Publications
2024
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(2024) Journal of Physical Chemistry C. 128, 37, p. 15443-15450 Abstract
In search of different chemical mechanisms for electro-freezing of supercooled water (SCW), we describe here the differences between the inert electrodes of Co and Ni and their ability to be converted into ice makers. The formation of crystalline domains by anodic electrochemical treatment of cobalt in 10 μM KOH solution triggers ice nucleation of supercooled water at ∼−3 °C without the application of voltage. X-ray diffraction and XPS demonstrate that these crystalline domains are composed of oxyhydroxide molecules. As predicted by their Moiré fringes, crystals of hexagonal ice grow epitaxially on top of these domains along their [002] direction. In contrast, Ni anodes under similar electrochemical treatment do not form crystalline domains of the oxyhydroxides and therefore do not yield ice-nucleating surfaces.
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2023
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(2023) Journal of the American Chemical Society. 145, 34, p. 18904-18911 Abstract
This work reports that the octahedral hydrated Al3+ and Mg2+ ions operate within electrolytic cells as kosmotropic (long-range order-making) \u201cice makers\u201d of supercooled water (SCW). 10-5 M solutions of hydrated Al3+ and Mg2+ ions each trigger, near the cathode (−20 ± 5 V), electro-freezing of SCW at −4 °C. The hydrated Al3+ ions do so with 100% efficiency, whereas the Mg2+ ions induce icing with 40% efficiency. In contrast, hydrated Na+ ions, under the same experimental conditions, do not induce icing differently than pure water. As such, our study shows that the role played by Al3+ and Mg2+ ions in water electro-freezing is impacted by two synchronous effects: (1) a geometric effect due to the octahedral packing of the coordinated water molecules around the metallic ions, and (2) the degree of polarization which these two ions induce and thereby acidify the coordinated water molecules, which in turn imparts them with an ice-like structure. Long-duration molecular dynamics (MD) simulations of the Al3+ and Mg2+ indeed reveal the formation of \u201cice-like\u201d hexagons in the vicinity of these ions. Furthermore, the MD shows that these hexagons and the electric fields of the coordinate water molecules give rise to ultimate icing. As such, the MD simulations provide a rational explanation for the order-making properties of these ions during electro-freezing.
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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) Angewandte Chemie - International Edition. 61, 49, e202213955. Abstract
Design of pyroelectric crystals decoupled from piezoelectricity is not only a topic of scientific curiosity but also demonstrates effects in principle that have the potential to be technologically advantageous. Here we report a new method for the design of such materials. Thus, the co-doping of centrosymmetric crystals with tailor-made guest molecules, as illustrated by the doping of α-glycine with different amino acids (Threonine, Alanine and Serine). The polarization of those crystals displays two distinct contributions, one arising from the difference in dipole moments between guest and host and the other from the displacement of host molecules from their symmetry-related positions. These contributions exhibit different temperature dependences and response to mechanical deformation. Thus, providing a proof of concept for the ability to design pyroelectric materials with reduced piezoelectric coefficient (d22) to a minimal value, below the resolution limit of the method (
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(2022) Accounts of Chemical Research. Abstract
ConspectusThe ability to control the icing temperature of supercooled water (SCW) is of supreme importance in subfields of pure and applied sciences. The ice freezing of SCW can be influenced heterogeneously by electric effects, a process known as electrofreezing. This effect was first discovered during the 19th century; however, its mechanism is still under debate. In this Account we demonstrate, by capitalizing on the properties of polar crystals, that heterogeneous electrofreezing of SCW is a chemical process influenced by an electric field and specific ions. Polar crystals possess a net dipole moment. In addition, they are pyroelectric, displaying short-lived surface charges at their hemihedral faces at the two poles of the crystals as a result of temperature changes. Accordingly, during cooling or heating, an electric field is created, which is negated by the attraction of compensating charges from the environment. This process had an impact in the following experiments. The icing temperatures of SCW within crevices of polar crystals are higher in comparison to icing temperatures within crevices of nonpolar analogs. The role played by the electric effect was extricated from other effects by the performance of icing experiments on the surfaces of pyroelectric quasi-amorphous SrTiO3. During those studies it was found that on positively charged surfaces the icing temperature of SCW is elevated, whereas on negatively charged surfaces it is reduced. Following investigations discovered that the icing temperature of SCW is impacted by an ionic current created within a hydrated layer on top of hydrophilic faces residing parallel to the polar axes of the crystals. In the absence of such current on analogous hydrophobic surfaces, the pyroelectric effect does not influence the icing temperature of SCW. Those results implied that electrofreezing of SCW is a process influenced by specific compensating ions attracted by the pyroelectric field from the aqueous solution. When freezing experiments are performed in an open atmosphere, bicarbonate and hydronium ions, created by the dissolution of atmospheric CO2 in water, influence the icing temperature. The bicarbonate ions, when attracted by positively charged pyroelectric surfaces, elevate the icing temperature, whereas their counterparts, hydronium ions, when attracted by the negatively charged surfaces reduce the icing temperature. Molecular dynamic simulations suggested that bicarbonate ions, concentrated within the near positively charged interfacial layer, self-assemble with water molecules to create stabilized slightly distorted "ice-like"hexagonal assemblies which mimic the hexagons of the crystals of ice. This occurs by replacing, within those ice-like hexagons, two hydrogen bonds of water by C-O bonds of the HCO3- ion. On the basis of these simulations, it was predicted and experimentally confirmed that other trigonal planar ions such as NO3-, guanidinium+, and the quasi-hexagonal biguanidinium+ ion elevate the icing temperature. These ions were coined as "ice makers". Other ions including hydronium, Cl-, and SO4-2 interfere with the formation of ice-like assemblies and operate as "ice breakers". The higher icing temperatures induced within the crevices of the hydrophobic polar crystals in comparison to the nonpolar analogs can be attributed to the proton ordering of the water molecules. In contrast, the icing temperatures on related hydrophilic surfaces are influenced both by compensating charges and by proton ordering.
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(2022) Crystal Growth and Design. 22, 1, p. 43-47 Abstract
Following our previous investigations on the electrofreezing mechanism of supercooled water on pyroelectric crystal surfaces, we discovered that electrofreezing is a process involving the attraction and arrangement of specific ionic charges by an electric field. We found two classes of ions: the trigonal planar ions that raise the icing temperature, or \u201cice-makers\u201d, and ions of different structures that reduce the icing temperature, or \u201cice-breakers\u201d. In the search for more efficient promoters for electrofreezing, we anticipated that molecules that have the propensity to self-assemble with water to form hexagonal clusters might be better ice nucleators. Through icing experiments performed directly on the hemihedral faces of pyroelectric crystals of LiTaO3, we found that ions of biguanide elevate the icing temperature of supercooled water when concentrated near the negatively charged crystals interfacial water layer, either upon cooling or upon heating. On the other hand, the analogous guanylurea ions, which presumably assume configurations with deviations from planarity, operate as \u201cice-breakers\u201d.
2020
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(2020) Materials. 13, 20, p. 1-6 4663. Abstract
Surface pyroelectricity and piezoelectricity induced by water incorporation during growth in α-glycine were investigated. Using the periodic temperature change technique, we have determined the thickness (~280 µm) of the near surface layer (NSL) and its pyroelectric coefficient (160 pC/(K × cm2) at 23◦C) independently. The thickness of NSL remains nearly constant till 60◦C and the pyroelectric effect vanishes abruptly by 70◦C. The piezoelectric effect, 0.1 pm/V at 23◦C measured with an interferometer, followed the same temperature dependence as the pyroelectric effect. Abrupt disappearance of both effects at 70◦C is irreversible and suggests that water incorporation to α-glycine forms a well defined near surface phase, which is different form α-glycine because it is polar but it too close to α-glycine to be distinguished by X-ray diffraction (XRD). The secondary pyroelectric effect was found to be
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(2020) Angewandte Chemie - International Edition. 59, 36, p. 15570-15574 Abstract
By performing icing experiments on hydrophilic and hydrophobic surfaces of pyroelectric amino acids and on the x-cut faces of LiTaO3, we discovered that the effect of electrofreezing of super cooled water is triggered by ions of carbonic acid. During the cooling of the hydrophilic pyroelectric crystals, a continuous water layer is created between the charged hemihedral faces, as confirmed by impedance measurements. As a result, a current of carbonic acid ions, produced by dissolved environmental CO2, flows through the wetted layer towards the hemihedral faces and elevates the icing temperature. This proposed mechanism is based on the following: (i) on hydrophilic surfaces, water with dissolved CO2(pH 4) freezes at higher temperatures than pure water of pH 7. (ii) In the absence of the ionic current, achieved by linking the two hemihedral faces of hydrophilic crystals by a conductive paint, water of the two pH levels freeze at the same temperature. (iii) On hydrophobic crystals with similar pyroelectric coefficients, where there is no continuous wetted layer, no electrofreezing effect is observed.
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(2020) Angewandte Chemie - International Edition. 59, 36, p. 15575-15579 Abstract
Electrofreezing experiments of super-cooled water (SCW) with different ions, performed directly on the charged hemihedral faces of pyroelectric LiTaO(3)and AgI crystals, in the presence and in the absence of pyroelectric charge are reported. It is demonstrated that bicarbonate (HCO3-) ions elevate the icing temperature near the positively charged faces. In contrast, the hydronium (H3O+) slightly reduces the icing temperature. Molecular dynamics simulations suggest that the hydrated trigonal planar HCO(3)(-)ions self-assemble with water molecules near the surface of the AgI crystal as clusters of slightly different configuration from those of the ice-like hexagons. These clusters, however, have a tendency to serve as embryonic nuclei for ice crystallization. Consequently, we predicted and experimentally confirmed that the trigonal planar ions of NO(3)(-)and guanidinium (Gdm(+)), at appropriate concentrations, elevate the icing temperature near the positive and negative charged surfaces, respectively. On the other hand, the Cl(-)and SO(4)(2-)ions of different configurations reduce the icing temperature.
2019
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(2019) Journal of Physical Chemistry C. 123, 50, p. 30443-30446 Abstract
Electrofreezing studies date back to the late 19th century. Since then, it has been intensively investigated, yet the mechanism of this phenomenon is still under dispute. In the presented work, we use a device composed of electrodes covered by a thin protective dielectric layer in order to generate a large electric field/surface charge and separating their effects from those of the electric current/electrochemical reactions. We demonstrate that a surface charge density of up to ∼75 nC/mm2 and an electric field of up to ∼1 × 108 V/m, the maximum attainable without water decomposition, do not have an effect on the freezing of supercooled water. These results prove that at supercooling smaller than 11°, even a very large electric field does not order water molecules into an ice-like configuration.
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(2019) Advanced Materials. 31, 44, 1904733. Abstract
Symmetry-imposed restrictions on the number of available pyroelectric and piezoelectric materials remain a major limitation as 22 out of 32 crystallographic material classes exhibit neither pyroelectricity nor piezoelectricity. Yet, by breaking the lattice symmetry it is possible to circumvent this limitation. Here, using a unique technique for measuring transient currents upon rapid heating, direct experimental evidence is provided that despite the fact that bulk SrTiO3 is not pyroelectric, the (100) surface of TiO2-terminated SrTiO3 is intrinsically pyroelectric at room temperature. The pyroelectric layer is found to be ≈1 nm thick and, surprisingly, its polarization is comparable with that of strongly polar materials such as BaTiO3. The pyroelectric effect can be tuned ON/OFF by the formation or removal of a nanometric SiO2 layer. Using density functional theory, the pyroelectricity is found to be a result of polar surface relaxation, which can be suppressed by varying the lattice symmetry breaking using a SiO2 capping layer. The observation of pyroelectricity emerging at the SrTiO3 surface also implies that it is intrinsically piezoelectric. These findings may pave the way for observing and tailoring piezo- and pyroelectricity in any material through appropriate breaking of symmetry at surfaces and artificial nanostructures such as heterointerfaces and superlattices.
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(2019) Organic & Biomolecular Chemistry. 17, 17, p. 4200-4203 Abstract
Chiral crystals of the only achiral proteinogenic a-amino acid, glycine induced the asymmetric autocatalysis with amplification of enantiomeric excess (ee). The P31 crystals of.-glycine, which display positive Cotton effect (CD) at around 215 nm, mediate the asymmetric autocatalysis to yield (R)-pyrimidyl alkanol with high ee. In contrast, the enantiomorphic P32 crystals, which display negative Cotton effect, afford (S)-alkanol after the significant amplification of ee by asymmetric autocatalysis.
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(2019) Chemical Science. 10, 20, p. 5246-5250 Abstract
Enantiospecific crystallization of the three amino acids asparagine (Asn), glutamic acid hydrochloride (Glu·HCl) and threonine (Thr), induced by ferromagnetic (FM) substrates, is reported. The FM substrates were prepared by evaporating nickel capped with a thin gold layer on standard silicon wafers. Magnets were positioned underneath the substrate with either their North (N) or South (S) poles pointing up. Asymmetric induction, controlled by the magnetic substrates, was demonstrated for the crystallization of the pure enantiomers and was then extended for the racemic mixtures of Asn and Glu·HCl. In the case of the solution of the pure enantiomers, the l enantiomer was crystallized preferentially at one pole of the magnet and the d enantiomer at the other. Consequently, the racemates of Asn and Glu·HCl undergo separation under the influence of the magnetic substrate. With Thr, however, despite the enantiospecific interactions of the pure enantiomers with the FM, no separation of the emerging crystals could be achieved with the racemates, although they crystallize as conglomerates, implying differences taking place in the crystallization step. The results reported here are not directly related to the magnetic field, but rather to the aligned spins within the ferromagnets. The findings provide a novel method for resolving enantiomers by crystallization and offer a new perspective for a possible role played by magnetic substrates regarding the origin of chirality in nature.
2018
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(2018) Angewandte Chemie - International Edition. 57, 24, p. 7076-7079 Abstract
The pyroelectricity of AgI crystals strongly affects the icing temperature of super-cooled water, as disentangled from that of epitaxy. This deduction was achieved by the design of polar crystalline ceramic pellets of AgI, with experimentally determined sense of polarity. These pellets are suitable for measuring both their pyroelectric properties as well as the icing temperature of super-cooled water, separately on each of the expressed Ag+ and I- hemihedral surfaces. The positive pyroelectric charge at the silver-enriched side elevates the icing temperature, whereas the negative charge at the iodide side decreases that temperature. Moreover, the effect of pyroelectric charge remains dominant despite the presence of contaminants on both the silver and the iodide-enriched surfaces. Consequently an electrochemical process for ice nucleation is suggested, which might be of relevance for understanding the role played by electric charges in heterogeneous icing processes in general.
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(2018) Accounts of Chemical Research. 51, 5, p. 1238-1248 Abstract
Crystals are physical arrays delineated by polar surfaces and often contain imperfections of a polar nature. Understanding the structure of such defects on the molecular level is of topical importance since they strongly affect the macroscopic properties of materials. Moreover, polar imperfections in crystals can be created intentionally and specifically designed by doping nonpolar crystals with \u201ctailor-made\u201d additives as dopants, since their incorporation generally takes place in a polar mode. Insertion of dopants also induces a polar deformation of neighboring host molecules, resulting in the creation of polar domains within the crystals. The contribution of the distorted host molecules to the polarity of such domains should be substantial, particularly in crystals composed of molecules with large dipole moments, such as the zwitterionic amino acids, which possess dipole moments as high as ∼14 D. Polar materials are pyroelectric, i.e., they generate surface charge as a result of temperature change. With the application of recent very sensitive instruments for measuring electric currents, coupled with theoretical computations, it has become possible to determine the structure of polar imperfections, including surfaces, at a molecular level. The detection of pyroelectricity requires attachment of electrodes, which might induce various artifacts and modify the surface of the crystal. Therefore, a new method for contactless pyroelectric measurement using X-ray photoelectron spectroscopy was developed and compared to the traditional periodic temperature change technique. Here we describe the molecular-level determination of the structure of imperfections of different natures in molecular crystals and how they affect the macroscopic properties of the crystals, with the following specific examples: (i) Experimental support for the nonclassical crystal growth mechanism as provided by the detection of pyroelectricity from near-surface solvated polar layers present at different faces of nonpolar amino acid crystals. (ii) Enantiomeric disorder in dl-alanine crystals disclosed by detection of anomalously strong pyroelectricity along their nonpolar directions. The presence of such disorder, which is not revealed by accurate diffraction techniques, explains the riddle of their needlelike morphology. (iii) The design of mixed polar crystals of l-asparagine·H2O/l-aspartic acid with controlled degrees of polarity, as determined by pyroelectricity and X-ray diffraction, and their use in mechanistic studies of electrofreezing of supercooled water. (iv) Pyroelectricity coupled with dispersion-corrected density functional theory calculations and molecular dynamics simulations as an analytical method for the molecular-level determination of the structure of polar domains created by doping of α-glycine crystals with different l-amino acids at concentrations below 0.5%. (v) Selective insertion of minute amounts of alcohols within the bulk of α-glycine crystals, elucidating their role as inducers of the metastable β-glycine polymorph. In conclusion, the various examples demonstrate that although these imperfections are present in minute amounts, they can be detected by the sensitive pyroelectric measurement, and by combining them with theoretical computations one can elucidate their diverse emerging functionalities.
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(2018) Angewandte Chemie - International Edition. 57, 18, p. 4965-4969 Abstract
Metastable polymorphs commonly emerge when the formation of the stable analogues is inhibited by using different solvents or auxiliaries. Herein, we report that when glycine is grown in aqueous solutions in the presence of low concentrations of different co-solvents, only alcohols and acetone, unlike water and acetic acid, are selectively incorporated in minute amounts within the bulk of the α-polymorph. These findings demonstrate that although water binds more strongly to the growing face of the crystal, alcohols and acetone are exclusively incorporated, and thus serve as efficient inhibitors of this polymorph, leading to the precipitation of the β-form. These solvents then create polar domains detectable by pyroelectric measurements and impedance spectroscopy. These results suggest that in the control of crystal polymorphism with co-solvents, one should consider also the different desolvation rates in addition to the energy of binding to the growing faces of the crystal.
2016
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(2016) Journal of the American Chemical Society. 138, 44, p. 14756-14763 Abstract
The riddle of anomalous polar behavior of the centrosymmetric crystal of α-glycine is resolved by the discovery of a polar, several hundred nanometer thick hydrated layer, created at the {010} faces during crystal growth. This layer was detected by two independent pyroelectric analytical methods: (i) periodic temperature change technique (Chynoweth) at ambient conditions and (ii) contactless X-ray photoelectron spectroscopy under ultrahigh vacuum. The total polarization of the surface layer is extremely large, yielding ≈1 μC·cm-2, and is preserved in ultrahigh vacuum, but disappears upon heating to 100 °C. Molecular dynamics simulations corroborate the formation of polar hydrated layers at the sub-microsecond time scale, however with a thickness of only several nanometers, not several hundred. This inconsistency might be reconciled by invoking a three-step nonclassical crystal growth mechanism comprising (i) docking of clusters from the supersaturated solution onto the evolving crystal, (ii) surface recognition and polar induction, and (iii) annealing and dehydration, followed by site-selective recrystallization.
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(2016) Nature Communications. 7, 13351. Abstract
Doping is a primary tool for the modification of the properties of materials. Occlusion of guest molecules in crystals generally reduces their symmetry by the creation of polar domains, which engender polarization and pyroelectricity in the doped crystals. Here we describe a molecular-level determination of the structure of such polar domains, as created by low dopant concentrations (
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(2016) Journal of Physical Chemistry Letters. 7, 1, p. 43-46 Abstract
Polar crystals, which display pyroelectricity, have a propensity to elevate, in a heterogeneous nucleation, without epitaxy, the freezing temperature of supercooled water (SCW). Upon cooling, such crystals accumulate an electric charge at their surfaces, which creates weak electric fields,
2015
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(2015) Angewandte Chemie (International ed. in English). 54, 46, p. 13566-13570 Abstract
Young's moduli of selected amino acid molecular crystals were studied both experimentally and computationally using nanoindentation and dispersion-corrected density functional theory. The Young modulus is found to be strongly facet-dependent, with some facets exhibiting exceptionally high values (as large as 44 GPa). The magnitude of Young's modulus is strongly correlated with the relative orientation between the underlying hydrogen-bonding network and the measured facet. Furthermore, we show computationally that the Young modulus can be as large as 70-90 GPa if facets perpendicular to the primary direction of the hydrogen-bonding network can be stabilized. This value is remarkably high for a molecular solid and suggests the design of hydrogen-bond networks as a route for rational design of ultra-stiff molecular solids.
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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) Crystal Growth & Design. 15, 5, p. 2445-2451 Abstract
As part of an ongoing program on the design of functional materials with a varying degree of polarity, we investigated the processes of conversion of a nonpolar host, l-asparagine monohydrate crystal (space group P212121), into a conglomerate of mixed polar sectors when grown in the presence of varying amounts of l-aspartic acid guest at the glass-aqueous/solution interface. The structure, composition, and the reduction of symmetry of the mixed crystals were confirmed with pyroelectric coefficient measurements, X-ray diffraction, and HPLC analysis and supported by atom-atom potential energy computations. The pyroelectricity measured at the (010) and (01¯0) faces imply the formation of hybrid crystals with top and bottom parts having opposite polarities. Pyroelectric coefficients measured at these two faces as a function of the occluded guest concentration increase linearly up to 8 wt %/wt of guest, followed by an enhancement of polarity at 8-12 wt %/wt of guest and subsequent reduction upon an increase up to 16 wt %/wt of guest. An interpretation of the magnitude of the pyroelectric effect on guest concentration is proposed.
2014
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(2014) Crystal Growth & Design. 14, 8, p. 3839-3848 Abstract
Display of pyroelectricity along nonpolar directions of crystals or from surfaces implies structural disorder or presence of polar surface layers. Nonpolar {210} faces of polar dl-alanine crystals display far larger pyroelectric effect than that at the polar {001} faces. Similarly, pyroelectricity is reported from {110} faces of centrosymmetric crystals of dl-aspartic acid. The origin of the disorder is due to an interchange of enantiomers at specific chiral crystal sites as supported by atom-atom potential energy computations and by pyroelectric effect observed on the nonpolar crystals of l-alanine intentionally doped with opposite enantiomers. These results should explain the riddle of the needle-like morphology of dl-alanine. The {100} faces of dl-serine and the {021} faces of dl-glutamic acid monohydrate crystals exhibit pyroelectricity due to surface wetting, whereas pyroelectricity originating from the {210} faces of enantiomerically doped l-alanine crystals could be deciphered as arising both from surface wetting and enantiomeric bulk disorder.
2013
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(2013) Angewandte Chemie - International Edition. 52, 25, p. 6513-6516 Abstract
Surface pyroelectricity: Centrosymmetric crystals of α-glycine display an anomalous quadrupole-like pyroelectric current. This observation implies the formation of water-glycine hybrid polar layers at the (010) faces of the α-glycine crystals (see picture).
2012
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(2012) Physical Chemistry Chemical Physics. 14, 31, p. 11185-11186 Abstract
Dr Williams (AIP Adv., 2012, 2, 010701) suggested that cleaning Teflon by high pressure oxygen plasma may have affected our result that Cu 2+ and Pd 2+ ions can be absorbed but not chemically reduced by a Teflon surface rubbed by PMMA (Phys. Chem. Chem. Phys., 2012, 14, 5551). In response, we show that this treatment does not affect the adsorption of Cu 2+ and Pd 2+. We reaffirm our statement that Cu 2+ and Pd 2+ ions can be adsorbed by a Teflon surface only after rubbing with the organic polymers, not before rubbing.
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(2012) Chemical Communications. 48, 48, p. 6031-6033 Abstract
Crystal specimens of the γ-polymorph of achiral glycine which crystallize in space groups P31 and P32 as determined by the anomalous X-ray scattering method are shown to be laevorotatory and dextrorotatory, respectively, as determined by optical rotation of the crystals.
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(2012) Physical Chemistry Chemical Physics. 14, 16, p. 5551-5557 Abstract
It has recently been reported that Teflon and polyethylene (PE) if rubbed by polymethylmethacrylate (PMMA) or Nylon as well as non-rubbed PMMA and Nylon induce "redox" reactions, including those of the reduction of Pd +2 and Cu +2 ions. On this basis, it was deduced that these dielectric materials may hold ≅10 13-10 14 of "hidden" electrons cm -2, a value at least three orders of magnitude higher than the charge that a dielectric surface can accumulate without being discharged in air. The "hidden" electrons were termed "cryptoelectrons". In variance to these reports, we offer here an alternative interpretation. Our model is supported by X-ray photoelectron spectroscopy, contact angle and vibrating electrode (modified Kelvin probe) measurements performed on representative examples. Rubbing of the polymers was found to transfer polymer fragments between the rubbed surfaces altering their physical properties. The transferred polymer fragments promote adsorption of Cu 2+ and Pd 2+ ions. It was found that Teflon and PE rubbed with PMMA and Nylon, and non-rubbed PMMA and non-rubbed Nylon do not induce "redox" reactions of Cu 2+ and Pd 2+ ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd 2+ to Pd 0 by electrons, as detected by catalytic activity of Pd 0 in a Cu-plating bath, can be alternatively explained by reduction of adsorbed Pd 2+ by the reducing agents of the bath itself. Based on these findings, we support the hypothesis that charging of dielectric polymers is due to ions or free radicals rather than electrons and there is no evidence to invoke a hypothesis of "cryptoelectrons".
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) Angewandte Chemie - International Edition. 50, 25, p. 5654-5657 Abstract
Rub-a-dub-dub: The hypothesis of contact electrification through the transfer of cryptoelectrons was tested by scrutinizing the evidence for the reduction of Pd2+ and Cu2+ by static charges on rubbed Teflon. X-ray photoelectron spectroscopy studies indicated that neither of these ions is reduced (see picture; black: Pd2+ adsorbed, red: Pd 0 adsorbed and then reduced by formadehyde, green: mixture of Pd 2+ and Pd0 arising from partial reduction after 2 h under the XPS probe) by the static charge. The presented alternative interpretation challenges the role of cryptoelectrons.
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(2011) Chemical Reviews. 111, 5, p. 3236-3267 Abstract
Some of the experiments that have demonstrated the feasibility of stochastic 'mirror-symmetry breaking' and efficient processes of amplification of chirality through the organization of molecules into periodic one-, two-, and three-dimensional supramolecular architectures are presented. Related total asymmetric transformations have been extended for the formation of chiral J-aggregates in the form of fiber-like associates in solution via self-assembly of achiral cyanine dyes or achiral diprotonated meso-tetraphenylsulfonato- porphyrins in aqueous solutions. Absolute asymmetric synthesis has been demonstrated for several photochemical reactions by applying the concept of crystal engineering. Diene molecules containing two different double bonds have been engineered to crystallize within appropriate enantiomorphous packing arrangements needed for the observation of such syntheses.
2010
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(2010) Science. 327, 5966, p. 672-675 Abstract
Although ice melts and water freezes under equilibrium conditions at 0°C, water can be supercooled under homogeneous conditions in a clean environment down to -40°C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO 3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at -11°C freeze immediately when this surface is heated to -8°C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.
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(2010) Origins of Life and Evolution of Biospheres. 40, 1, p. 51-63 Abstract
Thioesters of α-amino acids are considered as plausible monomers for the generation of the primeval peptides. DL-Leucine-thioethyl esters (LeuSEt), where the L-enantiomer was tagged with deuterium atoms, undergo polycondensation in water or in bicarbonate or imidazole buffer solutions to yield mainly heterochiral (atactic) peptides and diketopiperazine, as analyzed by MALDI-TOF and ESI mass-spectrometry. In variance, when polymerization of DL(d10)-Leu, first activated with N,N-carbonyldiimidazole, then initiated with ethanethiol or with DL(d3)-LeuSEt yielded a library of peptides up to 30 detectable residues where those of homochiral sequence (isotactic) are the dominant diastereoisomers. At these conditions, racemic β-sheets are formed and operate as stereoselective templates in the process of chain-elongation. Isotopic L:L(d10)-Leu co-peptides were obtained in the polymerization of L(d10)-Leu with L-LeuSEt. By contrast, mixtures of oligo-D-Leu and oligo-L(d10)-Leu were obtained in the polymerization of mixtures of D-LeuSEt with activated L(d10)-Leu. Isotactic co-peptides containing Leu and Val residues were formed in the polymerization of mixtures of activated DL(d8)-Val with DL(d3)-LeuSEt in water, implying that the racemic β-sheets exert regio-enantio-selection but not chemo-selection. A reaction pathway is suggested, where LeuSEt operates both as initiator of the reaction as well as a multimer.
2009
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(2009) Accounts of Chemical Research. 42, 8, p. 1128-1140 Abstract
The origin of life is a historical event that has left no relevant fossils; therefore, it is unrealistic to reconstruct the chronology of its occurrence. instead, by performing laboratory experiments under conditions that resemble the prebiotic world, one might validate feasible reaction pathways and reconstruct model systems of artificial life. Creating such life in a test tube should go a long way toward removing the shroud of mystery over how it began naturally. The riddle of the appearance of natural proteins and nucleic adds-that is, biopolymers wholly consisting of homochiral subunits (L-amino acids and D-Sugars, respectively)-from the unanimated racemic prebiotic world is still unsolved. There are two hypotheses concerning the sequence of their emergence: one maintains that long homochiral (isotactic) peptides must have been formed after the appearance of the first living systems, whereas the other presumes that such biopolymers preceded the primeval enzymes. The latter scenario necessitates, however, the operation of nonlinear synthetic routes, because the polymerization of racemates in ideal solutions yields chains composed of residues of either handedness. In this Account, we suggest applying lessons learned from crystal chemistry, in which molecules from isotropic media are converted into crystals with three-dimensional (3D) periodic order, to understand how the generation of homochiral peptides from racemic alpha-amino acids might be achieved, despite its seemingly overwhelming complexity. We describe systems that include the self-assembly of activated alpha-amino acids either in two-dimensional (2D) or in 3D crystals, followed by a partial lattice-controlled polymerization at the crystal-aqueous solution interface. We also discuss the polymerization of mixtures of activated hydrophobic racemic alpha-amino acids in aqueous solutions, as initiated by primary amines or thiols. The distribution of the diastereomeric oligopeptides was analyzed by matrix-a
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(2009) Journal of the American Chemical Society. 131, 7, p. 2588-2596 Abstract
Glycine nanocrystals, grown in aligned nanometer-scale cylindrical pores of nanoporous polystyrene-poly(dimethyl acrylamide) monoliths by evaporation of imbibed aqueous solutions, adopt preferred orientations with their fast-growth axes aligned parallel with the pore direction. X-ray diffraction analysis revealed the exclusive formation of the metastable beta-polymorph, with crystal size comparable with the 22 nm pore diameter, in contrast to the formation of alpha-glycine in the absence of nanoscale confinement. When grown from aqueous solutions alone, the nanocrystals were oriented with their [010] and [0 (1) over bar0] axes, the native fast growth directions of the (+) and (-) enantiomorphs of beta-glycine, respectively, aligned parallel with the pore direction. In contrast, crystallization in the presence of racemic mixtures of chiral auxiliaries known to inhibit growth along the [010] and [0 (1) over bar0] directions of the enantiomorphs produced beta-glycine nanocrystals with their [001] axes nearly parallel to the pore direction. Enantiopure auxiliaries that inhibit crystallization along the native fast growth direction of only one of the enantiomorphs allow the other enantiomorph to grow with the [010] axis parallel to the cylinder. Collectively, these results demonstrate that crystal growth occurs such that the fast-growing direction, which can be altered by adding chiral auxiliaries, is parallel to the pore direction. This behavior can be attributed to a competition between differently aligned crystals due to critical size effects, the minimization of the surface energy of specific crystal planes, and a more effective reduction of the excess free energy associated with supersaturated conditions when the crystal grows with its fast-growth axis unimpeded by pore walls. These observations suggest that the beta-glycine nanocrystals form by homogeneous nucleation, with minimal influence of the pore walls on orientation.
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2008
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(2008) Chemistry-A European Journal. 14, 35, p. 10999-11009 Abstract
As part of our program on biochirogenesis of homochiral peptides from racemic precursors, we report the feasibility of obtaining peptides with homochiral sequences composed of up to 25 residues of the same handedness in the polymerization of racemic valine or leucine N-carboxyanhydrides in aqueous solutions, as initiated by amines. The composition of the oligopeptides was determined by MALDITOF mass spectrometry, and the sequences of some of the heterochiral diastereoisomers were studied by MALDI-TOF MS/MS performed on samples in which the S enantiomers of the monomer were tagged with deuterium atoms. The process comprises several steps: 1) a Markov mechanism of asymmetric induction in the early stages of the polymerization yields libraries of racemic oligopeptides enriched with isotactic diastereoisomers, together with oligopeptide sequences containing enantiomeric blocks of homochiral residues; 2) the short peptides self-assemble into racemic colloidal architectures that serve as regio-enantioselective templates in the ensuing process of chain elongation; 3) homochiral residues of the amino acids located at the periphery of these colloidal aggregates exert efficient enantioselection, which results in the formation of long isotactic oligopeptides. The final diastereoisomeric distribution of the peptides depends upon the composition of the templates, which is determined by the concentration of the initiator. The racemic mixtures of isotactic peptides can be desymmetrized by using enantiopure methyl esters of α-amino acids as initiators.
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(2008) Journal of the American Chemical Society. 130, 27, p. 8651-8659 Abstract
As part of our studies on the biochirogenesis of peptides of homochiral sequence during early evolution, the formation of oligopeptides composed of 14-24 residues of the same handedness in the polymerization of DL-leucine (Leu), DL-phenylalanine (Phe), and DL-valine (Val) in aqueous solutions, by activation with N,N-carbonyldiimidazole and then initiation with a primary amine, in a one-pot reaction, was demonstrated by MALDI-TOF MS using deuterium enantio-labeled α-amino acids. The formation of long isotactic peptides is rationalized by the following steps occurring in tandem: (i) creation of a library of short diasteroisomeric oligopeptides containing isotactic peptides in excess in comparison to a binomial kinetics, as a result of an asymmetric induction exerted by the N-terminal residue of a given handedness; (ii) precipitation of the less soluble racemic isotactic penta- and hexapeptides in the form of β-sheets that are delineated by homochiral rims; (iii) regio-enantiospecific chain elongation occurring heterogeneously at the β-sheets/solution interface. Polymerization of L-Leu with L-isoleucine (Ile) or L-Phe with L-1N-Me-histidine yielded mixtures of copeptides containing both residues. In contrast, in the polymerization of the corresponding mixtures of L- + D-α-amino acids, the long oligopeptides were composed mainly from oligo-L-Leu and oligo-D-Ile in the first system and oligo-D-Phe in the second. Furthermore, in the polymerization of mixtures of hydrophobic racemic α-amino acids DL-Leu, DL-Val, and DL-Phe and with added racemic DL-alanine and DL-tyrosine, copeptides of homochiral sequences are most dominantly represented. Possible routes for a spontaneous "mirror-symmetry breaking" process of the racemic mixtures of homochiral peptides are presented.
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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) 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) 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) Origins of Life and Evolution of Biospheres. 37, 4-5, p. 371-377 Abstract
Some experimental aspects concerning either the enantio-enrichment of α-amino acids starting from non-racemic mixtures or the formation of homochiral oligopeptides and oligonucleotides starting from racemic precursors are described. In principle, it is possible that more than one of these processes had played a role in chirobiogenesis.
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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) Chemistry-A European Journal. 13, 36, p. 10140-10151 Abstract
As part of our program on the biochirogenesis of homochiral peptides, we report the formation of racemic parallel (p) β sheets composed of alternating R and S chains of up to 14-15 repeat units of the same handedness through the polymerisation of (R,S)-valine N-carboxyanhydride (NCA) crystals suspended in aqueous solutions of a primary amine as the initiator. The occurrence of such a lattice-controlled reaction accompanied by a reduction in volume implies the operation of a mechanism that differs from that of the common solid-state polymerisation in vinyl systems. The topotacticity of the reaction is explained through the operation of a multistep nonlinear process comprising lattice control coupled with an asymmetric induction in the formation of homochiral short peptides followed by their self-assembly into racemic p β sheets, which operate as efficient templates in the ensuing process of enantioselective chain elongation at the polymer/crystal interface. The composition of the diastereoisomeric libraries of oligopeptides was determined by MALDI-TOF and MALDI-TOF-TOF MS analyses of the products obtained from monomers enantioselectively labelled with deuterium. The structure of the p β sheets could be determined by initiating the polymerisation reaction with water-soluble esters of enantiopure α-amino acids or short peptides. The same reaction performed with the monomer crystals suspended in hexane yielded a complex mixture of diastereoisomeric oligopeptides, thus highlighting the indispensable role played by water in controlling the stereoselectivity of the reaction. By contrast, polymerisation of (R,S)-leucine NCA crystals, with a different packing arrangement that presumably does not endorse the formation of periodic peptide templates, yielded, both in aqueous and hexane suspensions, libraries of peptides dominated by heterochiral diastereoisomers.
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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.
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(2007) ANGEWANDTE CHEMIE-INTERNATIONAL EDITION. 46, 20, p. 3710-3713 Abstract
(Graph Presented) A pattern on the sheets: Long oligopeptides of homochiral sequence (see mass spectrum of enantioselectively deuterated compounds) were obtained in the polymerization of racemic N-carboxyanhydrides of valine (ValNCA) or leucine (LeuNCA) in aqueous solutions, through self-assembly into racemic antiparallel β-sheet templates.
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(2007) Chirality. 19, 8, p. 612-624 Abstract
As part of our program on the search of possible prebiotic routes for the formation of oligopeptides of homochiral sequence (isotactic) from racemic precursors in aqueous environment, we report the polymerization of racemic crystals of phenylalanine N-carboxyanhydrides, enantioselectively tagged with five deuterium atoms, suspended in water containing various amine initiators. Racemic mixtures of isotactic oligopeptides, comprising up to 25 repeat units of the same handedness, as the dominant component for each length, were observed in a MALDI-TOF mass spectrometry analysis. The racemic mixtures of the peptides could be desymmetrized by initiating the polymerization reaction with water-soluble methyl esters of either enantiopure α-amino acids or dipeptides. A three-step mechanism is proposed to account for these results: (i) Surface recognition of the chiral initiator by the chiral sites present at specific faces of the crystal; (ii) Oligopeptide elongation at the polymer/crystal interface; and (iii) Self-assembly of the short isotactic peptides into racemic antiparallel β-sheets as templates followed by cross-enantiomeric impediment in the growth of enantiomeric chains at the peptide β-sheet/crystal interface.
2006
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(2006) Origins of Life and Evolution of the Biosphere. 36, 2, p. 151-170 Abstract
The present article challenges reports claiming to have demonstrated the Parity Violating Energetic Difference (PVED) between enantiomorphous D- and L-crystals. Apart from PVED, the presence of minute quantities and differing profiles of impurities incorporated during their different history of preparation will affect the physical properties of D- and L-crystals. These impurities are anticipated to play a much greater role in affecting crystallization behavior than PVED. The effect of impurities on the growth and dissolution of enantiomorphous crystals is illustrated with some representative examples. Shinitzky et al. (2002) reported recently dramatic differences in the growth and dissolution properties of the D- and L-crystals of tyrosine. We have repeated these experiments using commercial samples from different sources and employing a validated enantioselective gas chromatographic technique. We attribute Shinitzky's findings either to the use of inappropriate analytical techniques for the determination of enantiomeric composition and/or to the presence of unidentified contaminants in the commercial tyrosine samples. Related caveats hold also for the recently published claims by Shinitzky (2006) and Scolnik et al. (2006) to have observed experimentally PVED between enantiomeric helices of poly-glutamic acid composed of 24 repeating units.
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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) Chemical Communications. 43, p. 5432-5434 Abstract
Nε-stearoyl-lysine-ethyl-ester (C18-OE-Lys) operates as an efficient desymmetrizing agent for the generation of homochiral oligopeptides via a reaction catalyzed by silver ions in two-dimensional (2D) quasi-racemic crystallites of the corresponding thio-ester (C 18-TE-Lys) self-assembled on water.
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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) Chemistry-A European Journal. 11, 10, p. 3039-3048 Abstract
The formation of diastereoisomeric libraries of oligopeptides through the heterogeneous polymerization of racemic crystals of phenylalanine N-carboxyanhydride (PheNCA) is reported. The diastereoisomeric compositions of the oligopeptides formed on polymerization of (R,S) crystals incorporating the deuterium-tagged S enantiomer were determined by MALDI-TOF mass spectrometry. The racemic mixtures of the oligopeptides longer than pentamers are represented primarily by diastereoisomers of homochiral sequence and with peptides containing only one heterochiral repeating unit. A mechanism comprising the following three sequential steps to account for this unusual observation is proposed: 1) formation of dimers and trimers at a partially damaged liquid/ solid interface, 2) chain propagation that takes place within the bulk of the crystal through a lattice-controlled "zipper-like" mechanism between homochiral molecules arranged in a head-to-tail motif to yield crystalline antiparallel β-sheets of alternating oligopeptide chains of homochiral sequence of opposite handedness, and 3) enantiomeric cross-inhibition that results in chain termination. Induced desymmetrization of the racemic mixtures of the formed peptides was achieved by the polymerization of the mixed quasi-racemic crystals of (R)-PheNCA, ((S)-PheNCA), and (S)-ThieNCA (3-(2-thienyl)-alanine N-carboxyanhydride) of various compositions. These experiments resulted in the formation of non-racemic libraries of oligopeptides composed of homochiral chains of (R)-Phe and copolymers of randomly distributed (S)-Phe and (S)-Thie sequences. From these findings, we propose a stochastic model for the generation of libraries of nonracemic mixtures of oligopeptides from the polymerization of host (R,S)-PheNCA with racemic mixtures of other guest NCA amino acids dissolved in limited quantities in the crystal.
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) Journal Of Physical Chemistry B. 107, 18, p. 4245-4252 Abstract
CdS quantum particles (QPs) assembled at predetermined distances from a gold substrate are prepared within a Langmuir-Blodgett film that forms an organic host matrix. The system is characterized by controlled surface charging (CSC) in X-ray photoelectron spectroscopy (XPS) and complementary methods, successfully resolving the discrete QP layers. A quantitative study of substrate-QPs charge-transfer channels is provided by laser-intensity dependent contact potential difference (CPD) measurements. The extracted electron-transfer rate constants exhibit marked differences in electron transfer from the film toward the substrate versus the backward process. The charging of the hybrid film was found to be either negative or positive depending on the intensity of the laser that photoexcites the QPs.
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(2003) Angewandte Chemie : international edition. 42, 19, p. 2157-2161 Abstract
Reactions within crystals: Homochiral oligo(phenylalanine)s (see MALDI-TOF mass spectrum) were obtained from a racemic monomer by utilizing a "zipper-like" mechanism in the solid state (see packing diagram). By generating such species within crystalline architectures, the formation of oligopeptides of up to 17 units in length is plausible.
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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, 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.
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(2003) Helvetica Chimica Acta. 86, 11, p. 3851-3866 Abstract
Racemic S-ethyl thioesters of Nε-stearoyllysine (= S-ethyl (R,S)-2-amino-6-(stearoylamino)hexanethioate) and S-ethyl thioesters of γ-stearyl glutamic acid (=stearyl (R,S)-4-amino-5-(ethylsulfanyl)-5-oxopentanoate) self-assemble as separated two-dimensional crystalline monolayers within an achiral phospholipid environment of racemic 1,2-dipalmitoylglycerol (DPG) and 1,2-dipalmitoylglycero-3-phosphoethanolamine (DPPE), as demonstrated by grazing-incidence X-ray-diffraction (GIXD) measurements performed on the surface of H2O. Lattice-controlled polycondensation within these crystallites with deuterium-enantiolabeled monomers was initiated by injecting aqueous solutions of Ag+ or I2/KI beneath the monolayers, which yielded mixtures of diastereoisomeric oligopeptides containing up to six to eight repeating units, as analyzed by MALDI-TOF mass spectrometry. Analysis of the diastereoisomeric distribution showed an enhanced relative abundance of the oligopeptides with homochiral sequences containing three or more repeating units. Within the DPPE monolayers, the nucleophilic amino group of the phospholipid operates as an initiator of polymerization at the periphery of the monomer two-dimensional crystallites. Enhanced relative abundance of enantiomerically enriched homochiral oligopeptides was obtained by the polycondensation of nonracemic monomers. This enhancement indicated a phase separation into racemic and enantiomorphous monomer crystallites within the phospholipid environment, although this separation could not be observed directly by GIXD. A possible role that might have been played by crystalline assemblies for the abiotic generation and amplification of oligopeptides with homochiral sequences is discussed.
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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.
2002
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(2002) Advanced Functional Materials. 12, 10, p. 713-718 Abstract
Elliptically shaped (Pb1-xCdx)S nanoparticles (NPs) of average size 2.3 × 2.9 nm (minor axis × major axis) have been prepared via reaction of a solid [oligo(p-phenylene-ethynylene) dicarboxylate]Pb0.9Cd0.1 salt matrix, with gaseous H2S. A significantly long emission lifetime, with multi-exponential behavior, is detected in time-resolved photoluminescence measurements, substantially different from the decay patterns of pure PbS and CdS NPs within the same organic matrix. Evidence for the co-existence of Cd and Pb within the same particle is provided by light-induced X-ray photoelectron spectroscopy.
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(2002) Journal Of Physical Chemistry B. 106, 35, p. 9070-9078 Abstract
We investigated the electronic properties of hybrid inorganic/organic thin films where the inorganic components are monodisperse CdS quantum size particles (diameters 2.5 and 5 nm). For this purpose, low-energy photoelectron spectroscopy was applied in which the energy of photoelectrons, ejected from the metal substrate or from the films themselves, is measured. The electronic properties of hybrid Langmuir-Blodgett films were studied, when the films were deposited on gold or silicon surfaces and contained particles of different sizes arranged in layers at different distances from the substrate. For comparison films containing only the organic matrix were examined, so the effect of the quantum particles (QPs) on the electronic properties of the whole film could be resolved. The present study proves the importance of the organization of hybrid structure on its electronic properties. It is clearly demonstrated that the QPs cannot be considered as insolated species and that their electronic properties are affected by the structure of the film as a whole and by the substrate. It is shown that the QP size affects both the photoelectron emission yield and the scattering efficiency of electrons from the QPs.
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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) 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) ChemPhysChem. 3, 4, p. 343-349 Abstract
The magneto-optical properties of chromophore/semiconductor nanocrystalline superstructures were examined using optically detected magnetic resonance (ODMR) and time-resolved photoluminescence (PL) spectroscopy. The samples consisted of CdS or PbS nanocrystals separated by conjugated organic chains, forming three-dimensional superstructures. The ODMR measurements revealed that the magneto-optical properties of the superstructures are mainly controlled by the individual characteristic of the nanocrystals. The ODMR spectra were compared with simulated curves generated by the diagonalization of a spin Hamiltonian, indicating the existence of the following luminescence events. a) Recombination between electron-hole pairs trapped at a stoichiometric defect (metal or sulfur vacancies) or oxygen adatom sites at the surface of the nanocrystals. These electron - hole pairs showed anisotropic g-factors and weak exchange interactions. b) Bound exciton emission, from strongly coupled electron-hole pairs trapped at intrinsic stoichiometric or structural defects at the core of the nanocrystals. The existence of the two overlapping luminescence events is further confirmed by the acceptance of biexponential PL decay processes.
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(2002) Chemistry-A European Journal. 8, 6, p. 1413-1423 Abstract
Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of π-conjugated, rodlike dicarboxylic acids are reported. The synthesis of the composites, either as three-dimensional crystals or as thin films at the air-water interface, comprises a two-step process: 1) The preparation of the Cd salts 6(Cd), 8(Cd) or Pb salts 6(Pb), 8(Pb) of the oligo(p-phenyleneethynylene)dicarboxylic acids 6(H), 8(H), in which the metal ions are arranged in ribbons and are separated by the long axis of the organic molecules, as demonstrated by X-ray powder diffraction analysis of the solids and grazing incidence X-ray diffraction analysis of the films on water. 2) Topotactic solid/gas reaction of these salts with H2S to convert the metal ions into Q-particles of CdS or PbS embedded in the organic matrix that consists of the acids 6 (H) and 8 (H). These hybrid materials have been characterized by X-ray photoelectron spectroscopy and transmission electron microscopy.
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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) Journal Of Physical Chemistry B. 105, 29, p. 6792-6797 Abstract
The spectroscopic properties of new molecular-wire/nanocrystalline (NCs) superstructures have been examined. The molecular-wires consisted of (phenylene)n-(acetylene)n-1-dicarboxylate conjugated chains, with a length varying between 1.9 nm and 3.3 nm. The NCs component consisted of either PbS or CdS, with an average diameter ranging between 2.0 and 5.0 nm. The photoluminescence (PL) spectra at 1.4 K, the PL-excitation, and the time-resolved PL measurements showed a typical quantum size effect of the individual NCs, and additional collective effects of the superstructure. Simulation of the luminescence decay processes suggested an energy transfer between the molecular wires and the NCs, by an exciton diffusion mechanism. Furthermore, direct excitation into the NCs band edge showed an unexpected red-shift, a consequence of a molecular wires-NCs electronic interaction.
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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) Advanced Materials. 13, 8, p. 584-587 Abstract
The electronic properties of hybrid Langmuir-Blodgett (LB) films deposited on gold surfaces containing CdS particles of different sizes arranged in layers within the cascade-like arhitectures were studied by applying low-energy photoelectron spectroscopy (LEPS). It was demonstrated that thio acids can be applied to prepare stable hybrid films.
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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) 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.
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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) 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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2000
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(2000) Journal Of Physical Chemistry B. 104, 36, p. 8631-8634 Abstract
We report on the direct observation of collective electronic properties in assembled CdS quantum particles (QPs) arranged in periodic layers. Within each layer the QPs are of the same average size, either 2.5 or 5 nm, and the layers are arranged in a cascade-like pattern. The electronic properties of the QPs were studied using a new method, the attenuated low energy photoelectron spectroscopy (A-LEPS), in which a `pump' laser excites the QPs and a `probe' laser ejects photoelectrons from the QPs and from the metal substrate. The A-LEPS method provides information about the populated electronic states of the QPs (including the splitting between the light/heavy hole and split-off bands) and how these states depend on the interparticle interactions.
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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.
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(2000) Advanced Materials. 12, 4, p. 302-306 Abstract
CdS and Cd1-xMnxS nanoparticles arranged in patterns within acrylamide were prepared via the topotactic reaction of the corresponding metal thioalkanoates with gaseous ammonia. The organization of the particles was improved in some systems and induced in others in the presence of alkanoates that act as site-directing nucleating centers.
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(2000) Tetrahedron. 56, 36, p. 6645-6649 Abstract
An industrially feasible kinetic process for the conversion of D,L racemic mixtures that crystallize in the form of conglomerates into optically pure materials was elaborated. The method takes into consideration the presence of a single-enantiomer polymer that causes crystals that match it in chirality to lag far behind their enantiomers both in growth and in dissolution. This phase lag is exploited in a repeated cycle of growth and dissolution to collect crystals of one kind after partial growth and of the other before complete dissolution. Three major steps are involved: (a) Kinetically controlled crystallization of a racemic conglomerate in the presence of chiral polymers at temperature T1, modeled on the basis of the structure and morphology of the 3D crystals. The polymer inhibits the precipitation of the undesired enantiomorph, collecting the desired one by filtration; (b) Preferential dissolution of added racemic mixture of crystals of the substrate in the mother liquor enriched with the undesired enantiomer and the chiral polymer at temperature T2. This step takes advantage of concentration-gradients of the two enantiomers in the filtrate and the enatioselective dissolution induced by the chiral polymer; (c) The separation by filtration followed by racemization of the undesired enantiomer for additional cycles of resolution. The process is illustrated for the conversion of D,L methionine·HCl that crystallizes in the form of a conglomerate that displays twinning of enantiomorphous lamellae into the corresponding L or D Met·HCl in kilograms scale. (C) 2000 Elsevier Science Ltd.
1999
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(1999) Journal of the American Chemical Society. 121, 41, p. 9589-9598 Abstract
A method is described for the preparation of hybrid organic/inorganic structures where the inorganic component comprises semiconductor nanoparticles aligned in periodic layers within three-dimensional (3-D) crystalline powders and Langmuir-Blodgett (LB) films. The preparation process comprises the organization of metal ions in the form of periodic arrays within 3-D crystals or the LB films, followed by a topotactic gas/solid reaction. The method is illustrated for the organization of CdS nanoparticles within alkanoic acids. The order of the nanoparticles is achieved by introducing site directing nucleation centers of Cd thioalkanoates within Cd alkanoates, in the form of solid solutions. The formed particles are attached to the organic matrix via -C(O)S-Cd-S- bonds. The structure of those supramolecular architectures has been characterized by a variety of complementary methods, including transmission electron microscopy (TEM) and electron diffraction (ED), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and other spectroscopic measurements.
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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, 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) MRS Proceedings. 576, p. 159-164 Abstract
The preparation of lead sulfide and cadmium sulfide quantum size particles arranged in periodic layers within organic matrices is described. Superlattices of the particles have been generated in crystals or thin films of long chain amphiphilic acids by topotactic gas-solid reactions of the lead or cadmium salts, that pack in layer structures, with H2S gas. X-ray powder diffraction and transmission electron microscopy reveal that the crystalline order of the reactant has been partially retained in the organic-inorganic composite. This approach is demonstrated by examples of organic acids bearing an aromatic ring along the hydrocarbon chain, such as derivatives of phenyl propionic and benzoic acids.
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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
The structure of monolayers of the n-alkane C50H102 on water was studied. The orientational, conformational and packing properties of these molecules were explored on the water surface by reflection-absorption Fourier transform infrared spectroscopy (FT-IR) and grazing incidence X-ray diffraction (GIXD) directly on the water surface, as well as by cryo-TEM on vitreous ice and scanning force microscopy (SFM) of the films transferred onto mica support.
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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) 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.
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(1998) Advanced Materials. 10, 2, p. 121-125 Abstract
Quantum dots of CdS and PbS can be generated in thin films and 3D crystalline powders by taking advantage of the packing arrangement of metal alpha,omega-alkanedicarboxylates (see previous communication). The Figure is a transmission electron micrograph of the CdS particles formed in the 3D crystalline powder of cadmium C-12 dicarboxylate.
1997
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(1997) Science. 278, 5346, p. 2100-2102 Abstract
Catalytically active Langmuir-Blodgett films of a rhodium complex were prepared and characterized to determine the possible effect of the molecular order of metal complexes on catalytic activity. The hydrogenation of carbon- oxygen double bonds was used as a model reaction. The complex in solution exhibited low catalytic activity, whereas it was highly active in the film. The catalytic activity was found to be highly dependent on the orientation of the complex within the film. The reactions were also highly selective with regard to the substrate. These observations and the observed rate dependence on temperature strongly implicate the molecular order of a metal complex as an important dimension in catalysis.
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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) 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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(1997) Pure and Applied Chemistry. 69, 2, p. 255-256 Abstract
Biography: Meir Lahav studied at the Hebrew University of Jerusalem, and received the M.Sc. in 1963 in the field of organic polymers, and the Ph.D. in 1967 for studies of organic solid state reactions. After postdoctoral work with P.D. Bartlett at Harvard he joined the Weizmann Institute of Science in 1971, where he is currently Head of the Department of Materials and Interfaces, and Director of The Minerva G.M.J. Schmidt Interdisciplinary Center of Supramolecular Architectures. He is incumbent of the M. Thatcher chair of chemistry. He has been the recipient of the Kolthoff Prize of Chemistry from the Technion in Haifa and the Prelog Medal for Stereochemistry (with L. Leiserowitz) from the E.T.H. in Zurich, and was Centenary Lecturer of the Royal Society of Chemistry. He has research interests in the origin of chirality, stereochemistry, mechanistic study of solid state reactions, crystal engineering, thin organic films, and related fields.
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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) 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) 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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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]
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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.
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) 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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(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) 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) 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 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) 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) 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, 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.
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) 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) 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.
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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.
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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(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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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) 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.
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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.
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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(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) 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, 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.
1989
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(1989) Chemical Physics Letters. 164, 5, p. 539-544 Abstract
Monolayers of two nitroaniline-terminated amphiphilic compounds that differ from each other only by a methylene group in the hydrocarbon chain are studied at the air-water interface by optical second harmonic generation and ellipsometry. The nitroanilino groups exhibit significantly different ordering in the two monolayers at any surface density. This is the first clear demonstration that a small difference in the hydrocarbon chain length could lead to a large difference in the orientational ordering of a terminal group in an amphiphilic monolayer.
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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) 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) 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) 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
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) 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.
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(1987) Tetrahedron. 43, 7, p. 1281-1298 Abstract
A number of possible model systems for the spontaneous generation and amplification of net chirality in chemical systems by the process of crystallization are presented. In one of these a chiral resolved product (S') [or (R') generated from crystals of a multimolecular complex {Sn} or {Rn}, forms by ligand exchange diastereomeric complexes (Sn-1 S') and (Rn-1 S') which act as growth inhibitors of the parent conglomerate crystals, but at different extents. The Nickel complex of α-amino caprolactam (ACL) grown in ethanol, in basic conditions, has been investigated as an appropriate system for such a model. Kinetic, structural and morphological studies shed light on the mechanism of the asymmetric induction and demonstrated the feasibility of a complete cycle of generation and amplification of net chirality in this chemical system or others of the same type.
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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.
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) 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.
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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. 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) 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.
1985
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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, 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) 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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(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) 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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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. 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.
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.
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) 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) Journal of the American Chemical Society. 104, 17, p. 4610-4617 Abstract
A new general and efficient method for kinetic resolution of racemic conglomerates by crystallization in the presence of "tailor-made" additives is described. The process is explained in terms of stereoselective adsorption of the resolved additive at the surface of the growing crystals of the enantiomer of the same absolute configuration, resulting in a drastic decrease in their rate of growth and thus allowing preferential crystallization of the opposite enantiomer ("rule of reversal"). Some empirical r olutions reported in the literature are rationalized through this mechanism, and appropriate additives for the resolution of new systems are designed and successfully applied. The crystallization of the conglomerates (R,S)-glutarr1ic acid hydrochloride (GluHCl), (R,S)-threonine (Thr), (R,S)-(p-hydroxyphenyl)glycine p-toluenesulfonate (pHpgpTs)J and (R ,S )-aspa ragine hydrate (AsnH2O) in the presence of other amino acids, used as additives, has been studied in particular. It is demonstrated that the additives are occluded in the bulk of the homochiral crystal in typical amounts of 0.5-1.5%, while they are not found in the bulk of the crystals of the antipode. The possible role of the additives in nucleation and dissolution of the affected crystals is considered. A new method for the assignment of absolute configuration of chiral molecules is proposed.
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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, 12, p. 3422-3429 Abstract
An \u201cabsolute asymmetric synthesis\u201d with quantitative enantiomeric yield, via the process of crystallization of a nonchiral compound in a chiral crystal followed by a topochemical photoreaction, has been successfully executed. The needed crystalline chiral phases, composed of unsymmetrically disubstituted dienes and with the two different double bonds correctly juxtaposed for asymmetric (2π + 2π) photodimerization and photopolymerization along a translational axis, were designed. The starting point for this matrix engineering was the crystal structure of the chiral monomer 1. After inspection of the shortest contacts made by the chiral sec-butyl group inside the crystal, we examined some hypothetical transfers of methyls to and from this group, with the nearest neighboring molecules; such transfers might generate phases isomorphous to 1 but composed of achiral monomers. Four systems were considered promising candidates, namely the 1:1 mixture of monomers 3 and 4 and monomers 8, 9, and 11. Three of these behave in the predicted way. Large single crystals of achiral monomer 9 were grown and irradiated, yielding, in a number of independent experiments, dimers and oligomers of either chirality, with a quantitative enantiomeric yield within the limits of experimental error.
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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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(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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(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. 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 American Chemical Society. 104, 12, p. 3429-3434 Abstract
Monomeric dienes 17, which pack in chiral crystal structures and yield upon irradiation chiral cyclobutane dimers, trimers, and oligomers, have been crystallized in the presence of small amounts of their photoproducts (\u201cimpurities\u201d). An efficient asymmetric induction was observed in these crystallizations; the absolute configuration of the phase precipitating in excess is always found to be opposite to that of the crystalline phase from which the impurity was generated. A mechanism is proposed in which small amounts (∼1%) of the chiral additives are stereospecifically adsorbed onto the growing sites of the stereochemically similar crystal, thereby delaying its growth. Experiments yielding evidence in support of this mechanism are presented. Possible applications of the results of this study to the optical resolution of conglomerates in general in the presence of \u201ctailor-made\u201d impurities are discussed.
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1981
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(1981) Origins of Life. 11, 1-2, p. 107-118 Abstract
Experiments are described in which enantiomerically pure dimers, trimers and oligomers, generated from non chiral monomers by crystallization and topochemical reaction in suitably designed chiral crystals, induce preferential crystallization of the sarent monomer in the enantiomorphous chiral phase of opposite absolutute configuration. (We coin the term "inversion rule" for this effect). A general mechanism of amplification of optical activity by crystallization is proposed on the basis of these results, involving selective adsorption of resolved impurities on the surface of chiral crystals of similar stereochemistry, resulting in a decrease of the growth rate of the affected enantiomer and consequent preferential crystallization of the antipode. The implications of this mechanism to the generation and amplification of chiralily in a closed system are discussed.
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(1981) Macromolecular Chemistry And Physics-Makromolekulare Chemie. 4, S19811, p. 37-46 Abstract
A model for the generation and amplification of optical activity, involving crystallization of achiral monomers in chiral crystals combined with topochemical polymerization, is described. Taking into consideration some empirical rules of crystalpacking, seven achiral or chiral racemic monomers were selected as reactants for these studies, which were indeed found to pack in chiral crystals with the motif suitable for a topochemical solidstate polymerization with quantitative enantiomeric yield. The chiral products (dimers, trimers, and oligomers), obtained in a first successful generation experiment, proved to be powerful asymmetry inducing agents in a fresh crystallization of the starting materials. From the totally consistent results of a large number of experiments, the following \u201cinversion rule\u201d was derived: dimers, trimers, and oligomers generated in a \u201cd\u201d crystal always induce preferential crystallization of the monomers in the enantiomorphic \u201cl\u201d phase and vice versa. A mechanism of preferential adsorption of the chiral impurity on that crystalline phase which is stereochemically similar to it is proposed, and evidence for the validity of this hypothesis is presented. Viewed in more general terms, the inversion rule can be extended to include conglomerates as well, and examples from the literature as well as new systems have been demonstrated to undergo resolution via this mechanism. Finally the implications of this mechanism for the possible resolution of racemic polymers will be discussed.
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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) 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) Accounts of Chemical Research. 12, 6, p. 191-197 Abstract
The possibility of utilizing the chirality of crystals to achieve asymmetric synthesis was considered as early as 1908,1 but successful approaches awaited the better understanding of, and experience with, organic reactions which proceed in the crystalline state. Processes in which the crystalline lattice determines the course of reaction have been termed \u201ctopochemically controlled reactions\u201d.
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(1979) Journal of the American Chemical Society. 101, 8, p. 2152-2156 Abstract
An \u201cabsolute\u201d asymmetric synthesis of chiral dimers and polymers of variable optical yield and an asymmetric synthesis with quantitative enantiomeric yield starting from a monomer of low optical purity are described. Irradiation of polycrystalline ethyl 2-cyano-3-(p-sec-butyl-3-'(E)-propenoate)phenyl(E)-propenoate (1) of 60% optical purity or above leads to the formation of chiral dimers and oligomers with quantitative enantiomeric yield. Carefully crystallized samples of 1 of 22-60% optical purity lead to enantiomeric excesses of 90% or above. Racemic 1 crystallizes from the melt either in the form of a stable eutectic mixture or in the form of an isomorphous, chiral, metastable phase of different microcomposition. A method for the characterization of the two phases, based on photodimerization and NMR analysis of the diastereomeric dimers, is described. Irradiation of samples of the chiral metastable phase with one enantiomorphous crystal form in excess leads to an \u201cabsolute\u201d asymmetric synthesis with variable optical yield, of both signs. Possible routes to the achievement of an \u201cabsolute\u201d asymmetric synthesis with quantitative enantiomeric yield, in systems related to 1, are discussed.
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(1979) Pure and Applied Chemistry. 51, 6, p. 1269-1284 Abstract
An \u201cabsolute\u201d asymmetric synthesis of chiral dimers and polymers has been accomplished through a process of crystallisation of an achiral monomer in a chiral crystal, followed by a topochemical photopolymerisation. Once (2π+2π) photocycloadditon had been selected as a suitable reaction, we designed the chiral crystalline motifs needed for a reaction with quantitative enantiomeric yield. Unsymmetrically disubstituted dienes, translationally related in a chiral crystal, and where reaction occurs between non-equivalent double bonds, separated by 4.0A, fit the suggested model best. The execution of the synthesis involved the following steps. By applying some empirical rules of \u201ccrystal engineering\u201d, we selected a family of chiral resolved molecules (which guarantee a chiral crystalline motif), among which R(-) or S(+) ethyl-2-cyano-3(p-sec-butyl-3'-E-propenoate)-phenyl-E-propenoate-1 was found to fulfil all the stated requirements, yielding photodimers and polymers with quantitative diasterioisomeric yield. Finally, on the basis of the crystal structure of 1 we have designed at least two achiral molecules which pack in chiral structural motifs isomorphic with 1. Irradiation of crystals grown from the melt, led to polymers with high, probably quantitative optical yields. A possible model for an amplification of the generated chirality will be discussed.
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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, 9, p. 2838-2844 Abstract
The first solid-state asymmetric synthesis with quantitative diastereoisomeric yield, and with the asymmetric induction being entirely due to the chiral crystalline environment, has been designed and performed. Irradiation of crystalline enantiomerically pure (S)-(+)- or (R)-()-ethyl 2-cyano-3-(p-sec-butyl-3'-(E)-propenoate)phenyl-(E)-propenoate (1) at 5 °C leads to the formation of chiral cyclobutane dimers, trimers, and oligomers in quantitative (>97%) diastereoisomeric yield, as established by 1H and 13C NMR spectroscopy. The absolute configuration of the products follows from the crystal structure of the reacting enantiomeric crystal. On the other hand, the corresponding enantiomerically pure 2, in its β crystal modification, yields on irradiation chiral dimers with no detectable asymmetric induction; this is interpreted as being due to the quasiracemic space group of the reactant crystal.
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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.
1976
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(1976) Molecular Crystals And Liquid Crystals. 32, 1, p. 137-141 Abstract
In recent years there has been a growing interest in asymmetric synthesis via topochemically-controlled reactions in chiral crystak2 In such reactions the sole chiral influence derives from the asymmetric environment of the molecules in 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.
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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.
1975
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(1975) Journal of Molecular Evolution. 6, 2, p. 99-115 Abstract
Many molecules which are achiral can crystallize in chiral (enantiomorphic) crystals and, under suitable conditions, crystals of only one chirality may be obtained. The formation of right- or left- handed crystals in excess is equally probable. Lattice-controlled (topochemical) photochemical or thermal solid-state reactions may then afford stable, optically active products. In the presence of the chiral products, achiral reactants may preferentially produce crystals of one chirality, leading to a feedback mechanism for the generation and amplification of optical activity. Amplification of optical activity can also be achieved by solid-state reactions. The optical synthesis of biologically relevant compounds by such routes may be envisaged.
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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.
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(1975) Journal of the Chemical Society, Chemical Communications. 12, p. 471-473 Abstract
The first synthesis of a chiral polymer by a topochemical reaction in a one-component chiral crystal, and the first asymmetric synthesis by reaction in a crystal of racemic composition are reported.
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(1975) Journal of the Chemical Society, Chemical Communications. 12, p. 491-492 Abstract
The stereocourse of the reaction of solid dihalogenoadipodinitriles with gaseous ammonia has been investigated in terms of the conformation of the reacting molecules in the crystal; under the heterogeneous conditions of the reaction, in certain systems, an equilibrium between different rotamers occurs before an anti-type elimination takes place.
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REACTIONS IN CHIRAL CRYSTALS. PRINCIPLES GOVERNING ASYMMETRIC SYNTHESIS VIA TOPOCHEMICALLY CONTROLLED SOLID-STATE PHOTODIMERIZATION.(1975) Molecular Crystals And Liquid Crystals. 29, 2, p. 187-200 Abstract
The chemical and crystallographic requirements for utilizing solid-state photodimerization reactions in the medium of chiral crystals to realize asymmetric syntheses have been considered. Asymmetric syntheses are possible from a variety of reactions in the monocomponent or two-component crystals which have been considered and in favorable cases quantitative optical yields are anticipated. These principles can be extended to other reactions including the formation of chiral cyclobutane photopolymers.
1974
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(1974) Journal of the Chemical Society, Perkin Transactions 2. 4, p. 428-432 Abstract
Solid dimethyl and dibenzyl esters of meso-β,β-dichloro- and meso-β,β-dibromo-adipates react quantitatively with gaseous dry ammonia, methylamine, and dimethylamine, yielding stereospecifically the corresponding diesters of trans,trans-hexa-2,4-dienedioic acid. When the same reactants were combined in ethanol or dimethylformamide solutions, a mixture of 2,4-trans,cis- and trans,trans-diesters and -diamides was obtained. Possible routes for the reaction are discussed.
1973
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(1973) Journal of the Chemical Society, Perkin Transactions 2. 8, p. 1095-1100 Abstract
The relative yields of the photodimers from mixed crystals of various acrylic acids and acrylamides have been determined as functions of the wavelength of the exciting light and of the composition of the mixed crystal. In one system the yields were also determined for various times of irradiation. The yield vs. composition data were compared with data calculated on the basis of various assumptions as to the structures of the mixed crystals and the mechanism of the reaction. The wavelength effect and the fair agreement of measured and calculated yields for different compositions show that the reaction involves one excited and one non-excited molecule, and occurs at or near the site of absorption of light. Crotonic acid and amide are considered to react only as ground state partners in the reaction.
1972
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(1972) Israel Journal of Chemistry. 10, 2, p. 585-599 Abstract
Irradiation (λ >300 nm) of eight twocomponent solid solutions of βarylacrylamides, prepared by melting and crystallising equimolar quantities of the amides trArCH:CHCONE where Ar=C6H5, pMeOC6H4, pClC6H4, pMeC6H4, and 2thienyl, which appeared in the α crystal form, gave three photoproducts for each system studied: two αtruxillic homodimers, derived from the respective monomers, and an αtruxillic heterodimer. No other isomers were found. Similarly, twocomponent solid solutions were prepared by melting and crystallising ringsubstituted cinnamic acids (trArCH:CHCO2H, where Ar=C6H5 (αform), pClC6H4(β), pMeC6H4(α), pMeOC6H4(γ), oClC6H4(β) and oCH3C6H4(γ)) and irradiated. A solid solution of two αtype acids (each photodimerising in the solid to αtruxillic acid) behaved like the amides, affording two αtruxillic homodimers and an αtruxillic heterodimer. A solid solution of an αtype acid with a βtype acid (photodimerising to βtruxinic acids) gave six photoproducts: three αtruxillic acid dimers and three βtruxinic acid dimers (i.e., four homodimers and two heterodimers). Irradiation of a solution of a γtype acid (nonphotodimerising in the solid state) with an αtype acid afforded αtruxillic dimers (two homodimers and one heterodimer). The above results are consistent with mutual miscibility of the two components which gives rise to mixed crystal formation and subsequent photobehaviour within each crystal which is characteristic of the particular lattice type. However, irradiation of the mixed crystals from pmethoxycinnamic acid (γtype) and pchlorocinnamic acid (βtype), which afford two αtruxillic dimers (methoxyhomodimer and methoxy, chloroheterodimer) and two βtruxinic dimers (chlorohomodimer and methoxy, chloroheterodimer) indicates that new phases, unobserved for the pure compounds, can also be generated. The new dimers were all characterised by the NMR and mass spectra of their dimethyl ester derivatives and, when possible, by conversion to known compounds.
1971
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(1971) Journal of the Chemical Society B: Physical Organic. p. 1552-1564 Abstract
Solid penta-1,3-diene-1-carboxylic acid (4), penta-1,3-diene-1-carboxamide (5), buta-1,3-diene-1,4-dicarbonitrile (6), styrylacrylic acid (10), its methyl ester (46), and amide (49) (all trans,trans-configurated), all photodimerise on irradiation (λ > 290 nm), to divinylcyclobutane (C4) derivatives. The structures of the fully characterised photoproducts from (4), (5), (6), and (49) and the light-stability of (N-phenyl)styrylacrylamide (53), are predictable from the known or postulated packing arrangements of their monomers. Cyclo-octa-1,5-trans,trans-dienes, although topochemically and symmetry-allowed from monomers which crystallise with parallel butadiene chains [(4), (5), (6), and possibly (10)], are not observed. Although the formation of C8-cyclodimers cannot be rigorously excluded our results neither support nor require their presence. The (all-ax)-cyclo-octa-1,5-cis,cis-diene (C8) derivatives formed during irradiation of (4), (5), and (10) are not primary photoproducts but arise from thermal (Cope) rearrangements of photochemically produced cis-1,2-divinylcyclobutanes.
1967
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(1967) Journal of the Chemical Society B: Physical Organic. p. 239-243 Abstract
β-2-Furylacrylic acid irradiated with λ > 290 mμ gives an oligomer of D.P. ∼30 as well as dimer identified chemically and by its n.m.r. spectrum as the furyl analogue of β-truxinic acid. β-2-Thienylacrylic acid is dimorphic: its (thermodynamically) stable modification polymerises while the metastable form gives the β-truxinic dimer and small amounts of oligomer. 5-Methyl-2-thienylacrylic acid yields oligomer. β-3-Pyridylacrylic acid dimerises to the β-truxinic analogue; β-3-indolylacrylic acid is light-stable. According to the u.v. spectra all oligomers contain partially reduced ring systems. The photo-behaviour of this series of compounds is interpreted in terms of the packing arrangement of the monomers and of the reactivity of the ring systems: the formation of β-truxinic dimers follows from the parallelism of monomers related by a 4 Å translation axis; polymerisation is initiated by the attack of photo-excited exocyclic 2 bonds on the lefthand of both sideC=C2 bonds on the righthand of side sign groups on sufficiently close double bonds of reactive (furan, thiophen) ring systems.
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(1967) Journal of the Chemical Society B: Physical Organic. p. 312-317 Abstract
trans,trans-Muconic acid, its monomethyl ester, and cis,cis-isomer, all of which crystallise in cells with shortest axes of 4 Å, react in the solid state on irradiation with λ > 290 mμ. At room temperature all give vinyl-substituted cyclobutanes of symmetry m, the configuration of which reproduces the geometry of the monomer molecules both in the attachment of the ring substituents and in the configuration of the vinyl side-chain. The all-trans-acid and its monomethyl ester give, in addition to dimers, various amounts of oligomers which, according to the spectroscopic evidence, are formed by both 1,4- and 1,2-addition of the butadiene chains. Dimethyl trans,trans-muconate, which does not crystallise in a 4 Å cell, is stable to λ > 290 mμ at 25°.