Publications

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.

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.

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.

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.

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.

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.

Heme detoxification is a critical step in the life cycle of malaria-causing 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.

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 beta-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 beta-hematin. A low-temperature XRPD study of beta-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 beta-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.

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.

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.

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC(12)H(24)SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-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 alpha-4T on the gold surface adopts a 2D crystal structure.

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. (C) 2007 Elsevier Ltd. All rights reserved.

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.

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.

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.

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.

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.

A grazing-incidence X-ray-diffraction (G1XD) study of the self-assembly, on water, of nonracemic gamma-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 alpha-amino acid amphiphile into racemic and enantiomorphous two-dimensional crystallites within the phospholipid domains. The packing arrangements of the two a-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 N-epsilon-stearoyllysine spontaneously segregated into two-dimensional enantiomorphous domains within the DPPE environment that induced a change in the till direction of the hydrocarbon chains of the alpha-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 alpha-amino acids on plain water or within phospholipid monolayers.

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)-N-epsilon-stearoyl-thio-lysine with similar to10% (R,S)-N-epsilon-stearoyl-lysine, and (R, S)-N-alpha-carboxyanhydride of N-epsilon-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-of-flight (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.

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 . H(2)O. 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.

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 Angstrom (2) ) and the attached hydrocarbon chain (20 Angstrom (2)). 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.

Identification of the phase transition (see Figure) that occurs at the early stages of the topotactic transformation of Langmuir-Blodgett films of Cd or Pb behenate and arachidate with H2S has led to clarification of the structures of the hybrid films, previously in question. The ordered phase is stabilized by the addition of thio-carboxylic acids to the system.

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.

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. Copyright (C) 2000 John Wiley & Sons, Ltd.

Insertion of the 115-bis(ethynylene)benzene unit as a rigid spacer into a linear alkyl chain, thus separating the two resulting stems by 9 Angstrom, 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 crystalline 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 Angstrom separation between alkyl chains, yields the conventional herringbone arrangement.

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.

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. Tn 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).

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).

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.

The structure of mixed Langmuir films of the normal alkane C(32)H(66) With different quantities of the normal alcohol C(31)H(63)OH Or the diol HO(CH(2))(30)OH 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 C(31)H(63)OH On the other hand, segregation of the two components was observed in the mixed systems of the alkane and HO(CH(2))(30)OH. 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.

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 beta-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. (C) 1998 Wiley-Liss, Inc.

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).

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 similar to 450 water molecules are necessary to form a stable ice cluster at the onset of induced ice nucleation at a temperature just below 0 degrees C.

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 deposition of quaterthiophene (S-4), quinquethiophene (S-5). and sexithiophene (S-6) from chloroform solutions on the 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 (GID) directly on the water surface. S-4 forms two polymorphic crystalline multilayers. in polymorph alpha, of structure very similar to that of the three-dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23 degrees from the normal to the water surface. In polymorph beta the long molecular axis is perpendicular to the water surface. S-5 self-ageregates at the water surface to form mixtures of monolayers and bilayers of the beta polymorph; S-6 forms primarily crystalline monolayers of both alpha and beta 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.

A correlation is presented between the crystalline structure of monolayers and multilayers of alpha,omega-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.

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.

Keywords: Chemistry, Inorganic & Nuclear; Chemistry, Physical; Crystallography

The induced formation of multilayer crystallites al 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 n-alkanes spread from solution onto the water interface are described and X-ray structural investigations discussed which indicate that the full crystal symmetry is present even in layers that are only a few unit cells thick.

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.

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]

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.

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 gamma-form of glycine (see Figure) from aqueous solutions. The ideas behind this work are presented, together with results.

The advent of well collimated, high intensity synchrotron 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, alpha-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 occurring 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 successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.

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.

The structure of an uncompressed monolayer of C31H63OH, which induces freezing of supercooled water drops at similar to-2 degrees C, was studied by grazing incidence X-ray diffraction in the temperature range +6 degrees 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 degrees C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Angstrom, 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 degrees C.

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 CaCI2 solutions at pH values 8.85 and 10, respectively, adjusted with ammonia, and subphase temperatures in the range 5 to 9-degrees-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.

The two-dimensional (2D) crystal structures of self-aggregated clusters of amphiphilic alcohols (tJ„tl2ji+iOH, n =23, 30, 31, and Ci9H39C02(CH2),,0H, 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 (C„H2n+iOH, C19H39C02C„H2n0H) are very similar. The molecules pack in a rectangular unit cell of average dimensions a =5.0
A, b - 7.5 A for the normal alcohols C„H2n+iOH and a - 5.7 A, b =7.5 A for C^f^CC^CnH^OH. The plane group symmetry is essentially p\g\ for the normal alcohols C„H2,+iOH and essentially pi lg 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 C„H2n+iOH 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 C„H2n+iOH permitted a reliable estimate of 0.07 A* 12 for the molecular mean-squared motion parallel to the water surface. The absolute orientations of the molecules Ci9H39C02CnH2«0H were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of Cigl^CChCnH^OH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.

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.

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.

Tailor-made molecular 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, 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.

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 C16H33OH 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 C31H63OH monolayer has also been demonstrated by ED.

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.

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-degrees-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-degrees along the short 5.53-angstrom axis of the unit cell and oriented so as to form the orthogonal O(perpendicular-to) 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.

The hydrophobic faces of single crystals of a series of pairs of racemic and chiral-resolved hydrophobic alpha-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-degrees to 5-degrees-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.

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 host-guest 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 "absolute" asymmetric synthesis inside centrosymmetric crystals.

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.

X-ray 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.

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.

Monolayers of aliphatic long-chain alcohols induced nudeation 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 (CMH2. + 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 dusters 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.

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.

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 '.

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.

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 (£)-cinnamamide (space group P2{/c) in the presence of (£)-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-H—O hydrogen bonds between host molecules by 0—0 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 (\R,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.

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.

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 Gin 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.

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 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 bonds. 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.

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.

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.

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 [CS, C6eq, C6.,], C6eq, and [C6eq, CS] 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 CS 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'=O' 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'=O' group and the potentially reactive steroid C-H bond are 3.5 A for Q, .. H 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°.

The crystalline host-guest channel inclusion complexes 5:2 (DCA) deoxycholic acid-acetophenone (C6H 5C'OCH 3) and 3:1 DCA-m-chloroacetophenone (CLC6H 4 C'OCH 3) each yield on UV irradiation a photoproduct via addition of guest to the steroid tertiary carbon atom CS with the formation of a new chiral carbon center C' (OH )(CH 3)( C 6H 5)(DC A) 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- C6H 5COC D•3 The inclusion compounds DCA¬ C6H5COCH3 and DCA-C LC6H 4COCH 3 each 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 CS. 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.

Retardation of growth of organic crystals in the presence of additives in solution has been investigated in terms of their crystal structures and the stereochemistry of the additives. A simple correlation has been established between affected directions of growth and structure of the additive. This mecnanism has been successfully applied for monitoring, in a controlled manner, the morphology of organic crystals; this is illustrated for benzamides and benzoic acids. Furthermore, the absolute configura- tion of polar crystals and chiral molecules has been directly assigned. This is illustrated for the polar crystal of tpcinnamoyl alanine, and for the molecule of threonine, through morphological changes this molecule induces in the centrosymmetric crystal of serine.

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.

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.

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).

A new chemical mass-spectrometric method for mapping short range order in crystalline solid-solutions 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 sec-butyl group. While the two polymorphs of 1 and polymorph α of 2 form random solid-solutions, 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.

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.

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 P[1 with combining macron], 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[double bond, length half m-dash]O 1·204, C–O 1·310 Å; the hydroxy-hydrogen was clearly evident. The molecules form a continuous array of O–H ⋯ O[double bond, length as m-dash] 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[triple bond, length half m-dash]C 1·182, 1·178, and C–CO2H 1·441, 1·441 Å.

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, C–OH 1·310, and C–NH2 1·323 Å.

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[triple bond, length half m-dash]C 1·175, 1·179; [triple bond, length half m-dash]C–C 1·455, 1·453; C–CO2H 1·484, 1·506; C[double bond, length as m-dash]O 1·221, 1·214; C–OH 1·307, 1·259 Å. The C(:O)·C·C⋮C system is syn-planar.

The molecules are hydrogen-bonded (O–H ⋯ 2·69 and 2·65 Å) in pseudo-centrosymmetric pairs. In the crystal there are no intermolecular ethynyl-H ⋯ O[double bond, length half m-dash]C interactions; rather, the acetylenic; C–H bond points at the C[triple bond, length half m-dash]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.

Die Kristallstruktur der Titelverbindung wurde mit Hilfe der SEARCH-Methode gelöst und aufgrund von 4050 (einschließlich 1484 „unbeobachteten”︁) Reflexen (sin Θ/λ ≤ 0.66) und 150 starken Reflexen im Bereich 0.66

Die beobachteten Schwankungen der aromatischen Bindungslängen und -winkel werden durch Hybridisierungsunterschiede der mit C oder H substituierten Ringkohlenstoffatome erklärt.

The crystal and molecular structures of trans,trans-muconodinitrile have been solved from 623 three-dimensional intensities measured with Cu kα 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.