268 Abstract
Crystals formed in biological tissues often adopt remarkable morphologies that are thought to be determined mainly by the shapes of the confined spaces in which they grow. Another possible way of controlling crystal shape, demonstrated only in vitro, is by means of specialized proteins preferentially interacting with certain crystal faces. In so doing, they reduce the rate of growth in these directions and consequently change the overall crystal shape. In an X-ray diffraction study of the distribution of defects within the lattice of calcite crystals produced by certain sponges, we show that a remarkable correlation exists between the defect patterns or crystal texture and the macroscopic morphology of the spicules. This was observed in two cases in which proteins are present within the spicule crystal, but not in a third case where such intracrystalline proteins are absent. Furthermore, one of the spicules exhibited marked differences in texture even within families of structurally identical crystal planes, demonstrating that the organisms exert exquisite control over the microenvironment in which crystals grow. We conclude that highly controlled intercalation of specialized proteins inside the crystals is an additional means by which organisms control spicule growth.
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(1995) JOURNAL OF PHYSICAL CHEMISTRY. 99, 16, p. 6036-6045 Abstract
A monolayer of 4-(octadecyloxy)benzoic acid at the air-solution interface has been used to induce oriented nucleation of three-dimensional (3-D) crystals of 4-hydroxybenzoic acid (HBA) monohydrate.1 The two-dimensional (2-D) crystalline structure of this monolayer in a self-assembled uncompressed state, both on a water subphase and on a solution containing HBA, was determined by grazing incidence X-ray diffraction (GID) using synchrotron radiation. Crystalline domains of large coherence length were formed on water subphase at 14°C, and the fraction of the monolayer in the ordered state was much increased on HBA solution. The agreement between the observed and calculated GID data was found to be sensitive to the conformation and orientation of the monolayer molecules. These results demonstrate the ordered binding of the HBA solute to the amphiphile benzoic acid head groups to form hydrogen-bonded cyclic dimers between the two carboxylic acid moieties. The GID method was complemented by other surface sensitive techniques such as specular X-ray reflectivity, surface pressure-area isotherms, and nonlinear optics. A resulting model arrangement of the monolayer, based on the molecular packing, has been proposed for the induced nucleation of the 3-D crystals of 4-hydroxybenzoic acid monohydrate attached to the monolayer.
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Wasserman, catenane chemist [5](1995) Chemical & Engineering News. 73, 12, p. 5 Abstract
We enjoyed reading the article by Ron Dagani (C&EN, Aug. 29, 1994, page 28) on the current field of catenanes. We were dismayed by his reference to "a chemist" who synthesized the first catenane in 1960. That pioneering study was performed by Ed Wasserman, who is considered as the originator of this field of science. Wasserman not only synthesized the first catenane, but also elaborated on the topological aspects of this class of materials. Wasserman, who is well known for his original contributions in pure and applied chemistry, is currently at DuPont Experimental Station, Wilmington, Del. [1st paragraph]
1994
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(1994) Advanced Materials. 6, 12, p. 956-959 Abstract
The spontaneous aggregation of the bolaamphiphile α,ωdocosanediol into embryonic threedimensional crystallites at the airwater interface can be inhibited by additives. A study with the aim of determining the necessary properties of such supplements is reported, which uses additives with molecular recognition properties and methods such as Xray reflectivity, cryotransmission electron microscopy, and atomic force microscopy in order to detect structural changes at the nanometer level.
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(1994) Advanced Materials. 6, 12, p. 952-956 Abstract
Crystal polymorphism is important because of the different physical and mechanical properties of the polymorphs. Based on a stereochemical approach to controlling crystal polymorphism using stereospecific nucleation inhibitors, auxiliaries have been designed for the precipitation of the yform of glycine (see Figure) from aqueous solutions. The ideas behind this work are presented, together with results. (Figure Presented.)
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(1994) Physics Reports-Review Section Of Physics Letters. 246, 5, p. 251-313 Abstract
The advent of well collimated, high intensity synchroton X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air-liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves and surface fluorescence techniques, and review recent results obtained for Langmuir films from their use. The methods have been successfully applied for the elucidation of the structure of crystalline aggregates of amphiphilic molecules at the water surface such as alcohols, carboxylic acids and their salts, α-amino acids and phospholipids. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occuring by phase transitions. Furthermore, via the surface X-ray methods, new light is shed on the structure of the underlying attached solvent or solute ionic layer. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counter-ionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals which had been succesfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.
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(1994) Chemistry of Materials. 6, 8, p. 1258-1268 Abstract
Crystallization of R,S-alanine in the presence of small amounts of other resolved α-amino acids results in the precipitation of the racemic compound as twinned crystals displaying a propeller-like morphology of 222 symmetry, in contrast to needlelike crystals of mm2 symmetry when grown from pure aqueous solution. This unusual twinning is explained by assuming that the supersaturated solutions contain ordered clusters of structures akin to the mature crystals of (R,S)-, (R)-, and (S)-alanine. The resolved α-amino acid additive, e.g., R, stereospecifically inhibits growth of the R,S and R nuclei but not of the S nuclei. The latter serve as a template onto which the (R,S) crystals nucleate and grow. A model which takes into consideration the structural similarity between the racemic and chiral resolved forms of alanine to explain the twinning is presented. An X-ray diffraction analysis of a twinned crystal specimen is in agreement with a gradual change in structure, from a chiral crystal of space group P212121 to the racemic compound of space group Pna21. To substantiate the model, crystals of alanine were grown from optically enriched solutions containing excess of one of the alanine enantiomers, which also induce the twinning phenomenon and an overall reduction in crystal symmetry. The relevance of these observations to the early stages of crystal nucleation is discussed.
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(1994) Molecular Crystals and Liquid Crystals. 248, 1, p. 185-198 Abstract
Composite materials in which the organic host is stiffened by guest particles, are widely used in nature and are produced synthetically by man. Organisms also produce a different type of composite in which the host is a crystal and the guests are macromolecules occluded in an orderly fashion within the crystal. The best studied examples, to date, are biogenic calcite crystals, and in particular those formed by the echinoderms. In vitro experiments with calcite crystals grown in the presence of echinoderm intracrystalline proteins, show that these macromolecules are occluded inside the crystal on specific planes, and their presence alters the mechanical properties of the crystal host. Furthermore, the proteins also influence the crystal textural properties. Model studies using crystals of dicarboxylic acid salts grown in the presence of intracrystalline proteins show that the coherence length is reduced in directions perpendicular to the planes on which the proteins adsorb. We found anisotropic effects in almost all the biogenic calcite crystals we examined. Furthermore, we noted an interesting relationship between the variations in coherence length in the different crystallographic directions and the gross morphology of the single crystal elements, suggesting that these proteins may also function in determining the morphology of the crystal during growth. These novel single crystal-protein composites may be just one example of strategies used in nature for producing materials with special properties.
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(1994) JOURNAL OF PHYSICAL CHEMISTRY. 98, 19, p. 4970-4972 Abstract
Spreading of the bolaamphiphile alpha,omega-docosanediol, HO(CH2)(22)OH, on water results in spontaneous aggregation into embryonic 3-D crystallites. These crystallites have been analyzed by cryo-transmission electron microscopy, scanning force microscopy, X-ray reflectivity, and external reflection Fourier transform infrared spectroscopy. The molecules are aligned in an almost perpendicular fashion in a rectangular cell, and the crystallites are about five molecular layers thick.
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Design of Two-Dimensional Crystals as Models for Probing the Structure of the Solid-Liquid Interface(1994) Journal of the American Chemical Society. 116, 8, p. 3271-3278 Abstract
Crystals of β-alanine and the α-form of glycine, when grown in the presence of 3% molar CdCl2, display morphologies which are different from those obtained from pure water solution. This effect was interpreted in terms of binding of Cd2+and Cl-ions to the exposed CO2- and NH3+ moieties at the various faces, so changing their relative rates of growth and leading to the development of new faces. The structure of the crystal face of β-alanine most affected by the presence of CdCl2was mimicked by a monolayer containing an equimolar mixture of two different amphiphiles octadecylamine (C18H37NH2) and stearic acid (C17H35COOH) spread on aqueous solution. Binding of the Cd2+and Cl-ions to such a monolayer has been investigated. X-ray specular reflectivity measurements reveal that the mixed monolayer is fully bound by Cd2+and Cl-ions when their concentration reaches 0.1 M. Grazing incidence X-ray diffraction measurements, using synchrotron radiation, of the mixed monolayer on pure water and on a 0.1 M CdCl2solution provide strong evidence that the monolayer CO2-and NH3+head groups are arranged in ordered array and the Cd2+and Cl-ions are bound to the monolayer head groups at ordered sites.
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(1994) Langmuir. 10, 3, p. 830-836 Abstract
Grazing incidence X-ray diffraction (GID) studies were performed on self-assembled monolayers of carboxylic acids CnH2n+1CO2H (n = 13,19,29) spread over CdCl2 or CaCl2 solutions at pH values 8.85 and 10, respectively, adjusted with ammonia, and subphase temperatures in the range 5 to 9 °C. The GID results show the tendency for spontaneous two-dimensional (2-D) crystalline aggregation of the molecules which pack in a rectangular cell with the hydrocarbon chains tending to be vertically aligned with respect to the solution surface. The molecular chains form a herring bone arrangement with the two molecules in the unit cell related essentially by glide symmetry. Detailed Bragg rod fitting analysis shows however, that the two molecules are crystallographically independent so that the plane group symmetry is chiral p1. For n= 29, it was possible to determine the extent of molecular thermal motion parallel to the water surface and which proved to be in the same range as in layered 3-D crystal structures containing aliphatic chains. The GID data did not provide definite evidence for ordered ion binding although the extent of crystallinity is enhanced by the presence of the bound counterion. For example, myristic acid (n = 13) yielded the GID pattern only when spread over CdCl2 but not on pure water. A simple packing model is provided for ordered binding of counterions with high degree of coordination to the oxygen atoms of the carboxylate head groups. The proposed model is also in keeping with the observed lower area per amphiphilic molecule than over pure water.
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(1994) Langmuir. 10, 3, p. 819-829 Abstract
Uncompressed arachidic acid films spread over 10−3 M cadmium chloride solution (pH 8.8 adjusted with ammonia) spontaneously form two-dimensional (2-D) crystalline clusters with coherence lengths of ≈1000 Å at 9 °C. Ten distinct low-order in-plane diffraction peaks and three high-order peaks were observed with grazing incidence X-ray diffraction (GID) using synchrotron radiation. Seven low-order peaks were attributed to scattering only from a crystalline cadmium layer and the remaining peaks to scattering primarily from the arachidate layer. The molecules in the arachidate layer arrange in a pseudorectangular unit cell with dimensions a = 4.60 Å and b = 8.37 Å and γ = 93.4° with the chains tilted 11° along the b axis. The chains of the two crystallographically independent molecules in the unit cell are related by pseudoglide symmetry along the b axis yielding the herringbone motif. The reflections from the cadmium layer were indexed according to a supercell a8 = 2a, b8 = 3(-a + b)/2. Analysis of X-ray specular reflectivity measurements and the GID data indicated that the counterionic layer consists of a CdOH+ species, bound to the arachidate layer in a stoichometry close to 1:1. The probable formation of a cadmium-ammonia complex at the high pH = 8.8 was strongly suggested by the X-ray reflectivity measurements employing CH3NH2, (CH3)2NH, and (CH3)3N as alternative counterions. The arrangements of the arachidate chains and of the Cd ions were each determined to near atomic resolution by fitting the GID data, but the relative offset between the arachidate and Cd \u201clattices\u201d was difficult to ascertain.
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(1994) Journal of the American Chemical Society. 116, 4, p. 1192-1204 Abstract
The two-dimensional (2D) crystal structures of self-aggregated clusters of amphipbilic alcohols (CnH2n+1OH, n = 23, 30, 31, and C19H39CO2(CH2)nOH, n = 9, 10) on pure water at a temperature of 5 °C have been determined to near-atomic resolution in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazing incidence synchrotron X-ray diffraction (GID) data, complemented by lattice energy calculations. The GID data of the different monolayers within each of the two series (CnH2n+1OH, C19H39CO2CnH2nOH) are very similar. The molecules pack in a rectangular unit cell of average dimensions a = 5.0 Å, b = 7.5 Å for the normal alcohols CnH2n+1OH and a = 5.7 Å, b = 7.5 Å for C19H39CO2CnH2nOH. The plane group symmetry is essentially p1g1 for the normal alcohols CnH2n+1OH and essentially p1g1 for the other group. The molecular chains are tilted from the vertical by an average angle of 9°, in the direction of the b axis, for CnH2n+1OH and by 29°, in the direction of the a axis, for the other molecular type. The molecular chains related by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of CnH2n+1OH permitted a reliable estimate of 0.07 Å2 for the molecular mean-squared motion parallel to the water surface. The a bsolute orientations of the molecules C19H39CO2CnH2nOH were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of C19H39CO2CnH2nOH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.
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(1994) Journal of the American Chemical Society. 116, 4, p. 1179-1191 Abstract
The induced freezing of drops of supercooled water covered by uncompressed monolayers of the aliphatic alcohols CnH2n+1OH, n = 16-31 was investigated. The ice-nucleation temperatures were found to depend on the length of the hydrocarbon chain and the parity of the number of carbon atoms in the chain. A gradual increase of the freezing point from -14 °C leveling off to a plateau at -7.5 °C from n= 22 onward was observed for the even series. Alcohols with n odd raised the freezing temperature from -11 to-1 °Cforn= 17-31. Fatty acids (CnH2n+1+1COOH) or alcohols bearing a fluorocarbon chain or a steroidal backbone as the hydrophobic part induced nucleation of ice at much lower temperatures. Powder X-ray diffraction measurements showed that the monolayer C31H63OH induced formation of hexagonal ice with its (001) face parallel to the monolayer surface. Grazing incidence X-ray diffraction (GID) studies of uncompressed monolayers of CnH2n+1OH with n = 16, 20, 23, 30, 31 on water at 5 °C revealed two-dimensional (2D) crysalline self assembly and yielded their packing arrangements to near atomic resolution.1 The extent of 2D crystalline order (i.e. coherence length) and the amount of crystalline material were less for monolayers with n = 16 and 20 than for monolayers with n = 23, 30, 31 according to a GID analysis, the latter property being independently confirmed by FT-IR. The best lattice match to ice was for n = 30 and 31, the lattice match to ice being poorer for n = 20 and 16, in terms of area per molecule, correlating with their less efficient ice-nucleation behavior. The packing arrangements of the hydrocarbon chains of monolayers C30H63OH and C31H63OH are very similar, but for the orientation of their CH2OH moieties, which are different, resulting in different ice-nucleation behavior. Further evidence for the role played by the orientation of the C-OH group is the ice-nucleating properties of alcohols CH3(CH2)nCOX(CH2)mOH (X = O, NH). In these systems the freezing point was found to depend on the parity of m but not on n. The 2D crystal structures of two ester-alcohols n = 19, m = 9, 10 on water were determined by GID and lattice energy calculations.1Finally, mixtures of aliphatic alcohols with different chain lengths (by up to four CH2 groups) substantially reduced the ice-nucleation temperature, implying drastic reduction in the lateral order of the 2D nets of alcohol OH groups. On the other hand, up to 50% fluorocarbon alcohol in a mixture with hydrocarbon alcohol did not reduce the freezing temperature of ice, indicating phase separation of crystalline domains.
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(1994) Neutron and synchrotron radiation for condensed matter studies : applications to soft condensed matter and biology. p. 47-68 Abstract
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(1994) JOURNAL OF PHYSICAL CHEMISTRY. 98, 15, p. 4087-4093 Abstract
The structure of an uncompressed monolayer of C31H63OH, which induces freezing of supercooled water drops at ∼-2°C, was studied by grazing incidence X-ray diffraction in the temperature range +6°C to freezing of the water subphase. As the water was cooled, the hydrocarbon chains underwent a change in tilt angle. The monolayer preserved its structure on the epitaxially-grown surface of hexagonal ice (after freezing) and, after the ice was melted, subsequent to the temperature being raised above 0°C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Å, which is in agreement with the extent of match between the lattice of the monolayer and the ab lattice of ice. This yields some information on the critical size of the ice nucleus bound to the monolayer, just below 0°C.
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(1994) Hydrogen Bond Networks. Vol. 435. p. 395-402 Abstract
Two approaches are used to study suggested mechanisms for heterogeneous ice nucleation. In one, an epitaxial-like mechanism is studied by using Langmuir monolayers of aliphatic alcohols C n H2n+1OH (n=16 to 31) spread on supercooled water drops. The packing arrangement of several of these uncompressed monolayers on water, have been determined by grazing incidence x-ray diffraction (GID) and correlated with the structure of water molecules at the (001) face of hexagonal ice. The roles played by lattice match between the monolayers and ice, the coherence length of the amphiphile crystallites and the orientation of the hydroxyl groups at the interface in determining the nucleation efficiencies are specifically addressed. In another induced nucleation of ice by an electric field is studied using hydrophobic surfaces of chiral resolved and racemic α-amino acid crystals onto which water vapor has been condensed and nucleated into ice.
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(1994) Computational Approaches In Supramolecular Chemistry. Vol. 426. p. 411-418 Abstract
Two approaches are used to study suggested mechanisms for heterogeneous ice nucleation. In one, an epitaxial-like mechanism is studied by using Langmuir monolayers of aliphatic alcohols C n H 2n +1OH (n=16 to 31) spread on supercooled water drops. The packing arrangement of several of these uncompressed.monolayers on water, have been determined by grazing incidence x-ray diffraction (GID) and correlated with the structure of water molecules at the (001) face of hexagonal ice. The roles played by lattice match between the monolayers and ice, the coherence length of the amphiphile crystallites and the orientation of the hydroxyl groups at the interface in determining the nucleation efficiencies are specifically addressed. In another induced nucleation of ice by an electric field is studied using hydrophobic surfaces of chiral resolved and racemic α-amino acid crystals onto which water vapor has been condensed and nucleated into ice.
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(1994) Science. 264, 5165, p. 1566-1570 Abstract
The spontaneous formation and coexistence of crystalline polymorphic trilayer domains in amphiphilic films at air-liquid interfaces is demonstrated by grazing incidence synchrotron x-ray diffraction. These polymorphic crystallites may serve as models for the early stages of crystal nucleation and growth, helping to elucidate the manner in which additives influence the progress of crystal nucleation, growth, and polymorphism and suggesting ways of selectively generating and controlling multilayers on liquid surfaces. Auxiliary molecules have been designed to selectively inhibit development of the polymorphs, leading primarily to a single phase monolayer.
1993
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(1993) Journal of the American Chemical Society. 115, 24, p. 11110-11118 Abstract
At low temperature (512 °C), uncompressed films of insoluble amphiphilic molecules C13H39X, where the head group X contains one (CONH2, 1) or two (CONHC2H4CONH2, 2) amide groups, spontaneously form two-dimensional (2D) crystalline clusters over aqueous subphases containing soluble amide or carboxylic acid molecules. These crystallites were detected and their structures were studied using grazing incidence X-ray diffraction (GID). In the presence of subphases containing carboxylic acid (RCO2H, R = H, CH2Cl) at sufficiently high concentrations, a loss of diffraction signal was observed for 1, while amide and less concentrated acid subphases did not show such a destructive effect. The effect of the subphase solute molecules was understood in terms of the different ways in which the solutes hydrogen bond to the amide head groups of the amphiphiles. Both amide and acid solute molecules can form hydrogen-bonded cyclic dimers with the amide head groups. With an amide subphase, such dimers lead to an extension of the hydrogen-bonding network of the crystallites, and thus enhance its stability, but acid molecules may also bind to the monolayer at low concentrations with less than full occupancy. At high acid concentration, and thus more extensive formation of cyclic dimers between carboxylic acid and amphiphilic amide molecules, repulsive interactions between lone pair electrons on oxygen atoms of bound acid molecules inhibit formation of ordered arrays of these dimers and lead to a lack of diffraction signal. In 2, the second amide group strengthens the crystallites to the extent that there is no decrease in crystallinity over a 1 M formic acid subphase. The shape of the intensity profiles of the Bragg rods and the specular X-ray reflectivity measurements of 2 indicate formation of molecular trilayers.
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(1993) Journal Of Physics D-Applied Physics. 26, 8, p. B22-B22 Abstract
Tailor-made molecular auxiliaries for the control of nucleatiw and growth of molecular crystals may be classified into two broad categories: Inhibitors and promoters, Tailor-made inhibitors of crystal growth can be used for a variety of purposes which include morphological engineering, reduction of crystal symmetry, assignment of absolute structure of polar crystals, elucidation of the effect of solvent on crystal growth and crystallization of a desired polymorph. As for crystal growth promoters, Langmuir monolayers on water have been used to induce growth of 3D crystals at the monolayer-solution interface by means of structural match, molecular complementarity or an electrostatic interaction. The 2D crystalline structures of these monolayers have been studied by grazing-incidence x-ray diffraction (gixd). It has become possible to monitor, by gixd, the growth and dissolution of self-aggregated crystalline monolayers affected by the interaction of solvent molecules in the aqueous subphase with the monolayer headgroups.
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(1993) Faraday Discussions. 95, p. 307-327 Abstract
Principles are outlined for symmetry lowering of a mixed crystal composed of host and tailor-made additive molecules, based on selective occlusion of the latter through a subset of surface sites of the growing crystal, the symmetry of the surface generally being lower than that of the bulk. A survey is given of the various methods and approaches used to detect the reduction in symmetry. These include changes in crystal morphology, detection of enantiomeric segregation of chiral additives in 'centrosymmetric' crystals, generation of second-harmonic optical signals, optical birefringence, asymmetric photoreactions in the crystalline state and X-ray and neutron diffraction. The last two methods are applied to mixed crystals of cinnamamide (host) and thienylacrylamide (additive). The diffraction analysis demonstrated that the mixed crystals are composed of six sectors of reduced symmetry, from monoclinic centrosymmetric P21/c to triclinic P1 in four sectors and possibly Pc in the remaining two. The X-ray diffraction data were not sufficiently accurate to permit assignment of the absolute structures of the P1 sectors with the use of anomalous X-ray scattering. Thus, by this method one could not ascertain the absolute orientation of the guest molecules on the surface sites through which they were selectively occluded. This ambiguity was resolved by assignment of the absolute configuration of the chiral heterophotodimers, between host and guest, in enantiomeric excess in the P1 sectors, after irradiation with UV light. These results led to the definite conclusion that the selective occlusion of thienylacrylamide arises from a replacement of attractive C-H⋯π (electron) interactions between host molecules by repulsive sulfur (lone-pair electron)⋯π (electron) interactions between guest and host at the crystal surfaces.
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(1993) JOURNAL OF PHYSICAL CHEMISTRY. 97, 49, p. 12848-12857 Abstract
Mixed monolayers containing two different amphiphiles C19H39CONHCH2CH2CO2H (A) and C19H39CONH2 (B) were designed in order to form a two-dimensional (2-D) crystalline solid solution in which the A-type molecules form domains within the sea of B-type molecules. A "continuous" 2-D arrangement of the aliphatic chains was expected, driven by the amide hydrogen bonding requirement; a tendency for the formation of the embeded A-type domains should be provided by the interactions between the -CH2CH2CO2H head group moieties. The mixed monolayers served to promote the oriented nucleation of silver propionate 3-D crystals attached at the monolayer-solution interface. Only the A-type domains induced silver propionate crystallization whereas the B-type domains were essentially inert. The mixed A + B monolayers were found to be efficient nucleators down to a 1:10 molar ratio, providing proof for the existence of A-type domains. Additional information such as the structure and ion-binding properties of the mixed monolayers was furnished by specular X-ray reflectivity and grazing incidence X-ray diffraction using synchrotron radiation.
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(1993) Science. 259, 5096, p. 776-779 Abstract
Textures of calcite crystals from a variety of mineralized tissues belonging to organisms from four phyla were examined with high-resolution synchrotron x-ray radiation. Significant differences in coherence length and angular spread were observed between taxonomic groups. Crystals from polycrystalline skeletal ensembles were more perfect than those that function as single-crystal elements. Different anisotropic effects on crystal texture were observed for sea urchin and mollusk calcite crystals, whereas none was found for the foraminifer, Patellina, and the control calcite crystals. These results show that the manipulation of crystal texture in different organisms is under biological control and that crystal textures in some tissues are adapted to function. A better understanding of this apparently widespread biological phenomenon may provide new insights for improving synthetic crystal-containing materials.
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(1993) Journal of Physical Chemistry. 97, 34, p. 8692-8695 Abstract
The method of cryo-transmission electron microscopy has been applied to study twinning behavior of mixed monolayers of 3-(eicosanoylamino) propionic acid and eicosanoamide spread on water and CdCl2 aqueous solution. The monolayers were deposited with a thin layer of aqueous subphase on grids which were vitrified by plunging into liquid ethane. Various electron diffraction patterns were observed and interpreted in terms of formation of two-dimensional (2-D) crystallites which display single and multiple twinning. Morphological features of the monolayer films were also obtained from bright and dark field images.
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(1993) Journal of Physical Chemistry. 97, 19, p. 5162-5170 Abstract
A unique set of proteins extracted from a variety of invertebrate calcitic mineralized tissues is able to selectively interact in vitro with certain crystal faces and not others. This was previously demonstrated by observing changes in morphology of crystals grown in the presence of proteins as compared to those grown in the absence of proteins. Following interaction, the proteins are overgrown by the crystal and are subsequently occluded within the crystal itself. Here we address the fundamental question of whether or not the proteins also alter the crystal texture in an anisotropic manner. For this purpose we used high-resolution synchrotron X-ray diffraction to monitor changes in coherence length and angular spread. We studied the interactions of proteins extracted from the mineralized skeletal hard parts of sea urchins and mollusks, with crystals of two calcium dicarboxylic acids, calcium fumarate and calcium malonate, as well as the polymorph of CaCO3 calcite. For the calcium dicarboxylate crystals, we did demonstrate that the coherence lengths are reduced in the directions perpendicular to the planes onto which the proteins preferentially adsorb. In contrast the calcite crystals grown in the presence of the proteins exhibited an increase in angular spread compared to the controls, but no anisotropic effect in coherence length was detected. A biologically produced calcite crystal, on the other hand, showed a preferential reduction in coherence length in the direction of the c axis. Clearly in the case of calcite, the processes controlling crystal texture in the biological environment are more sophisticated than those in vitro. The detection of a reduction in coherence length in the directions perpendicular to the planes onto which the proteins preferentially adsorbed represents one of very few direct demonstrations that an additive that is able to selectively alter crystal morphology also affects crystal texture in an anisotropically specific manner. An understanding of this phenomenon may, in the future, improve our ability to control crystal texture in synthetic materials.
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(1993) Journal of Physical Chemistry. 97, 20, p. 5200-5203 Abstract
The spontaneous formation of highly crystalline two-dimensional films of uncompressed long chain amide and carboxylic acid amphiphiles spread on formamide has been demonstrated by grazing incidence X-ray diffraction. These results were compared to those found for the same amphiphiles on water. The observed diffraction data Of C29H59CONH2 over both water and formamide and those Of C29H59CO2H over formamide are strikingly similar, while those Of C29H59CO2H over water are different. Arachidamide (CH19H39CONH2) over water subphase forms a unimolecular layer with the molecules tilted by 18-degrees. Over formamide, the crystallites are made up, at least in part, of three molecular layers, with the molecules standing vertically. This was confirmed by X-ray specular reflectivity. These results are analogous to the well known, but as yet unexplained, influence which solvent may have on the growth of polymorphic modifications of three-dimensional crystals.
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(1993) Science. 261, 5123, p. 899-902 Abstract
A new approach is described for probing domains of ordered self-assemblies of amphiphilic monolayers at the aqueous solution interface. The method has potential importance for the study of membrane structure, Langmuir-Blodgett films, and nucleation processes of two-and three-dimensional crystals. Electron diffraction (ED) patterns indicative of two-dimensional crystalline self-assembly were obtained from samples, which were examined by cryo-electron microscopy, of monolayers of water-insoluble amphiphiles on vitrified aqueous substrates. The apparent hexagonal symmetry of an ED pattern from a C 16H33OH monolayer was interpreted in terms of multiple twinning. Monolayers of the C31H63OH and cadmium salt of C19H39CO2H that were studied by dark-field techniques displayed faceted two-dimensional crystallites with a maximal size of 1 to 2 micrometers. Epitaxial nucleation of hexagonal ice by the C 31H63OH monolayer has also been demonstrated by ED.
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(1993) Journal of Physical Chemistry. 97, 49, p. 12858-12861 Abstract
The crystalline structures of monolayers of the N-eicosanoyl-3-aminopropionic acid CH3-(CH2)18-CONH-(CH2) 2-CO2H and its mixtures with eicosanamide CH3-(CH2)18-CONH2 on a water subphase were determined, at near-atomic resolution, by grazing incidence X-ray diffraction (GID), using synchrotron radiation. The N-eicosanoyl-3-aminopropionic acid molecule must adopt a very particular conformation in order to ensure satisfactory packing requirements. The packing characteristics were tested by fitting the measured GID data using atomic coordinate models for various conformers, complemented by lattice energy calculations. The structure of the mixed monolayer in 1:1 and 1:2 molar ratios is different from that of each pure component. We propose that the two components do not form a random solid solution but rather the N-eicosanoyl-3-aminopropionic acid forms domains within the sea of eicosanamide molecules, stabilized by the interactions between the -(CH2)2-CO2H moieties.
1992
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(1992) Journal of the American Chemical Society. 114, 25, p. 9983-9989 Abstract
Recently, grazing-incidence X-ray diffraction studies of insoluble amphiphilic molecules have shown that molecules possessing fluorocarbon chains crystallize more efficiently on the surface of water that those possessing hydrocarbon chains. Here, we perform lattice energy calculations involving atomic electrostatic and van der Waals parameters on model two-dimensional crystals of hydrocarbon chains and fluorocarbon chains which possess crystalline arrangements similar to those of the corresponding amphiphilic films on water. The electrostatic parameters of CF2 groups were determined from an X-ray study of the deformation electron density of perfluoroglutaramide, using single-crystal low-temperature (approximately 100 K) X-ray diffraction data. The net charge on the fluorine atoms q = -0.14 e is almost twice that on hydrogen atoms q = +0.06 e, suggesting that intramolecular repulsion between fluorines will limit the possibility for conformational disorder in fluorocarbon chains. The calculated lattice energy of the model 2-D crystalline films of vertical fluorocarbon chains containing 20 carbons is lower by about 3.0 kcal per CF2 group than the lattice energy of the model 2-D crystalline films of vertical hydrocarbon chains with the same number of carbons. We conclude that the crystallization behavior of amphiphilic molecules with fluorocarbon chains is determined by a higher backbone stiffness and higher interchain attractive van der Waals forces than for molecules with hydrocarbon chains. In addition, we show that it is possible to simply correlate the calculated lattice energies and the observed crystalline self-assembly at the air-water interface in various amphiphilic systems, in particular the homologous series of carboxylic acid molecules CnH2n+1CO2H (n = 13, 19, 20, 21, 29): for more attractive lattice energies, the extent of crystalline order is larger.
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(1992) Angewandte Chemie-International Edition In English. 31, 2, p. 130-152 Abstract
The advent of wellcollimated, highintensity synchrotron Xray sources and the consequent development of surfacespecific Xray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the airliquid interface. These methods allowed for the first time the determination of the inplane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence Xray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained from them for Langmuir films. The methods have been successfully applied in the elucidation of the structure of crystalline aggregates of amphiphilic molecules such as alcohols, carboxylic acids and their salts, αamino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline selfaggregates as well as structural changes occurring by phase transitions. Furthermore, the surface Xray methods shed new light on the structure of the underlying ionic layer of attached solvent or solute species. Examples are given where singly or doubly charged ions bound to the twodimensional (2D) crystal form either an ordered or diffuse counterionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals, which had been successfully accomplished by epitaxiallike crystallization both in organic and inorganic crystals.
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(1992) Journal of Physical Chemistry. 96, 1, p. 15-16 Abstract
Different growth rates on the opposite hemihedral faces of polar, platelike crystals of N-n-octyl-D-gluconamide were observed. One face is hydrophobic and the other hydrophilic, in keeping with contact angles of various solvents on these faces which were measured. It was found that the different growth rates can be explained by the hydrophilicity and hydrophobicity of the crystal surfaces.
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(1992) Pure and Applied Chemistry. 64, 9, p. 1263-1270 Abstract
The structure and dynamics of embryonic aggregates at the air/water or air/solution interface en route to crystal nucleation have been probed by applying a variety of techniques including stereospecific photochemistry, Second Harmonic Generation (SHG), Grazing Incidence X-ray diffraction and oriented crystallization methodologies. This approach is illustrated by three different systems: oriented crystallization of 4-Hydroxybenzoic Acid Monohydrate (HBA) by 4-Alkoxybenzoic Acid, photochemistry of 4-Methoxy-E-cinnamic acid at the air/solution interface and the induced nucleation of ice by amphiphilic alcohols.
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(1992) Science. 256, 5058, p. 815-818 Abstract
The hydrophobic faces of single crystals of a series of pairs of racemic and chiral-resolved hydrophobic α-amino acids were used as a substrate, onto which water vapor has been cooled to freezing. The morphologies and molecular packing arrangements within each crystal pair are similar but only one of each pair exhibits a polar axis, parallel to the hydrophobic face exposed to water. Those crystals that have a polar axis induce a freezing point higher by 4° to 5°C than the corresponding crystals that do not have a polar axis. The results are interpreted in terms of an electric field mechanism that helps align the water molecules into ice-like clusters en route to crystallization.
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(1992) Journal of Physical Chemistry. 96, 25, p. 10380-10389 Abstract
A general approach is presented toward the determination at near atomic resolution of two-dimensional (2-D) crystalline structures of amphiphilic molecules at the air-water interface. It involves fitting of grazing incidence X-ray diffraction (GID) data from the 2-D crystal using molecular models, complemented by lattice energy calculations. In this way, one may eliminate ambiguities associated with assignment of the 2-D crystal symmetry, the molecular conformation, and the packing arrangement. The method was applied to the elucidation of the low temperature (5°C) structure of an uncompressed monolayer of triacontanoic acid C29H59CO2H. We deduced a packing of the molecules in a rectangular unit cell in plane group pg. The molecular chain axes are tilted from the vertical by 27° along the short 5.53-Å axis of the unit cell and oriented so as to form the orthogonal O⊥ packing type. The CO2H groups are not directly interlinked by O-H⋯O hydrogen bonds. A distinct improvement of the agreement between observed and calculated GID data was achieved by partial relaxation of the glide plane symmetry. The observed Bragg peaks suggested very anisotropic shapes for the crystallites. This anisotropy was correlated with the energy of attachment of a molecule to a specific crystallite facet.
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(1992) Israel Journal of Chemistry. 32, 1, p. 15-21 Abstract
A method is described for probing interactions between molecules at the surfaces of growing crystals. For this purpose, mixed crystals have been treated as unique hostguest systems where the guest molecules are occluded into the host crystal through stereospecific interactions occurring at the different growing surfaces of the host. Owing to the process of molecular recognition at such surfaces, the adsorption and occlusion of the guest molecules takes place at specific sites, resulting in a reduction in symmetry of the mixed crystal compared to the pure host. The concept of reduction in symmetry is general, and has been instrumental for probing subtle molecular interactions at the surfaces of growing crystals, for the transformation of centrosymmetric single crystals into crystals with polar arrangement, and for \u201cabsolute\u201d asymmetric synthesis inside centrosymmetric crystals.
1991
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(1991) Journal of the American Chemical Society. 113, 20, p. 7684-7691 Abstract
Uncompressed insoluble amphiphilic molecules possessing linear hydrocarbon chains CnH2n+1X (n = 23, 30, 31, X = OH; n = 29, X = COOH; and n = 19, X = CONH2) spontaneously form large two-dimensional (2-D) crystalline clusters over pure water at low temperature (5 °C). These 2-D crystallites were detected and their structures were solved using grazing incidence X-ray diffraction (GID). Their packing arrangements are described in terms of 2-D space-group symmetry and hydrocarbon-chain packing. All the crystal structures display rectangular unit cells containing two molecules that are probably related by glide symmetry in the 2-D space group pg for the alcohol (X = OH) and the acid (X = COOH) and by translation symmetry in the 2-D space group p1 for the amide (X = CONH2). The alcohol molecules are tilted by 811° from the vertical toward next-nearest neighbors, the tilt angle being dependent on the chain length. The amide and the acid molecules are tilted toward nearest neighbors by 18° and 26°, respectively. The positional correlation lengths of the crystallites were found to be anisotropic; they extend over only 3595 spacings parallel to the molecular tilt direction, but over 135270 spacings perpendicular to it. The similarity of chain packing in the 2-D crystallites and in three-dimensional (3-D) crystals of aliphatic amphiphilic molecules is clearly established. These crystallites may therefore, on the water surface, mimic crystallization mechanisms observed in 3-D systems.
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Direct Assignment of Absolute Configuration of Chiral Alkyl Gluconamides by Wettability Measurements(1991) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 30, 6, p. 696-698 Abstract
The contact angle of a crystal face affords information about the absolute configuration of the molecules forming the crystal. This could be demonstrated with the alkylgluconamides 1, which crystallize with the molecules arranged in layers. The angle of contact at the face consisting of the chainend alkyls is 7587°, while at the oppositely disposed face consisting of the CH2OH groups it is 4456°C. (Figure Presented.)
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(1991) Makromolekulare Chemie. Macromolecular Symposia. 46, 1, p. 89-96 Abstract
Xray diffraction methods for Langmuir films on the surface of water are briefly presented, together with recent results for docosanoic acid monolayers on pure water and for eicosanoic acid monolayers on an ionic subphase.
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(1991) Makromolekulare Chemie. Macromolecular Symposia. 46, 1, p. 125-132 Abstract
Monolayers of aliphatic long chain alcohols (CnH2n+1OH) were found to induce nucleation of ice at temperatures approaching O°C in contrast to watersoluble alcohols which are efficient antifreeze agents. The corresponding fatty acids and alcohols with bulky hydrophobic groups, induce ice nucleation at temperatures lower by as much as 12°C. The freezing point induced by the amphiphilic alcohols was found to be sensitive not only to area per molecule but also to chain length and parity, reaching higher temperatures for monolayers with n odd. Grazing incidence Xray diffraction studies performed on some of these alcohols at the air/water interface (at 5°C and at zero pressure), demonstrated the formation of crystalline twodimensional clusters with a distorted hexagonal cell whose dimensions resemble those of hexagonal ice. The catalysis of ice nucleation by these alcohol monolayers may be rationalized in terms of the structural match between the monolayer domains and the ab layer of hexagonal ice.
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(1991) Materials for Nonlinear Optics. Vol. 455. p. 472-483 (trueACS Symposium Series). Abstract
Two novel methodologies for the design of solid materials with poar axes by a kinetic controlled process are described. In the first approach we demonstrate that amphiphilic molecules bearing two amide groups along the hydrocarbon chain invariably deposite to yield Z-type Langmuir-Blodgett films. Attachment of hyperpolarizable molecules to such hydrocarbon chains resulted in the formation of films displaying frequency doubling. In the second methodology, crystallographic information has been used for the design of polymeric crystallization inhibitors of a stable non-polar polymorph of PAN [N-(2-acetamido-4-nitro-phenyl)- pyrrolidene]. As predicted, addition of minute amounts of polymer 15 to a supersaturated solution of PAN results in the precipitation of the metastable polar form which displays second harmonic generation.
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(1991) Science. 252, 5012, p. 1532-1536 Abstract
A crystalline counterionic layer at the interface between an electrolyte solution and a charged layer of insoluble amphiphilic molecules was observed with grazing incidence synchrotron x-ray diffraction. Uncompressed arachidic films spread over 10-3 molar cadmium chloride solution (pH 8.8) spontaneously form crystalline clusters with coherence lengths of ∼1000 angstroms at 9°C. Ten distinct diffraction peaks were observed, seven of which were attributed to scattering only from a crystalline Cd2+ layer and the other three to scattering primarily from the arachidate layer. The reflections from the Cd2+ layer were indexed according to a 2 x 3 supercell of the arachidate lattice with three Cd2+ ions per cadmium unit cell.
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(1991) Science. 253, 5020, p. 637-645 Abstract
Nucleation, growth, and dissolution of crystals have been studied by stereochemical approach involving molecular recognition at interfaces. A methodology is described for using "tailor-made" additives designed to interact stereospecifically with crystal surfaces during growth and dissolution. This procedure was instrumental in controlling crystal morphology and in revising the concept of the structure and symmetry of solid solutions. Consequently, it was applied to the transformation of centrosymmetric single crystals into solid solutions with polar arrangement displaying second-harmonic generation and to the performance of asymmetric synthesis of guest molecules inside centrosymmetric host crystals. The method has led to a discovery of a new "relay" mechanism explaining the effect of solvent on crystal growth. Finally, it allowed for the design of auxiliary molecules that act as promoters or inhibitors of crystal nucleation that can be used to resolve enantiomers and crystallize desired polymorphs.
1990
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(1990) Journal of the American Chemical Society. 112, 17, p. 6215-6220 Abstract
The effect of solvent on crystal growth and dissolution studied by two stereochemical strategies is described. The first approach involved the growth of two crystalline hydrates (asparagine monohydrate and rhamnose monohydrate) in the presence of primary alcohols as \u201ctailor-made\u201d solvents. It was demonstrated that stereospecific adsorption of the tailor-made solvent at a particular face inhibits its growth. The second approach, concerned the growth and dissolution of hemihedral faces at the opposite ends of the polar axes of (R, S)-alanine and γ-glycine in water, one pole exposing carboxylate groups and the other amino groups. Relatively fast growth and dissolution at the carboxylate end in water are explained in terms of strong solvent binding at a subset of surface sites and solvent repulsion at the remaining sites, so allowing for easy access of solute at the latter unhydrated surface sites followed by expulsion of water at the former. The repetitive succession of these steps results in a \u201crelay\u201d-type mechanism of crystal growth.
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Ice nucleation by alcohols arranged in monolayers at the surface of water drops(1990) Science. 250, 4983, p. 973-975 Abstract
Monolayers of aliphatic long-chain alcohols induced nucleation of ice at temperatures approaching 0°C, in contrast with water-soluble alcohols, which are effective antifreeze agents. The corresponding fatty acids, or alcohols with bulky hydrophobic groups, induce freezing at temperatures as much as 12°C lower. The freezing point induced by the amphiphilic alcohols was sensitive not only to surface area per molecule but, for the aliphatic series (CnH2n + 1OH), to chain length and parity. The freezing point for chains with n odd reached an asymptotic temperature of 0°C for an upper value of n = 31; for n even the freezing point reached a plateau of-8°C for n in the upper range of 22 to 30. The higher freezing point induced by the aliphatic alcohols is due to formation of ordered clusters in the uncompressed state as detected by grazing incidence synchrotron x-ray diffraction measurements. The diffraction data indicate a close lattice match with the ab layer of hexagonal ice.
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Intercalation of sea urchin proteins in calcite: Study of a crystalline composite material(1990) Science. 250, 4981, p. 664-667 Abstract
Sea urchin skeletal elements are composed of single crystals of calcite. Unlike their synthetic counterparts, these crystals do not have well-developed cleavage and are consequently much more resistant to fracture. This phenomenon is due in part to the presence of acidic glycoproteins occluded within the crystals. By means of x-ray diffraction with synchrotron radiation, it is shown that the presence of the protein in synthetic calcite only slightly decreases the coherence length but significantly increases the angular spread of perfect domains of the crystals. In biogenic calcite, the coherence length is 1/3 to 1/4 as much as that in synthetic calcite and the angular spread is 20 to 50 times as wide. It is proposed that the presence of macromolecules concentrated at mosaic boundaries that are oblique to cleavage planes is responsible for the change in fracture properties. These results may be important in the material sciences, because of the unusual nature of this material, namely, a composite based on the controlled intercalation of macromolecules inside single-crystal lattices.
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(1990) Journal of the American Chemical Society. 112, 21, p. 7724-7736 Abstract
In order to provide, on the molecular level, information on crystal nucleation of monolayers at air-water interfaces, the self-aggregation of lH,lH,2H,2H-perfiuorododecyl aspartate CF3(CF2)9(CH2)20COCH2CH(NH3+)C02~ (PFA) over water subphases at various pH values was studied using synchrotron X-ray grazing incidence diffraction (GID), including Bragg rods (BR) and reflectivity (XR) measurements. Two-dimensional crystalline domains with coherence lengths exceeding 1 500 A were detected for low surfactant surface densities and zero surface pressure. GID measurements reveal structural changes with subphase pH and composition. Structural models are proposed at high, neutral, and low pH. For water subphases containing KOH at pH > 11.2, the diffraction is consistent with molecules arranged in a hexagonal net and vertically aligned. Over pure water and acidic subphases containing HC1 at pH = 1.5, the molecules pack in a distorted hexagonal net with the fluorocarbon chains tilted from the vertical. The growth in time of the uncompressed crystallites over aqueous glycine solutions was directly monitored by GID. Compression and subsequent decompression of the monolayers over pure water and HC1 (pH = 1.5) subphases, for which the fluorocarbon chains are originally tilted, were found to reduce the crystallinity of the system considerably. By contrast, over KOH at pH > 11.2, the hexagonal net with vertically aligned molecules is preserved at all surface pressures and the crystalline order of the system is reduced upon compression but increases again upon release of pressure. Estimates of the degree of crystallinity of the monolayer were made over water for various states of compression and over KOH at pH >11.2 in the uncompressed state. The packing characteristics and the dynamics involved in the formation and partial destruction of the crystallites can be understood in terms of interaction between the hydrophilic ionic head groups of the monolayer and, if present, the attached molecules or ions (water, K+ or Cl"). Additional support for the packing arrangements proposed at high, neutral, and low pH was obtained from studies of the oriented growth of sodium chloride under PFA monolayers.
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(1990) Molecular Crystals and Liquid Crystals. 186, 1, p. 3-17 Abstract
The packing disorder in racemic valine is characterized by techniques previously used to control nucleation, growth, and dissolution of crystals. R,S-valine crystals were grown and dissolved in the presence of other racemic α-amino acid additives. We inferred the presence of disorder in R,S-valine crystals from the lack of enantiomeric segregation of the additives occluded inside growing crystals, and from the non-specific etch-pit formation on the faces of dissolving crystals. Subsequent X-ray diffraction studies showed the disorder to arise from \u201cflipping\u201d of hydrogen-bonded bilayers across interfaces which are linked by relatively weak hydrophobic interactions. Possible mechanisms for the disorder are discussed.
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(1990) Journal of the American Chemical Society. 112, 21, p. 7718-7724 Abstract
The formation of two-dimensional ordered aggregates of water-soluble hydrophobic α-amino acids at the air-solution interface has been demonstrated through the induced epitaxial nucleation of α-glycine crystals at these interfaces. This crystallization method, albeit indirect, has been found to be very sensitive to small changes in the structure of the presumed aggregates. The hydrophobic-amino acids used were divided into two classes: The first, including valine, leucine, phenylalanine, norleucine, isoleucine, and n-aminooctanoic acid, induced fast oriented crystallization of α-glycine, at the solution surface, whereas the second class, tert-butylglycine, neopentylglycine, and hexafluorovaline, did not. The comparison between the packing arrangement of the α-glycine crystalline face attached at the interface and that of various hydrophobic α-amino acid crystals, complemented by surface tension measurements, brought evidence in favor of the formation of structured domains which must carry precise enantioselective information to generate the oriented crystallization process. These results may be relevant to any process involving structural self-aggregation.
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(1990) Advanced Materials. 2, 1, p. 40-43 Abstract
Polar crystals can display pyroelectricity, piezoelectricity and frequency doubling leading to second harmonic generation. These metastable polar polymorphs can be selectively precipitated to the exclusion of stable, nonpolar forms by the use of polymeric crystallization auxiliaries. The approach is based on the hypothesis that crystal nuclei assume structures very similar to those of the crystals into which they develop.
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(1990) Journal of the American Chemical Society. 112, 7, p. 2498-2506 Abstract
A new series of amphiphilic molecules bearing two amide groups along the hydrocarbon chain form head-to-tail Z-type (polar) multilayers by the Langmuir-Blodgett (LB) technique, in contrast with the common amphiphiles that tend to deposit in the head-to-head, tail-to-tail Y-type fashion. Molecules with one amide group along the chain form Y-type or Z-type structures, depending on the location of the amide along the chain. p-Nitroaniline or merocyanine groups could be aligned in a polar sense, by attaching them covalently to such amphiphiles, to yield multilayers displaying second-harmonic generation (SHG). These films were characterized by ellipsometric, X-ray diffraction, IR, UV, and SHG measurements. The Z-type deposition correlates with the observed low water contact angles of the films, indicating formation of porous structures in which water may penetrate. This behavior is rationalized in terms of the hydrophilic nature of the amide groups that retain water by hydrogen bonding. In keeping with this mechanism, it was anticipated that amphiphiles bearing other polar groups, such as hydroxyls along the hydrocarbon chain, also form Z-type LB films.
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1989
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(1989) Thin Solid Films. 178, 1-2, p. 203-209 Abstract
In recent studies we have reported that the insertion of one, or preferably, two amide groups in the hydrocarbon chain of amphiphilic molecules yields porous monolayers which deposit in a genuine Z-type mode. In this paper, we extend our studies to amphiphiles with an amide group and a polymerizable diacetylene function in the chain. Monolayers of monomers and polymers of 10,12-non-acosadiynoyl-S-lysine (1) and 10,12-nonacosadiynoyl-S-ornithine (2) were deposited in a Z-type mode. The differences in the photopolymerization of these films seem to depend on the odd or even location of the amide group along the chain with respect to the headgroup. These Langmuir-Blodgett (LB) films were characterized by UV-visible and Fourier transform (FT) IR (reflection-absorption mode) spectroscopies and by low-angle X-ray diffraction measurements.
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(1989) Journal De Physique. 50, C7, p. C729-C737 Abstract
Monolayers of PFA, a fluorinated surfactant, on water were studied using synchrotron X-ray grazing incidence diffraction (GID) and reflectivity measurements (XR). The uncompressed monolayer is found to self-assemble into crystalline domains giving a strong GID peak. Upon compression and decompression a surface pressure driven solid-solid phase transition takes place. A hexagonal lattice with vertical molecules at high pressure undergoes a distortion due to a rigid molecular tilt as the pressure is decreased. Bragg rod scans provide conclusive evidence for a tilt of about 20 " at 10 mN/m and show the tilt to be towards nearest neighbours. The uncompressed monolayer has a molecular tilt of about 25 '.
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(1989) Pure and Applied Chemistry. 61, 4, p. 673-684 Abstract
Crystal nucleation of glycine and sodium chloride has been studied under floating Langmuir monolayers at air/solution interfaces, and at glass slides coated with Langmuir-Blodgett films. The packing arrangements of the polar head groups of the monolayers were varied in a controlled manner by introducing different groups in the hydrophobic tails. The structures of some of these monolayers were independently elucidated by grazing incidence X-ray diffraction and reflectivity measurements using synchrotron light. It was found that the crystals nucleated at the interface exposed a top layer of molecules with an arrangement similar or complementary to that of the polar head groups of the monolayer. These results imply that the packing of the polar head groups determines the morphology and the nucleation rate of the attached crystal.
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(1989) Angewandte Chemie (International ed.). 28, 4, p. 466-468 Abstract
A network of hydrogen bonds determines the structure of the complex A of nylon66 and perfluoroglutaric acid, HO2C(CF2)3CO2H. This explains why shorter or longer dicarboxylic acids do not form complexes with nylon66. The nylon partial structure differs from the structure of pure nylon not in the CC bond lengths but rather in the CCC angles, which result in a longer b axis. Complexes of type A are unusual, because they consist of a molecular mixture of otherwise immiscible hydrocarbons and fluorinated hydrocarbons. (Figure Presented.)
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(1989) Journal of the American Chemical Society. 111, 3, p. 1029-1034 Abstract
Controlled reduction in crystal symmetry has been accomplished and exploited to perform an absolute asymmetric photoreaction. The principle is based on selective introduction of a guest molecule into a centrosymmetric host structure, thus reducing the symmetry of the mixed crystal. Crystallization of (E)-cinnamamide (space group P21/c) in the presence of (E)-cinnamic acid results in a mixed crystal composed of two enantiomorphous halves each containing 0.5-1.0% of the acid. The replacement of N-HO hydrogen bonds between host molecules by OO lone-pair repulsion between host and guest at the chiral surfaces of the growing crystal is responsible for the stereoselective occlusion of the guest and the reduction in symmetry. Irradiation of each half separately yielded the optically active cyclobutane dimer la or lb in excess, with an enantiomeric yield in the range of 40-60%, varying from one single crystal to another. The absolute configuration of the chiral dimer la formed in excess at the +b pole (1R,2R,3R,4R) was independently assigned by the Bijvoet method, NMR, and crystal-etching experiments and is in agreement with our proposed mechanism of molecular recognition.
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(1989) Journal of the American Chemical Society. 111, 4, p. 1436-1445 Abstract
In a stereochemical approach aimed at the understanding of crystal nucleation on a molecular level, the oriented crystallization of glycine at air-solution interfaces covered with monolayers 1-12 of resolved a-amino acids has been studied. Three types of monolayers with different packing motifs of the polar head groups have been used. Coverage of a supersaturated aqueous glycine solution with monolayers 1 and 2 did not lead to crystallization at the interface; on the other hand, coverage with monolayers 3-8 yielded a fast crystallization with only partial orientation. Finally, monolayers 9-12 yielded a fast nucleation of glycine with complete orientation of the crystals. These results imply that the packing of the polar head groups determines the nucleation rate and the degree of orientation of the attached growing crystals. This conclusion is strongly substantiated by the assignment of the structures of monolayers 3 and 9 using grazing-angle X-ray diffraction and reflectivity measurements from a synchrotron light source. Crystallization experiments were performed on solid hydrophobic glass supports coated with Langmuir-Blodgett films of monolayers of 1, 3, 4, 6, 9, and 11; in all cases the results were similar to those observed with the corresponding Langmuir monolayers
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(1989) Journal of the American Chemical Society. 111, 3, p. 1035-1040 Abstract
We have demonstrated, for the first time by diffraction methods, that a solid solution composed of host and guest molecules can exhibit a crystal symmetry lower than that of the host. The study proves that the symmetry of a solid solution is dependent not only upon the host crystal structure and the guest molecular structure but also upon the surface structure and symmetry of the host crystal. The crystal structures of (S)-asparagine monohydrate [D2NCOCH2CH(ND3)CO2.D2O] and of the solid solution (0.848:0.152) (S)-asparagine/(S)-aspartic acid [DO2CCD2CD(ND3)CO2] monohydrate were refined by using neutron diffraction data obtained at 15 K. The space group of the pure host crystal is P212121(Z = 4), whereas that of the host/guest crystal is monoclinic P1211 with two molecular sites per asymmetric unit. The ratios of guest/host occupancies of the two independent sites are 0.173:0.827 and 0.132:0.868. The reduction in symmetry is in accordance with the preferred adsorption of guest aspartic acid on the (010) crystal face at half of the orthorhombic, symmetry-related surface sites. Aspartic acid mimics, at the preferred (010) surface sites, molecular asparagine, participating in all hydrogen bonds. At the less-favored (010) surface sites a \u201cnormal\u201d N-H.O(host) hydrogen bond is replaced by O(hydroxyl).O(host) repulsion between lone-pair electrons.
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(1989) Chemistry of Materials. 1, 1, p. 114-118 Abstract
Reduction of crystal symmetry via preferential occlusion of guest molecules through different subsets of crystal surface sites has been used to include centrosymmetric host crystals as a matrix for providing second-order nonlinear optical effects. The method is also applicable for generating a second harmonic signal along the principal nonpolar axis of polar crystals. This approach is illustrated by several examples. These optical measurements yield information on crystal growth processes and on the alignment of guest molecules in the host crystal.
1988
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(1988) Science. 241, 4872, p. 1475-1479 Abstract
Mixed single crystals composed of host and guest organic molecules of similar structures and shapes are shown to comprise sectors with different host-guest distributions and to have symmetries lower than that of the host crystal. These properties are determined by the structure of the guest and the surface structures of the crystal faces through which the guest molecules are occluded. This general concept is illustrated by studies of three mixed crystal systems, (E)-cinnamamide(E)-2-thienylacrylamide, (E)-cinnamamide(E)-3-thienylacrylamide, and (S)-asparagine(S)-aspartic acid, with x-ray and neutron diffraction and solid-state photochemistry.
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(1988) Thin Solid Films. 159, 1-2, p. 29-41 Abstract
We have determined the packing arrangement of a floating monolayer of palmitoyl-(R)-lysine at the air-water interface by grazing incidence X-ray diffraction and reflection measurements. These techniques utilize the unique properties of synchrotron radiation: high intensity within a small natural collimation. In the grazing angle diffraction experiment two peaks were detected in the "twodimensional powder" pattern from a monolayer of palmitoyl-(R)-lysine. Their widths indicated coherence lengths of 500 J~. The positions and intensities of these peaks allowed us to choose between various models and to determine the monolayer structure. The packing arrangement of the Qt-amino acid headgroups in the model proved to be very similar to that found in the crystal structures of the 0t form of glycine and several hydrophobic at-amino acids, thus providing a simple explanation for the oriented crystallization of 0t-glycine at monolayer-solution interfaces. The tilt of the molecule calculated from the model is consistent with the results from reflectivity measurements and X-ray powder diffraction data of the crystalline powder material. Reflectivity measurements indicate that at surface pressures as high as 30 mN m- 1 the monolayer covers only about 90% of the surface. Reflectivity measurements of the monolayer over water and over solutions of 29/o (S)-glutamine showed significant differences, indicating binding of the solute molecules to the monolayer.
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(1988) Journal of the American Chemical Society. 110, 2, p. 561-567 Abstract
A model for generation and amplification of optical activity, using the centrosymmetric crystals of glycine as substrates for total separation of occluded α-amio acids into enantiomeric territories, is described. The principle is based on the enantioselective occlusion of these additives through the enantiotopic (010) and (0l-0) faces of glycine crystals. Such crystals, when floating at the air/solution interface, and if correctly oriented, may incorporate only one of the two enantiomeric additives present in solution. We demonstrate that complete (010) or (01̅0) orientation may be induced by both kinetic and \u201chydrophobic\u201d effects. The former is achieved through inhibition of nucleation and growth of the say (010) oriented crystals [(010) face toward air], by (S) amino acids in solution, while the latter is due to induction of (01̅0) orientation by (S) hydrophobic amino acids. By symmetry the enantiomers induce opposite orientation. The two effects are investigated separately, and possible mechanisms are proposed. Combination of the two effects, which operate in the same direction, allows total orientation of glycine crystals and thus triggering of amplification starting with a solution containing leucine with an enantiomeric excess as low as 6%. The relevance of such a mechanism to other systems and to nucleation in general is discussed.
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(1988) Science. 242, 4883, p. 1286-1290 Abstract
A measurement and interpretation on a molecular level of a phase transition in an ordered Langmuir monolayer is reported. The diagram of surface pressure (π) versus molecular area of a monolayer of chiral (S)-[CF3-(CF2)9-(CH2)2-OCO-CH2-CH (NH3+)CO2-] over water shows a change in slope at about πs= 25 millinewtons per meter. Grazing-incidence x-ray diffraction and specular reflectivity measurements indicate a solid-solid phase transition at πs. The diffraction pattren at low pressures reveals two diffraction peaks of equal intensities, with lattice spacings d of 5.11 and 5.00 angstroms; these coalesce for π ≥πs. Structural models that fit the diffraction data show that at π> πs the molecules pack in a two-dimensional crystal with the molecules aligned vertically. At π
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(1988) Molecular Crystals And Liquid Crystals. 161, 1, p. 199-221 Abstract
A new class of monomeric and polymeric crystal dissolution inhibitors has been prepared taking into consideration the packing arrangements and the morphologies of organic crystals. The efficiency of these inhibitors has been demonstrated by comparative morphological studies of α-glycine and by the kinetic resolution of the racemic conglomerates of his·HCl·H
2O, threonine, glu·HCl, and 2,4-sec-phenethyl-3,5-dinitrobenzoate
1987
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(1987) Reactive Polymers, Ion Exchangers, Sorbents. 6, 2-3, p. 241-253 Abstract
The design of chiral polymers as stereospecific inhibitors for efficient kinetic resolution of conglomerates by crystallization is described. The selection of the polymers is done on the basis of the crystal structure and the morphology of the substrate. The resolutions with the polymers are better than those previously obtained with low molecular weight additives. This is illustrated here for the comparative resolutions of (R,S)-glutamic acid (Glu · HCl), (R,S)-threonine, (Thr), (R,S)-asparagine monohydrate, (Asn · H2O), and (R,S)-p-hydroxyphenylglycine-p-toluenesulfonate, (pHpgpTs). Further, (R,S)-histidine, which could not be resolved with the low molecular weight inhibitors, was successfully resolved with two polymeric reagents. The method is also applicable for molecular crystals lacking ionic and hydrogen bonds as is illustrated for the resolution of (R,S)-sec-phenethyl-3,5-dinitrobenzoate (PDNB). Finally, a simultaneous resolution of the enantiomers in a device composed of two compartments separated by a membrane is described.
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(1987) Proceedings of the National Academy of Sciences of the United States of America. 84, 14, p. 4737-4738 Abstract
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(1987) Nature. 328, 6125, p. 63-66 Abstract
Recently it has been demonstrated that a compressed monolayer of palmitoyl-(R)-lysine (Fig. 1) on the surface of water induces the orientated growth of α-glyeine crystals. This effect was inter-preted to be a structural match between the monolayer and the α-glycine. To test this hypothesis we undertook to determine the packing arrangement of the monolayer by grazing incidence X-ray diffraction and reflection. These techniques use the unique proper-ties of synchrotron radiation: high intensity within a small natural collimation. In the diffraction experiment two peaks were detect-able in the two-dimensional powder pattern from a monolayer of palmitoyl-(R)-lysine. The positions and intensities of these peaks allowed us to choose between various models and determine the monolayer structure. This is the first time that the crystal structure of a compressed surfactant monolayer at the airwater interface has been determined. The same techniques could be used for structural characterization of other monolayers of interest in fields as diverse as biological membranes and optical second harmonic generation. The packing arrangement of the α-amino acid head groups in the model proved to be very similar to that found in the crystal structures of α-glycine and several hydrophobic α-amino acids.
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(1987) Journal of the American Chemical Society. 109, 13, p. 3883-3893 Abstract
The structures of the partially reacted crystals of the two complexes (5:2) deoxycholic acid (DCA)-acetophenone and (8:3) DCA-p-fluoroacetophenone were determined by low-temperature (-170 °C) X-ray diffraction. the structure analyses indicated that both independent guest acetophenone molecules G and G in DCA-acetophenone react and that in DCA-p-fluoroacetophenone the guest molecule G reacts whereas the other guest molecule G remains unreacted because of packing of G and G molecules along the channel. the observed guest photoconversion in DCA-acetophenone is 40%; the maximum value according to packing considerations is 50%. In DCA-p-fluoroacetophenone the observed guest photoconversion is 35%, very close to the maximum theoretical yield of 33% based on packing considerations. the analysis demonstrated that during the course of photoconversion there is minimal motion of the (guest) phenyl ring, pronounced rotation of the acetyl group about the exocyclic C-C bond, and displacement of unreacted and reacted molecules. Atom-atom potential energy calculations showed that in DCA-acetophenone a guest ketyl radical with a pyramidal geometry may rotate by a full 180° without inducing prohibitively short intermolecular contacts.
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(1987) Journal of the American Chemical Society. 109, 6, p. 1869-1871 Abstract
Precipitation of metastable polymorphic crystalline phases is of topical importance in several fields of science. In previous studies we have described the design of low molecular and poly eric additives as enantioselective inhibitors of crystal nucleation and growth of conglomerates (i.e., racemic mixture of enan iomorphous crystals in monomorphic systems). The design took mto account the packing arrangement in the crystal and the orientation and conformation of the molecules vis-a-vis the various crystal faces. This approach is now being tested for use in the precipitation of metastable phases of polymorphic crystals by kinetic control, illustrated here with the design of polymeric inhibitors for the induced resolution of racemic histidine.
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(1987) JOURNAL OF PHYSICAL CHEMISTRY. 91, 2, p. 472-481 Abstract
The absolute direction of preferred growth of α-resorcinol in water along its polar c axis was established by the use of anomalous dispersion of X-rays and three independent methods involving growth and dissolution in the presence of "tailor-made" additives: induced changes in growth rate at the opposite poles of the crystal, anisotropic distribution of occluded additives along the polar axis, and formation of etch pits at preselected faces on partial dissolution of the crystal. It was found that the preferred growth in water, ether, and dimethyl sulfoxide occurs at that end of the crystal from which hydroxyl oxygen atoms are exposed, labeled the {011} faces. C-H and O-H groups emerge at the slow-growing {011} faces. The specific arrangement of the molecules at these faces, which are acidic in character, causes the formation of pockets at which solvent molecules may be easily bound by (resorcinol)O-HO(solvent) and (resorcinol)C-HO(solvent) interactions. On the other hand, the {011} faces, which are weakly basic, are relatively flat, but expose oxygen atoms which can strongly bind solvent water molecules by (water)O-H⋯O(resorcinol) bonds but ether and dimethyl sulfoxide by much weaker (solvent)C-H⋯O(resorcinol) bonds. Potential energy calculations suggest that water is more strongly bound, by 1 kcal/mol per site, to the {011} faces than to {011}. Overall, our results appear to indicate that we may correlate the preferred direction of growth with a weaker affinity for the solvent.
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(1987) Tetrahedron. 43, 7, p. 1449-1475 Abstract
The two inclusion complexes of guest acetophenone and m, chloro-acetophenone with host deoxycholic acid (DCA) each yielded on UV irradiation a single diastereomeric photoaddition product at steroid atom C5, of configuration S at the generated stereogenic centre (DCAOHCH3C6H4). Comparison of the host-guest arrangement at the sites of reaction with the S configuration at the new chiral atom C of the photoproducts indicates a net rotation of 180° by the guest acetyl group prior to C-C bond formation since the guest molecules expose their Re faces to the C5-H5 steroid bonds1. To elucidate this rotation, photochemical and crystallographic studies were performed on four DCA complexes with engineered guest ketone arrangements. The guests were p-fluoroacetophenone and (p-substituted) propiophenones (XC6H4CO2C2H5, X=H, F, Cl). The crystal structures of two isolated photoproducts are also reported. The channels of DCA-fluoroacetophenone contain two independent guest molecules G' and G, respectively exposing their Re and Si faces to the steroid C5-H5 bond vectors at the potential sites of reaction. Only one diastereomeric photoproduct was obtained with addition at atom C5, with absolute configuration S at the generated stereogenic centre, akin to that of the DCA-acetophenone complexes. The reaction is interpreted in terms of a 180° rotation of the acetyl group of the G' molecule; the absence of the R diastereomer is explained by a preclusion of photoaddition of guest G to DCA because of guest packing along the channel. The guest arrangements in the two complexes DCA-propiophenone and DCA-p-fluoropropiophenone are almost isomorphous. There are two independent guest molecules G and Ḡ related by a pseudo centre of inversion. Each of the two guest molecules exposes its Re face to a steroid C5-H5 bond at a site of reaction. Thus photoaddition takes place with and without 180° rotation of the propionyl group since the two complexes each yield the two diastereomeric photoproducts at C5. DCA-p-chloropropiophenone yields only one diastereomeric photoproduct at C5, with absolute configuration R about the new stereogenic centre. The crystal structure analysis suggests three independent guest molecules G, G' ana Ḡ; G and G' are related by pseudo two fold screw symmetry along the channel axis; Ḡ is related to G and G' by pseudo centre of inversion; once again the guests G, G' and Ḡ appear to expose their Re faces to the C5-H5 bonds at the sites of reaction.
1986
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STEREOSELECTIVE ETCHANTS FOR MOLECULAR-CRYSTALS - RESOLUTION OF ENANTIOMORPHS AND ASSIGNMENT OF ABSOLUTE STRUCTURE OF CHIRAL MOLECULES AND POLAR CRYSTALS(1986) Nouveau Journal De Chimie-New Journal Of Chemistry. 10, 12, p. 723-737 Abstract
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(1986) ChemPhysChem. 83, p. 831-840 Abstract
Photochemical reactions in crystals proceed under control of the structure, including absolute configuration, of the bulk, unless preceeded by energy transfer, in which case defects may be important. The reaction may be followed by phase separation. Two-phase systems allow wider scope for reactant selection. The presence during growth of additives, matched stereochemically to the crystal, modifies habit and growth-rate, with far-reaching consequences. Some partially ordered systems are considered.
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(1986) Pure and Applied Chemistry. 58, 6, p. 947-954 Abstract
A new stereochemical approach for the controlled nucleation. growth and dissolution of organic crystals with well designed auxiliaries is described. The method is comprised of the use of appropriate stereospecific inhibitors of growth and dissolution of one or more preselected faces of the crystal. This approach has been successfully exploited for the engineering of organic crystals with desired morphologies, for the kinetic resolution of conglomerates by the process of crystallization, and for inducing etch-pits at preselected faces of crystals. A correlation has been established between crystal morphology and crystal purity. Oriented growth of organic and inorganic crystals at interfaces has been accomplished with the assistance of designed Langmuir and Langmuir Blodgett films. The approach has recently been extended to the study of solvent effects on the morphology of crystals in general, and of polar crystals in particular.
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(1986) Molecular Crystals And Liquid Crystals. 137, 1, p. 67-86 Abstract
Various properties of polar crystals, in particular the effect of solvent on their growth, are directly related to the absolute structure of the crystal. The absolute structures of polar crystals composed of achiral molecules or racenic mixtures were assigned by three independent methods involving growth and dissolution in the presence of tailor made additives: Inhibition of crystal growth, anisotropic distribution of the occluded additive, and formation of etch pits on partial dissolution of the crystal. The method has been applied to crystals of α-resorcinol, (R, S) alanine and γ-glycine, each of which exhibits oxygen-rich and hydrogen-rich faces at the opposite ends of polar axes. All three crystals were found to grow almost unidirectionally from water at the oxygen-rich end of the crystal. The results are analysed in terms of solvent-surface interactions.
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(1986) Topics in Stereochemistry. Vol. 16. p. 1-85 Abstract
This chapter contains sections titled: Introduction; Statement of the Problem; The Bijvoet Method; A Bridge Between Crystal Structure, Crystal Morphology, and Molecular Chirality; Absolute Configuration from Chemical Reactivity in Polar and Enantiopolar Crystals; The Solvent Effect; Pyroelectricity, Piezoelectricity, and Optical Activity in Crystals; Determination of Absolute Configuration by Electron Microscopy; Conclusions
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(1986) Molecular Crystals And Liquid Crystals. 134, 1, p. 323-335 Abstract
Oriented growth of crystals of α-glycine and sodium chloride under compressed Langmuir monolayers at air-water interfaces was achieved. For α-glycine, a variety of monolayers containing resolved glycyl head groups and different hydrophobic moieties were used, thus establishing the structural requirements necessary for oriented crystallization. For sodium chloride, monolayers of positive, negative and zwitterionic charge induced crystal nucleation from faces of the type {100}, {110} and {111}, the latter two faces not being naturally occurring.
1985
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(1985) Journal of the American Chemical Society. 107, 13, p. 4058-4070 Abstract
The crystalline hostQuest channel inclusion complexes 5:2 (DCA) deoxycholic acid-acetophenone (C6H5C'Och3) and 3:1 DCA-m-chloroacetophenone (CLC6H4COCH3) each yield on UV irradiation a photoproduct via addition of guest to the steroid tertiary carbon atom C5 with the formation of a new chiral carbon center C'(OH)(CH3)(C6H5)(DCA) of S configuration. The crystal structures of the two host-guest complexes were determined by low-temperature (103 K) X-ray diffraction; a low-temperature (16 K) neutron study was made on DCA-C6H5COCD3. The inclusion compounds DCA-C6H5COCH3 and DCA-CLC6H4COCH3each contain two crystallographically independent guest molecules G and G' arranged along the channel axes such that both G and G' should form the same diastereomeric product at C5. A comparison of the stereochemistry of each of the two isolated photoproducts and the host-guest arrangements at the reaction sites in each corresponding complex indicates that photoaddition of the guest molecule to C5 takes place with a net rotation of 180° by the guest acetyl group.
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(1985) Journal of the American Chemical Society. 107, 13, p. 4043-4058 Abstract
Deoxycholic acid (DCA) forms channel inclusion complexes with acetone (host-guest molar ratio 5:3), diethyl ketone (2:1), and ethyl methyl ketone (2:1). UV irradiation of these three complexes led to stereospecific photoaddition of the guests at sites [C5, C6eq, C6ax], C6eqand [C6eq, C5] of the host, respectively. Crystal structures of DCA-acetone and DCA-ethyl methyl ketone at temperatures of 103 K were determined and of DCA-diethyl ketone at 293 K. The host structures are isomorphous; they form hydrogen-bonded bilayers which are juxtaposed by hydrophobic contacts to form inclusion channels delineated by four walls. The occluded ketones are sandwiched between the wide walls of the channel comprising the steroid rings A and B. The regio-and stereospecificity of these reactions are explained on topochemical grounds. The 3:1 complex of DCA-methyl pentyl ketone is very similar to that of DCA-ethyl methyl ketone. Photoirradiation leads to cleavage of the ketone, yielding acetone which subsequently adds to DCA at site C5 and C6. A different channel motif was engineered in which cyclohexanone was sandwiched between rings D and the steroid side chains, leading to photoaddition to site C16 of ring D. The 1:1 complex of (APA) apocholic acid-acetone is light stable. The X-ray structure analysis indicates a similar host bilayer structure as DCA. The acetone molecules are stacked up the channel axis, unlike in DCA-acetone, and are arranged such that the ketone C'=0' bond tends to be parallel to the nearest C-H of the steroid wall. According to this analysis the maximal distances between the ketone C'=0' group and the potentially reactive steroid C-H bond are 3.5 A for OH and 4.2 A for C'. C. The angle between the planar guest ketone group and the potentially reactive C-H bond of the host steroid was found to vary over a wide range from about 50° to 90°.
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(1985) Angewandte Chemie-International Edition In English. 24, 6, p. 466-485 Abstract
When crystals of organic compounds are grown in the presence of growth inhibitors, there is a change in crystal morphology. A stereochemical correlation exists between the crystal structure, its modified morphology, and the molecular structure of the inhibitor. This correlation has been successfully exploited for the efficient resolution of conglomerates, the engineering of organic crystals with desired morphologies, the direct and relative assignment of the absolute configurations of chiral molecules and crystals, and for the design of a new model for the spontaneous generation of optical activity. In an analogous way dissolution of organic crystals in the presence of these growth inhibitors induces etch pits at preselected faces. The effect of solvent on crystal growth has been analyzed in some model systems. The experimental results are complemented by atomatom potential energy calculations.
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(1985) Journal of the American Chemical Society. 107, 11, p. 3111-3122 Abstract
A general method for the controlled modification of the morphology of organic crystals is described. The method comprises stereospecific inhibition of growth of preselected faces with the assistance of "tailor-made" growth inhibitors. The change in crystal habit is explained by a mechanism which takes into consideration a stereochemical correlation between the molecular structure of the additive, the crystal structure of the substrate, and the affected growth directions. The method has been successfully applied to primary amides ((E)-cinnamide, benzamide, p-toluamide, and p-clorobenzamide), carboxylic acids ((E)-cinnamic acid and benzoic acid), and steroids (androsterone and epiandrosterone). Dramatic changes in habit were induced by replacement, at the crystal surface, of a substrate N-H ..,0 bond by an Q...Q repulsion, by replacement of an attractive C-H ..,0 interaction by a C-H ..,H or C-Cl..,0 repulsion, or by insertion of a methyl or chlorine substituent instead of hydrogen, etc. The induced morphological modifications were used to obtain information on molecular interactions and conformations of the adsorbed inhibitor at the crystal surface. Thus, we could show that adsorbed additives benzamide and picolinamide assume twisted and planar conformations, respectively, on the affected faces of host benzoic acid, which is planar in the solid. In contradistinction, p-aminobenzoic acid tends to be planar when interacting with the growing faces of the nonplanar p-aminobenzamide. The additive (E)-cinnamide assumes a syn-planar C=C-C=O conformation on the affected surface of host (E)-cinnamic acid, which tends to assume an antiplanar C=C-C=O conformation. The effect of various additives on crystal habit is accounted for and quantitatively evaluated by atom-atom potential energy calculations.
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SPONTANEOUS ASYMMETRIC TRANSFORMATIONS IN CHIRAL AND CENTROSYMMETRIC SINGLE-CRYSTALS(1985) Chemica Scripta. 25, p. 91-99 Abstract
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(1985) Israel Journal of Chemistry. 25, 3-4, p. 362-372 Abstract
The centrosymmetric monoclinic crystals of glycylglycine were grown in the presence of three dipeptides (racemic and resolved GlyAla, AlaGly and GlyLeu); the presence of each yielded a different morphology. It was found by a combined theoretical and experimental study that the conformation of the guest molecule is determined by the highly stereospecific intermolecular interactions between adsorbed additive and the molecules of particular surface layers of the growing crystals. Theoretical calculations predicted the preferred conformation of the leucine side chain of GlyLeu consistent with the change in morphology. Occluded racemic GlyLeu segregates along the b axis of the single crystals of GlyGly.
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(1985) Israel Journal of Chemistry. 25, 3-4, p. 353-361 Abstract
The stereoselective interactions in aqueous solution between the crystal faces of centrosymmetric (R,S) serine 1 and additives resolved and racemic threonine 2 and allothreonine 3 were studied by four independent techniques: change in crystal habit on growth, anisotropic distribution of occluded additive during the crystal growth, etch pit formation on partial dissolution and atomatom potential energy calculations. The effects of the two diastereomers 2 and 3 are radically different due to differences in adsorption: threonine is adsorbed on the {011} crystal faces; the (R) enantiomer is adsorbed at, and eventually occluded through the (011) and (011) crystal faces replacing an (R) serine molecule. By symmetry, (S) threonine is adsorbed and occluded through the enantiotopic (011) and (011) faces replacing (S) serine. In contrast, (R) and (S) allothreonine are both adsorbed primarily at the (100) and (100) faces. The results indicate that the serine moiety of adsorbed threonine and allothreonine assume the same conformation and position as the substrate molecule. The additive is adsorbed only at specific sites such that the CβCH3 groups emerge from the crystal surface. The observed effects lead in the case of threonine, but not allothreonine, to the assignment of its absolute configuration.
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(1985) Israel Journal of Chemistry. 25, 3-4, p. 334-337 Abstract
A model for the separation of enantiomers of several classes of compounds by copper complexes of N,Ndialkylαamino acids, based on chromatographic results and supported by crystal growth and dissolution studies, is described.
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(1985) Nature. 318, 6044, p. 353-356 Abstract
One enantiomer of an initially racemic mixture of α-amino acids can be preferentially removed from solution by selective incorporation into the opposite enantiotopic faces of growing centrosymmetric crystals of the α-form of glycine1,16. We report here that oriented growth of crystals of the α form of glycine has been achieved under chiral Langmuir monolayers comprising amphiphilic α-amino acids, by virtue of a structural match between the monolayer and the ac surface layer of the attached growing glycine crystals. Such monolayers of α-amino acid of R configuration, containing long hydrocarbon chains, induce glycine to crystallize with its (010) face attached to the monolayer, and by symmetry the corresponding S amino-acid monolayers induce attachment of the (0&1macr;0) face of glycine. Replacement of the hydrocarbon by a fluorocarbon chain induces analogous crystallizations, albeit, with only a partial degree of orientation, whereas monolayers of a resolved amino acid bearing a cholestanoyl moiety do not promote crystallization of glycine. Monolayers of the R, S-amino acids induce attachment of both (010) and (0&1macr;0) faces of glycine. These results on oriented crystal growth provide a new route for efficient amplification of optical activity of amino acids present in solution, through the enantioselective occlusion into the growing crystals of glycine1 at water-air interfaces covered by a monolayer.
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1984
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(1984) Nature. 310, 5973, p. 161-164 Abstract
Recent studies of crystal growth in the presence of specific additives have suggested a new approach to the generation and amplification of optical activity in initially racemic mixtures1-4. This approach is based on the observation that particular enantiomers can be selectively incorporated not only into chiral crystals5-7 but also into the opposite enantiotopic faces of centrosymmetric crystals3,4,8. If one such face of a centrosymmetric crystal is blocked, therefore, enantioselective occlusion into the opposite face will remove one enantiomer preferentially from the racemic mixture. We report here that this process can take place with centrosymmetric crystals of glycine, in a way that may be of relevance to the generation of optical activity during prebiotic evolution. Glycine crystals float in solution, such that only one face is available for growth. The addition of a particular enantiomer of a hydro-phobic amino acid orientates the floating crystals in such a way that the face that occludes amino acids of the opposite chirality is exposed to the solution. Thus, a statistical fluctuation in the orientation of crystals floating in an initially racemic solution could lead to the generation and amplification of optical activity.
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1983
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(1983) Journal of the American Chemical Society. 105, 22, p. 6615-6621 Abstract
A new method for the direct assignment of the absolute configuration of chiral molecules is described. The method comprises precipitation of centrosymmetric crystals composed of chiral or prochiral molecules, in the presence of racemic or resolved chiral additives. The additives are occluded via the enantiotopic faces of the centrosymmetric crystals, resulting in changes in morphology and enantiomeric segregation of the additives within the crystal. The absolute configurations of the additives are deduced from these two effects. The structural requirements for host and additive molecules in centrosymmetric triclinic, monoclinic, and orthorhombic space groups are outlined. The method has been successfully applied for determination of the absolute configuration of the α-amino acids by their effect on the centrosymmetric monoclinic crystals of prochiral glycine. The crystal morphology of pure glycine and the changes in its habit are interpreted quantitatively in terms of atom-atom potential energy calculations.
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(1983) Journal of the American Chemical Society. 105, 4, p. 765-767 Abstract
Doubly vs. singly hydrogen-bonded arrangements in crystalline N-acylated amino acids R-CONH-CHR-C02H are examined by energy calculations. The O(amide) is a significantly stronger proton acceptor than the O(carboxyl). Thus in one-third of the crystal structures the molecules form doubly hydrogen-bonded systems O-H-O(amide) and N-H-O(amide), despite the general preference for the maximum number of proton acceptor sites to participate in hydrogen bonds, dictated by molecular packing and conformation. The energy results are consistent with the tendency for the molecules to be interlinked via single hydrogen bonds O-H-O(amide) and N-H-O(carboxyl), rather than the reverse, and the total absence of the doubly hydrogen-bonded systems O-H-O(carboxyl) and N H-(carboxyl).
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GENERATION OF POSSIBLE CRYSTAL STRUCTURES OF PRIMARY AMIDES.(1983) Proceedings Of The Royal Society Of London Series A-Mathematical Physical And Engineering Sciences. 388, 1794, p. 133-175 Abstract
Procedures are outlined for generation of crystal structures of primary amide molecules by constructing the possible ways in which the molecules may pack. For each given one- or two-dimensional hydrogen-bonded array, ensembles of three-dimensional crystal structure are generated by considering the possible ways in which the arrays may be juxtaposed. Observed and generated hypothetical molecular arrangements are analyzed.
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(1983) Molecular Crystals And Liquid Crystals. 96, 1-4, p. 1-17 Abstract
Retardation of the growth of organic crystals in the presence of additives in solution was investigated in terms of the crystal structure and the stereochemistry of the additives. A simple correlation was established between affected directions of growth and structure of the additive. This mechanism was successfully applied to monitoring, in a controlled manner, the morphology of the organic crystals of benzamides and benzoic acids.
1982
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(1982) Angewandte Chemie-International Edition In English. 21, 8, p. 631-632 Abstract
The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 1336. DOI:
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(1982) Nature. 296, 5852, p. 21-26 Abstract
A method for resolution of conglomerates, based on morphological differences between the enantiomorphic crystals induced by tailor-made resolved impurities is described. The relationship between the crystal structure of the affected enantiomorph and the stereochemistry of the added impurity is analysed in the systems glutamic acid. HCl, asparagine. H2O and threonine in the presence of L- or D-amino acids, and it is found that the additive binds stereoselectively at the surface of the enantiomer of the same absolute configuration, occupying the same site of the amino acid groups of the host molecule. The modified side chains perturb in a second stage the regular growth of the crystal in the directions along which the side chains of the host are hydrogen bonded.
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(1982) Journal of the Chemical Society, Chemical Communications. 11, p. 584-587 Abstract
The ability of molecules, chiral by virtue of the presence of an s-butyl group, to display conformational polymorphism has been exploited for the performance of asymmetric syntheses of either sense on the same chiral molecule; this phenomenon is illustrated for the asymmetric solid-state polymerization of the monomer (R)-()-(1).
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(1982) Nature. 296, 5852, p. 27-34 Abstract
A method is described for direct determination of absolute configuration of chiral polar crystals, based on changes in crystal habit induced by tailor-made impurities. The method embodies the assignment of the direction of the chiral substrate molecule with respect to the crystal polar axis based on differences in the hemihedral faces of the substrate crystals as grown from solution in the presence and absence of impurities. The molecular packing requirements and the choice of impurities for application of this method in lysine and trans-cinnamoyl alanine are outlined.
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(1982) Journal of the American Chemical Society. 104, 15, p. 4052-4064 Abstract
Information contained in electron density distributions, derived from X-ray diffraction data, was exploited to improve the electrostatic parameters of atom-atom potential functions of the carboxyl group and thus to obtain a better estimate of the Coulomb intermolecular energies. The proton affinities of the carbonyl oxygen atom of the amide and carboxyl groups were compared in terms of electrostatic potential energy around these atoms and correlated with hydrogen-bonding properties. Energy calculations were undertaken to account for various characteristic packing motifs of carboxylic acid molecules, including the geometry of the catemer motif, order-disorder of the carboxyl dimer, preferred stacking of carboxyl dimers, the role played by C-H⋯O(carbonyl) interactions in stabilizing overall packing, characteristic packing of substituted benzoic acids, and the stability of the observed crystal structures of oxalic acid and formic acid, via the generation and analysis of ensembles of alternative crystal structures.
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(1982) Journal of the Chemical Society, Chemical Communications. 6, p. 326-328 Abstract
The differences in luminescence properties between the chiral resolved and the racemic compounds of some 9-anthroates have been exploited for the determination of the enantiomeric excess in these esters.
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Reactions in molecular inclusion complexes. 4. Biomimetic photohydroxylation of crystalline Deoxycholic-acid as catalyzed by occluded oxygen carrier(1982) Nouveau Journal De Chimie-New Journal Of Chemistry. 6, 2, p. 75-77 Abstract
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1981
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REACTIONS IN MOLECULAR INCLUSION COMPLEXES .3. THE REACTION PATHWAY OF THE PHOTOADDITION OF ACETOPHENONE TO DEOXYCHOLIC-ACID AS STUDIED BY X-RAYS IN A SYSTEM UNDERGOING A SINGLE-CRYSTAL SINGLE-CRYSTAL TRANSFORMATION(1981) Nouveau Journal De Chimie-New Journal Of Chemistry. 5, 10, p. 475-478 Abstract
1980
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(1980) Journal of the American Chemical Society. 102, 26, p. 7677-7690 Abstract
Atomic multipole moments of acetamide and formamide were derived from low-temperature X-ray diffraction data. These atomic moments comprised net atomic charges, dipoles, and quadrupoles. The derived molecular dipole moment of formamide compares well with that as obtained from other experiments. These atomic moments were used to calculate the Coulomb energies of various hydrogen-bonding motifs found in the crystals of primary amides and of cis and trans secondary amides, demonstrating how Coulomb forces determine hydrogen bond arrangements. For the most common hydrogen-bonding motifs the dominant contributions to the intermolecular Coulomb energy are charge-charge and charge-dipole terms. Comparisons of commonly and rarely observed hydrogen-bonding motifs demonstrate that Coulomb forces generally favor the former. The linearity of the N-HO bond, the N-(H)O=C angle, the degree of nonplanarity of the N-HO=C
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(1980) Journal of the American Chemical Society. 102, 8, p. 2768-2772 Abstract
A molecular model which accounts for stereoselective interactions within the series of chiral secondary amides RCONHCH(CH3)Ar is proposed. It assumes that a guest molecule intercalates between two host molecules in their 5-Å translation H-bonded array to form diastereomeric arrangements for the two enantiomers, differing in stability. The model explains various aspects of chiral recognition: (1) the gas chromatographic separation of enantiomers achieved with N-acyl-α-arylethylamines as chiral phases; (2) the difference in solubility of (R)- and (S)-N-trifluoroacetyl-α-(1-naphthyl)ethylamine in solid (R)-N-lauroyl-α-(1-naphthyl)ethylamine. The packing arrangement of certain diamide molecules was examined in order to reinforce the assumptions on which the explanation of the stereoselective effects was based. Energy computations support the stability relationships which follow from the model.
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(1980) Pure and Applied Chemistry. 52, 12, p. 2693-2704 Abstract
The crystalline inclusion complexes of deoxycholic acid 1 (DCA) and apocholic acid 2 ACA (choleic acids) are used as vehicles for the performance of biomimetic reactions between host steroid and included guest. The choleic acids generally appear in three different channel motifs, α, βand y, in orthorhombic crystals as determined by the size and shape of the guest molecule. Thus the nature of the channel wall can be controlled by the guest. U.V. irradiation of the complexes of DCA with linear aliphatic ketones, crystallized under argon, yields products of addition at sites C5 or C6, depending upon the relative orientation of the ketone to the steroid channel wall. On the other hand, cyclohexanone adds to site C16. The complex between DCA and prochiral acetophenone yields, on irradiation, the addition product at C5, with the formation of a new chiral centre. A comparison of the structure of DCA-acetophenone and the configuration about the new chiral centre indicates a net rotation of 180° by the acetyl group, upon reaction. To gain further insight into the reaction pathway the structure of the partially reacted crystal (these complexes do not disintegrate on reaction) was determined. It proved that during the course of conversion there was minimal motion of the phenyl ring, pronounced rotation of the acetyl group about the exocyclic C-C bond and, displacement of reacted and unreacted molecules. Furthermore the conformation of this photoproduct molecule about the new chiral centre in this partially reacted crystal differs radically from that in the crystal structure of the isolated photoproduct. Other systems which reveal similar photobehaviour are briefly described. Irradiation of several DCA complexes in the presence of molecular oxygen yields 5β hydroxy product of DCA. Possible routes for this reaction, with ketones as well as with other guests serving as oxygen carrier, are discussed.
1979
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(1979) Israel Journal of Chemistry. 18, 3-4, p. 385-389 Abstract
The deoxycholic acid (DCA)water (2:3) complex crystallises in space group P41212. The DCA molecules form hydrogenbonded layers which contain channels filled by chains of water molecules. These water molecules are linked by hydrogen bonds with each other and with the DCA molecules on the channel walls. The channel walls are thus hydrophilic unlike the hydrophobic channel walls found in the commonly observed orthorhombic P212121 inclusion complexes of DCA.
1978
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(1978) Journal of the American Chemical Society. 100, 18, p. 5879-5887 Abstract
A methodology for the study of the role of hydrogen bonding and other energetic factors in determining the symmetry and secondary and tertiary structures of hydrogen-bonded crystals is presented. Lattice energy minimization techniques are extended to the study of \u201chypothetical\u201d crystal packing modes in order to carry out this analysis. The comparison of these hypothetical crystal structures with observed structures is also proposed as a useful tool for testing the validity of potential functions. The model system chosen for this initial study is adipamide. The observed packing mode of adipamide is anomalous with respect to the family of primary amides in that it does not involve hydrogen-bonded rings. The hydrogen-bonding structure of adipamide appears to be less stable than the standard packing motif. A total of 16 hypothetical crystals, comprising various space groups, were constructed. These include the three typical modes of hydrogen-bonded networks (secondary structures) related by var ous symmetries to form structures common to the family of primary amides. The observed structure is calculated to be more stable than all hypothetical structures considered. Analysis of intra-and interlayer energies indicates that adipamide, by foregoing the most favorable hydrogen-bonding arrangement, achieves a better interlayer packing which more than compensates for the poorer hydrogen bonding. Several hypothetical structures are found to be within a kilocalorie or two of the experimental structure. This result is related to the tendency to form the maximum number of hydrogen bonds and polymorphism in crystals. Finally, the results are discussed in terms of symmetry considerations, both with regard to the experimental observation that amides rarely pack in orthorhombic and higher symmetries, and the need to relax symmetry constraints when studying hypothetical models.
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1977
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(1977) Journal of the Chemical Society, Chemical Communications. 24, p. 928-929 Abstract
Resolved 1-phenylpropyl and 1-phenylbutyl 9-anthroates, when crystallised from n-hexane, form 6:1 inclusion molecular complexes which are isostructural; X-ray analysis of the propyl derivative demonstrates the formation of a channel structure by the anthracenes, in which the n-hexane is included.
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1976
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(1976) Journal of the American Chemical Society. 98, 15, p. 4600-4612 Abstract
The energetics of rotation about the N-Cα (Φ) and Cα-C (Ψ) bonds of methyl groups in simple amide and peptide systems have been studied by experimental and theoretical methods. X-Ray crystal structure analyses of 12 molecular conformations indicated that the position of the minimum in Φ (C-N-Cα-H) was equal to 180° (i.e., C-H anti to the C-N bond). In Ψ (H-Cα-C-N) the minimum was found to be 0°, i.e., methyl C-H syn to the C-N bond, based on analysis of ten molecular structures. Variations from these rotational minima appeared to be induced by crystal forces. In order to better understand these phenomena, ab initio molecular orbital, and empirical force field calculations of the rotational potential surface, and lattice energy calculations of the effect of crystal forces on the conformation were carried out. Minimal basis set molecular orbital calculations as carried out here and by others seem to yield results in disagreement with the experimental observations. When extended basis set calculations were carried out it was found that the calculated rotational potential surface in Φ is compatible with the experimental results. The location of the minimum in Ψ is still not correct, however, although the barrier was found to be almost negligible (0.1-0.2 kcal/mol vs. ∼1 kcal/mol in the minimal basis sets). Lattice energy calculations on N-methylacetamide indicated that the crystal forces were of the same magnitude as those due to the rotational potential, in agreement with the experimental observation from various crystals that these forces seem to affect the intramolecular conformations. The minimized lattice energies at different Φ's and Ψ's were combined with the rotational potential energies as obtained from the various quantum mechanical methods in order to compare the predicted conformation with that observed. The empirical force field calculations using four previously derived different sets of potential functions (three of which having been obtained from fitting crystal data) all yielded the correct minimum in. However, in all potentials predicted a minimum in disagreement with the experimental results as in the case of the quantum mechanical calculations. Thus in Ψ all theoretical methods yield the same result, which seems to be at odds with the experimental observations. The results also indicated that a 12th power repulsion may be too \u201cstiff\u201d when applied to the short intramolecular interactions important in determining rotational potentials.
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(1976) Molecular Crystals And Liquid Crystals. 32, 1, p. 127-129 Abstract
In recent years there has been increased interest in organic chemistry to mimic the enzymatic reactions of nature in order to increase the power of synthetic methods1. Molecular crystals are very convenient vehicles for the performance of such reactions, since in them as in an enzyme, the geometrical contacts between reacting centers of the rigid molecules are strictly defined by the packing of the crystal.
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(1976) Israel Journal of Chemistry. 15, 1-2, p. 116-123 Abstract
A new chemical massspectrometric method for mapping short range order in crystalline solidsolutions of enantiomers undergoing topochemical reactions is described. We have applied this method to assigning the microstructure of the racemic mixtures of the diacrylates 1 and 2, each one in two dimorphic modifications and which undergo topochemical (2π + 2π) photocyclodimerisation and photocyclopolymerisation. All crystalline phases are disordered with respect to the enantiomeric secbutyl group. While the two polymorphs of 1 and polymorph α of 2 form random solidsolutions, polymorph β of 2 forms partially ordered crystals in which the degree of order depends on the method of crystallization. It could be established that in the latter system the degree of order across the two independent centres of inversion is measurably different. The scope and limitation of this new method are discussed.
1975
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(1975) Journal of the Chemical Society, Chemical Communications. 21, p. 864-865 Abstract
Solid-state thermolysis at 90°C or photolysis at ambient temperature of the molecular complex of deoxycholic acid with di-t-butyl diperoxycarbonate leads to a one-step regiospecific and stereospecific hydroxylation at position 5 of the acid.
1974
1973
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(1973) Angewandte Chemie-International Edition In English. 12, 2, p. 158-159 Abstract
Keywords: Chemistry, Multidisciplinary
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(1973) Journal of the Chemical Society, Perkin Transactions 2. 5, p. 503-508 Abstract
The benzamide-succinic acid adduct (I) crystallises in space group P21/c, with Z = 2 in a unit cell of dimensions: a = 9.194, b = 5.116, c = 19.480 Å, β = 105.66°. The structure was refined from three-dimensional film data to R 0.10 (1441 reflections). The furamide-oxalic acid adduct (II) crystallises in space group P1, with Z = 1 in a unit cell of dimensions: a = 7.459, b = 6.579, c = 8.064 Å, α = 92.19, β = 95.60, γ = 120.29°. The structure was refined from three-dimensional counter data to R 0.05 (1451 reflections). Both (I) and (II) form 2:1 molecular-complex units in which the amide and carboxy-groups are linked by cyclic hydrogen-bonded (asymmetric) pairs [(I) : O-H ⋯ O 2.60, N-H ⋯ O 2.98; (II) : O-H ⋯ O 2.53, N-H ⋯ O 2.92 Å]. The units of (I) are interlinked, along the 5 Å axis, by amide-amide hydrogen bonds (N-H ⋯ O 3.02 Å). Those of (II) are joined, along the 6.6 Å axis, by bifurcated N-H ⋯ O bonds [N-H ⋯ O(hydroxy) 3.10, N-H ⋯ O(carbonyl) 2.94 Å] between the furamide and oxalic acid fragments.
1972
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(1972) Zeitschrift fuer Kristallographie. 135, 3-4, p. 232-239 Abstract
Methods are outlined for various types of calculations for the four-circle diffractometer which include: (1) calculation of the Eulerian setting angles and (2), determination of the crystal-specimen orientation.
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1778-1785 Abstract
The crystal structure of pentafluorobenzoic acid has been solved from 1598 three-dimensional counter data, and refined by least-squares to R 0.04. Crystals are triclinic, space group P1, with Z = 2 in a unit cell of dimensions: a = 7.925, b = 8.637, c = 6.262 Å, α = 97.55, β = 89.65, and γ = 121.86°. Values of σ for bond lengths are 0.002 Å. The angle between the planar pentafluorophenyl group and the carboxy-group is 29.8°. The molecules form cyclic hydrogen-bonded dimers (OH ⋯ O 2.668 Å) in which the C-OH and C=O bond lengths are 1.278 and 1.221 Å. The structure of mesitoic acid was refined similarly to R 0.05 for 2060 reflections. Crystals are monoclinic, space group C2/c, Z = 8, in a unit cell of dimensions: a = 15.142(1), b = 7.026(1), c = 17.439(1) Å, β = 89.35 (1)°. The mean σ values of the heavy-and hydrogen-atom co-ordinates are 0.001 and 0.03 Å. The angle between the mesityl and carboxy-groups is 48.4°. The molecules form cyclic hydrogen-bonded pairs (OH ⋯ O 2.640 Å) in which the C-O bond lengths are nearly equal (1.275 and 1.248 Å). The molecular dimensions and the packing arrangements of both structures are discussed.
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1763-1768 Abstract
The crystal structures of the α- and β-forms of tetrolic acid have been determined from X-ray diffractometer data. Crystals of the α-form are triclinic, space group P1, Z = 2, a = 7.320, b = 5.099, c = 7.226 Å α = 83.97, β = 117.46, γ = 112.00°. The structure was refined to R 0.07 for 985 reflections. The molecules form hydrogen-bonded cyclic dimers (O ⋯ O 2.649 Å) in which the C-O bond lengths are almost equal (1.252, 1.265 Å). 'Half hydrogens' were attached to each oxygen, their parameters refined, and their presence substantiated by a difference-Fourier synthesis. The methyl hydrogen atoms were located. The structure of the monoclinic β-modification [a = 7.887, b = 7.121, c = 3.937 Å, β = 100.18°, Z = 2, P21] was refined from counter data to R 0.04 (687 reflections). The methyl hydrogen atoms were located. The bond lengths within the carboxylic group are distinct: C=O 1.204, C-O 1.310 Å; the hydroxy-hydrogen was clearly evident. The molecules form a continuous array of O-H ⋯ O= hydrogen bonds (O ⋯ O 2.655 Å) between molecules related by a two-fold screw axis similar to the arrangement in the structure of formic and acetic acids. The carbon - carbon bond lengths of the α- and β-forms are: H3C-C 1.458, 1.455, C≡C 1.182, 1.178, and C-CO2H 1.441, 1.441 Å.
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1750-1758 Abstract
The crystal structures of β-methyl-cis-, (III), β-chloro-cis-, (II), and β-chloro-trans-cinnamic, (I), acids have been determined from three-dimensional counter data by Patterson and Fourier methods. (I): a = 19.588, 6 = 5.374, c = 16.683 Å, β = 103.62°, Z= 8, space group A2/a, R 0.054 (1873 reflections): (II): a = 5.797, b = 24.247, c = 7.240 Å: β = 121.56° Z = 4, space group P21/c, R 0.046 (1917 reflections); (III): a = 7.504 b = 7.442, c = 17.056 Å, β = 107.00°, Z = 4, space group P21/c, R 0.066 (1987 reflections). The σ values of the experimental bond lengths are
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1758-1763 Abstract
The crystal structure of the title compound has ben solved from X-ray diffractometer data by direct methods and refined by least-squares to R 0·038 for 1182 independent reflections. Crystals are monoclinic, space group P21, with Z= 2 in a unit cell of dimensions: a= 7·367(1), b= 9·041(1), c= 3·742(1)Å, β= 99·39(1)°. The mean σ for bond lengths not involving hydrogen is 0·002 Å.
The hydrogen-bond network is two-dimensional. The amide and carboxy-groups of molecules related by translation form cyclic hydrogen-bonded (asymmetric) pairs (OH ⋯ O, 2·660; NH ⋯ O, 2·838 Å) generating chains which are interlinked, along the 21 axis, by NH ⋯ O (2·935 Å) bonds. The conformations of the groups C:C·C(NH2):O and C:C·C(OH):O are syn-planar, in contrast to the antiplanar C:C·C:O arrangements in the α- and β-forms of fumaric acid.
The bond lengths are: C[double bond, length half m-dash]C 1·320, C(OH)C 1·497, C(NH2)C 1·486, C(OH)[double bond, length as m-dash]O 1·209, C(NH2)[double bond, length as m-dash]O 1·247, COH 1·310, and CNH2 1·323 Å.
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1769-1772 Abstract
The crystal structure of anhydrous acetylenedicarboxylic acid has been solved from 1031 independent three-dimensional counter data and refined by least-squares methods to R 0.045. Crystals are monoclinic, space group P21/n, Z = 4, unit cell dimensions: a = 14.894, b = 6.420, c = 4.862 Å, β = 90.90°. σ For bond lengths involving heavy atoms only is estimated as 0.002 Å. The molecule is non-planar: the dihedral angle between the carboxy-groups is 57.8°. The four C-O bond lengths are nearly equal (1.254 ± 0.01 Å). The mean C-C bond length is 1.458 ± 0.001 Å; the experimental value of the -C≡C- bond is short (1.168 Å). The molecular dimensions and packing arrangement are discussed.
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(1972) Journal of the Chemical Society, Perkin Transactions 2. 12, p. 1772-1778 Abstract
The title compound crystallises in space group Pca21 with two molecules in the asymmetric unit, the cell having dimensions a = 8.060, b = 4.195, c = 25.433 Å. The phases were determined from a diffractometer data by a multi-solution method and the structure refined by a least-squares procedure to R 0.067 for 968 independent reflections. The bond lengths are C≡C 1.175, 1.179; ≡C-C 1.455, 1.453; C-CO2H 1.484, 1.506; C=O 1.221, 1.214; C-OH 1.307, 1.259 Å. The C(:O)·O·C⋮C system is syn-planar. The molecules are hydrogen-bonded (O-H ⋯ O 2.69 and 2.65 Å) in pseudo-centrosymmetric pairs. In the crystal there are no intermolecular ethynyl-H ⋯ O=C interactions: rather, the acetylenic C-H bond points at the C≡C triple bond, [H ⋯ C⋮C(midpoint) 3.0 Å]. The C(sp3)-H ⋯ bond is directed towards the carbonyl oxygen (C-H ⋯ O 3.45, and H ⋯ O 2.6 Å). But-3-ynoic acid also appears in a pseudo-orthorhombic form showing order-disorder. The relationship of the two forms is discussed.
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(1972) Israel Journal of Chemistry. 10, 2, p. 601-612 Abstract
trans, transMuconic acid C6H6O4 crystallises in the space group (Formula Presented.) , with a = 8.982, b = 9.895, c = 3.787 Å, α = 103.68, β=75.27, γ= 101.58°, and two centrosymmetric molecules per unit cell. The structure was solved from 1671 reflections measured by an ω/2θ scan with Mo Kα radiation on an IBM 1800 computercontrolled Siemens diffractometer, and refined anisotropically (on F2) to R = 0.078. The geometries in the carbon chains of the two independent molecules are essentially identical; their bond lengths match to within 0.003 Å. The carboxyl group of one molecule is statistically disordered (CO 1.257, 1.275 Å) whereas in the second molecule the bond lengths differ significantly (1.236, 1.294 Å), indicating a greater degree of order. A careful consideration of the packing mode allows us to interpret the difference between the two molecules.
1970
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(1970) Acta crystallographica. Section B, Structural crystallography and crystal chemistry. 26, 4, p. 442-443 Abstract
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(1970) Chemische Berichte. 103, 4, p. 1119-1131 Abstract
Die Kristallstruktur der Titelverbindung wurde mit Hilfe der SEARCHMethode gelöst und aufgrund von 4050 (einschließlich 1484 \u201eunbeobachteten\u201d) Reflexen (sin Θ/λ ≤ 0.66) und 150 starken Reflexen im Bereich 0.66 KαStrahlung (DifferenzFilterMethode) auf einem mit Lochstreifen gesteuerten SiemensDiffraktometer gemessen worden waren. Die Standardabweichung der kristallographisch unabhängigen Längen der Dreifachbindung (1.192 Å) sowie der exocyclischen (1.434 Å) und endocyclischen Bindungen der Benzolkerne (mit Symmetrie m statt 6) beträgt 0.003 Å. Die Abweichung der Kohlenstoffatome der Dreifachbindungen aus den Ebenen ihrer Benzolringe (maximal 0.103 Å) ist signifikant, da die Ringatome selbst durchschnittlich nur 0.004, maximal 0.010 Å außerhalb dieser Ebenen liegen. Die Konformation des Moleküls entspricht eher der Symmetrie m als ō m 2, wofür Packungseffekte verantwortlich gemacht werden. \u201eOvercrowding\u201d der dreifachgebundenen orthoKohlenstoffatome ist nicht klar erkenntlich. Die beobachteten Schwankungen der aromatischen Bindungslängen und winkel werden durch Hybridisierungsunterschiede der mit C oder H substituierten Ringkohlenstoffatome erklärt.
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(1970) Chemische Berichte. 103, 4, p. 1132-1156 Abstract
Die Kristallstrukturen von Hexamphenylen (1) und Pentamphenylen (2) wurden anhand dreidimensionaler (sin Θ/λ ≤0.66) Zählerdaten gelöst, die nach der DifferenzFilterMethode bei Raumtemperatur mit MoKαStrahlung auf einem SiemensDiffraktometer gemessen worden waren. Die Strukturen wurden mit einem BlockMatrixKleinsteQuadrateVerfahren zu R 0.047 (1) und 0.050 (2) verfeinert; die Standardabweichungen der Bindungslängen und winkel betragen 0.0024 Å bzw. 0.15°. Bei Annahme der Symmetrie m anstelle von 6 der Benzolkerne belaufen sich die Streuungen chemisch gleichwertiger Bindungen und Winkel auf 0.002 Å und 0.3°. Die Abweichungen der beobachteten Strukturen von den idealisierten Konformationen (1: ō m; 2 : 2) werden intermolekularen Packungseffekten zugeschrieben. Der \u201eovercrowding effect\u201d in beiden Molekeln (1 : C C 2.9582.968; HH 2.192.30 Å. 2: 2:8012.839; 1.842.19 Å) erzeugt eine Verdrillung von etwa 18° um die Achsen, die durch die Mittelpunkte der exocyclischen Bindungen gehen, und eine Deformation der Benzolkerne. Die Längen der exocyclischen Bindungen (1 : 1.487; 2 : 1.495 Å) werden diskutiert. Ein energetisch günstiger Inversionsmechanismus von 2 wird zur Erklärung der kernmagnetischen Äquivalenz der inneren Protonen vorgeschlagen.
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(1970) Journal of the Chemical Society B: Physical Organic. p. 497-504 Abstract
The electron-density sections in the planes of the benzene rings of penta-m-phenylene (PMP), hexa-m-phenylene (HMP), and 1:2,5:6,9:10-tribenzocyclododeca-1,5,9-triene-3,7,11-triyne (TTT) have been computed and averaged as well as the sections parallel to the ring planes at a distance of 0.4 Å. The results demonstrate σ-density in the centres of the (aromatic) C-C bonds and π-density above the ring planes. Sections passing through the aromatic and exocyclic bonds of the three compounds have been averaged and show 'π' density maxima at approximately 0.5 Å from their bond centres. The averaged electron-density distribution in the -C≡C- triple bond of TTT contains a trough of -0.2 e/Å3 in the centre as well as 'π' peak density of 0.1 e/Å3 at 0.6 Å distance from the centre of the bond. The anhydrous form of acetylenedicarboxylic acid (ADA) also exhibits a trough in the centre of its -C≡C- triple bond.
1969
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(1969) Journal of the Chemical Society A: Inorganic, Physical, Theoretical. p. 2372-2382 Abstract
The ideal hydrogen-bonded contact of amide groups is postulated as a function of the geometry of the CONH2 bonds and of the lone pairs of the oxygen atoms. The observed packing arrangements of primary amides are analysed in terms of this ideal contact and of three elements of symmetry (glide plane; two-fold screw axis; translation) generating hydrogen-bonded networks from centrosymmetric amide pairs. The inter-relationship between molecular shape and conformation and packing modes is considered. The geometry of NH ⋯ O lonepair contacts and the NH ⋯ O distances in the observed packing types are also discussed.
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(1969) Journal of the Chemical Society A: Inorganic, Physical, Theoretical. p. 2367-2371 Abstract
The crystal structure of trans-β-2-furylacrylamide has been solved from partial three-dimensional photographically recorded data by means of SEARCH and refined with anisotropic temperature factors for carbon, oxygen, and nitrogen to R 0·07 and R (the weighted value) 0·02. Bond lengths and angles between the heavy atoms have been determined with an estimated standard deviation of 0·005 Å; and 0·3°. Variations in the C=C and C-O bond lengths of the furan ring are attributed to large thermal vibrations of three ring-atoms. The furan ring is planar within 0·006 Å deviations from planarity of the furylvinyl system are probably not significant. The dihedral angle between this system and the amide group is 3°40. The crystal structure consists of glide-plane related centrosymmetric amide pairs located at (0,0,0), (1/2,1/2,0), (0,1/2,1/2), and (1/2,0,1/2), linked by a second set of hydrogen bonds along the a direction.
1967
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(1967) Journal of the Chemical Society B: Physical Organic. p. 290-296 Abstract
The crystal and molecular structures of dimethyl trans,trans-muconate have been determined from three-dimensional counter data taken at room temperature with Cu Kα radiation and corrected for absorption. The structure was solved from a sharpened Patterson section and refined by a full-matrix programme with anisotropic temperature factors of the heavy atoms to r = 0.014 and R = 0.058. Secondary-extinction corrections reduced R to 0.046 but did not affect the atomic parameters. Analysis of the thermal vibrations suggests that the molecule vibrates as a rigid body. The bond lengths, before and after correction for thermal vibration, are: C-C 1.450, 1.454; C-C 1.334, 1.341; C-O 1.203, 1.218; COC-OCH3 1.338, 1.347; CO-CH3 1.445, 1.449 Å; the e.s.d.s. are 0.002 Å. The average length of the C-H bond is 0.96 Å. Analysis of the bond angles of the methoxycarbonyl group suggests that the axis of the methyl group makes an angle of 5° with the O-C vector; the effect is interpreted as evidence for a "bent" bond. The packing arrangement is discussed.
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(1967) Journal of the Chemical Society B: Physical Organic. p. 329-334 Abstract
Methods have been developed for growing thin polycrystalline films, and oriented polycrystalline films. Experiments with the thin films allow measurement of the absorption spectra of the N-salicylideneanilines well into the ultraviolet, and thus permit comparison of the effects of temperature change on the "thermochromic" band and on the ultraviolet bands. In two anils studied the ultraviolet absorption is practically independent of temperature; the intensity of the thermochromic band is much lower than those of the ultraviolet bands. In N-(5-methoxy-3-nitrosalicylidene)aniline the results suggest a temperature-dependent equilibrium between two absorbing species in the crystal. The experiments with oriented films and polarised light indicate that the thermochromic transition is near in-plane long-axis polarised in the isolated molecule.
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(1967) Journal of the Chemical Society B: Physical Organic. p. 297-304 Abstract
The crystal structure of trans,trans-sorbamide has been solved from 1380 three-dimensional counter data (Cu Kα radiation), corrected for absorption, and refined by a least-squares method with anisotropic temperature factors of all but the hydrogen atoms. The "final" agreement factors were r = 0.009, R = 0.046 and 0.045, including and excluding |Fc| > kFt, respectively. The e.s.d. of heavy-atom and hydrogen co-ordinates are estimated as 0.002 and 0.03 Å, respectively. A rigid-body analysis of the molecule has been carried out, and the bond lengths have been corrected accordingly. The molecular dimensions and the packing arrangement are discussed.
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(1967) Journal of the Chemical Society B: Physical Organic. p. 305-311 Abstract
The crystal and molecular structures of trans,trans-muconodinitrile have been solved from 623 three-dimensional intensities measured with Cu k α radiation on a G.E. goniostat, and corrected for absorption and secondary extinction; the structure was refined anisotropically in all atoms to r = 0.009, R = 0.039. The bond lengths, measured with an e.s.d. of 0.003 Å, are compared with corresponding values derived from previous analyses of other butadiene derivatives. The packing arrangement is discussed.
1966
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(1966) The Journal of physics and chemistry of solids. 27, 9, p. 1453-1457 Abstract
A technique for the toroidally symmetrical implosion of non-metallic solids has been developed, and satisfactory recovery of shocked materials achieved. A procedure to quench a heat-sensitive high-pressure phase produced by a pressure pulse has been demonstrated. New examples of shock-induced phase changes have been found in the reconstructive transitions litharge → massicot,β → α lead dioxide, and cadmium sulfide (wurtzite → zincblende via the sodium chloride structure). A shock-induced ionic displacement transition has been observed in calcite → aragonite. By analogy with static experiments involving shear it is concluded that both types of phase transitions obtained here by the converging shockwave technique take place by a shear mechanism.