Publications

Conspectus The 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.

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.

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.

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 “tailor-made” 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.

The riddle of anomalous polar behavior of the centrosymmetric crystal of alpha-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 approximate to 1 mu C.cm(-2), and is preserved in ultrahigh vacuum, but disappears upon heating to 100 degrees 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.

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.

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 congruent to 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 Cu2+ and Pd2+ 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 Cu2+ and Pd2+ ions but adsorb these ions on their surfaces. Furthermore, the earlier reported reduction of Pd2+ 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 Pd2+ 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''.

Although ice melts and water freezes under equilibrium conditions at 0 degrees C, water can be supercooled under homogeneous conditions in a clean environment down to -40 degrees 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 LiTaO(3) 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 LiTaO(3) surface and remaining liquid at -11 degrees C freeze immediately when this surface is heated to -8 degrees 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.

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 a-amino acids as "tailor-made" additives in centrosymmetric crystals of glycine and serine, for the absolute structure of polar crystals of sugars and alpha-amino acids and consequently the absolute configuration of molecules packed in such crystals.

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.

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 alpha-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 P-sheets as templates followed by cross-enantiomeric impediment in the growth of enantiomeric chains at the peptide beta-sheet/crystal interface.

As part of our program on the biochirogenesis of homochiral peptides. we report the formation of racemic parallel (p) beta 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 beta 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 beta sheets could be determined by initiating the polymerisation reaction with water-soluble esters of enantiopure alpha-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.

In a recent paper. Srinivasan and Sherwood reported that alpha-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 a-resorcinol in the vapor phase, a process that does not exclude the inhibition of crystal growth by solvent.

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-(2thienyl)-alanine N-carboxyanhydride) of various compositions. These experiments resulted in the formation of nonracemic 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.

N(epsilon)-stearoyl-lysine-ethyl-ester (C(18)-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.

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.

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 a-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 NE-stearoyl-lysine thioethyl ester (C-18-TE-Lys), gamma-stearyl-glutamic thioethyl ester (C-18-TE-Glu), N-alpha-carboxyanhydride of gamma-stearyl-glutamic acid (C-18-Glu NCA) and gamma-stearyl-glutamic thioacid (C-18-thio-Glu). According to insitu grazing incidence X-ray diffraction measurements on the water surface, (R,S)-C-18-TE-Lys, (RA-C-18-TE-Glu, and (R,S)-C-18-Glu-NCA amphiphiles self-assembled into ordered racemic 2D crystallites. Oligopeptides 2-12 units long were obtained at the air-aqueous solution interface after injection of appropriate catalysts into the water subphase. The experimental relative abundance of oligopeptides with homochiral sequence generated from (R,S)-C-18-TE-Lys and (R,S)-C-18-TE-Glu, as determined by mass spectrometry on enantioselectively deuterium-labeled samples, was found to be significantly larger than that obtained from (R,S) C-18-thio-Glu which polymerizes randomly. An efficient chiral amplification was obtained in the polymerization of nonracemic mixtures of C-18-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.

Reactions within crystals: Homochiral oligo(phenylalanine)s (see MALDI-TOF mass spectrum) were obtained from a racemic monomer by utilizing a “zipperlike” 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.

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 Angstrom, 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.

We investigated the electronic properties of hybrid inorganic/organic thin films where the inorganic components are monodisperse US 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.

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 (GIRD) 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 x 2 and 3 x 3 silver(I) grid-type complexes generated at the air-solution interface. (C) 2002 Elsevier Science B.V. All rights reserved.

Composite materials of quantum particles (Q-particles) arranged in layers within crystalline powders of pi-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-phenyteneethynytene)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.

The growth of a cholesterol crystalline phase, three molecular layers thick at the air-water interface, was monitored by grazing incidence x-ray diffraction and x-ray reflectivity. Upon compression, a cholesterol film transforms from a monolayer of trigonal symmetry and low crystallinity to a trilayer, composed of a highly crystalline bilayer in a rectangular lattice and a disordered top cholesterol layer. This system undergoes a phase transition into a crystalline trilayer incorporating ordered water between the hydroxyl groups of the top and middle sterol layers in an arrangement akin to the triclinic 3-D crystal structure of cholesterol . H(2)O. By comparison, the cholesterol derivative stigmasterol transforms, upon compression, directly into a crystalline trilayer in the rectangular lattice. These results may contribute to an understanding of the onset of cholesterol crystallization in pathological lipid deposits.

Cholesterol and some cholesterol-like derivatives such as stigma-sterol and cholesterol acetate self-assemble into semicrystalline monolayers, bilayers, or trilayers on the surface of water, as has been reported previously. Here we have extended our thin film studies toward cholesteryl esters that, in their bulk crystals, pack as interdigitated bilayers due to the mismatch between the cross-sectional area of the rigid cholesterol moiety (40 Angstrom (2) ) and the attached hydrocarbon chain (20 Angstrom (2)). As shown by grazing incidence X-ray diffraction (GIXD), cholesteryl esters spontaneously self-assemble on the surface of water in order to form interdigitated bilayer films. The unit cell parameters of such bilayers at the water surface almost match those of their three-dimensional counterparts. Other experiments such as surface pressure vs area per molecule isotherms and ellipsometry measurements corroborate this result. To control the growth of the interdigitated films, we have used "tailor-made" additives (long-chain alcohols or acids) that fill the voids in the chain region of the cholesteryl ester layer caused by the structural mismatch, thereby inhibiting interdigitation. Using this strategy, we were able to form a mixed monolayer composed of the ester and the additive molecules in a 1:1 ratio, where the hydrocarbon chain of additive molecule intercalates between the ester chains and effectively inhibits the growth of the interdigitated bilayer.

The electronic properties of hybrid Langmuir-Blodgett (LB) films containing CdS quantum particles (QP) of different size arranged in "cascade like" architectures on gold are studied. The energy distribution of photoelectrons ejected from the gold substrate and transmitted through the LB films is reported. The electronic properties were found to depend on the structure and the organization of the layers relative to the gold substrate, and on the interaction between the metal substrate and the film.

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.

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 TI, 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 T-2. 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. All rights reserved.

The absolute orientations of the amphiphilic molecules alpha-hydroxy omega-bromo alcohols BrC(n)H(2n) OH, n = 21, 22, and the alkyl hydroxy esters C(m)H(2m+1C)COO(CH(2))(n)OH, 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 CH(3)-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.

To provide more direct information on the role played by "tailor-made" auxiliary molecules in the early stages of crystal nucleation, the interplay between dusters of polar headgroups of monolayers of the copper complexes S-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-N(epsilon)-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) alpha-amino acid on an aqueous solution of copper acetate followed by injection of the water-soluble (S') or (R') alpha-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 hans and cis configuration, respectively, according to comparative X-ray photoelectron spectroscopy (XPS) studies with appropriate cis and trans three-dimensional (3-D) alpha-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 alpha-amino acids of the opposite handedness.

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.

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.

Oriented crystalline films, similar to 11 -20 Angstrom, thick, of metal ion complexes of the grid type [Co4L4](8+).8PF(6)(-) and [Ag9L6](9+).9CF(3)SO(3)(-), 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.

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.

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.

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.

The formation of Langmuir monolayers at the air-water interface has long been believed to be limited to amphiphilic molecules containing a hydrophobic chain and a hydrophilic headgroup. Here we report the formation of crystalline mono- and multilayer self-assemblies of oligothiophenes, a class of aromatic nonamphiphilic molecules, self-aggregated at the air-water interface. As model systems we have examined the deposition of quaterthiophene (S-4), quinquethiophene (S-5). and sexithiophene (S-6) from chloroform solutions on the water surface. The structures of the films were determined by surface pressure-area isotherms, by scanning force microscopy (SFM) after transfer of the films onto atomically smooth mica, by cryo-transmission electron microscopy (Cryo-TEM) on vitreous ice, and by grazing incidence synchrotron X-ray diffraction (GID) directly on the water surface. S-4 forms two polymorphic crystalline multilayers. in polymorph alpha, of structure very similar to that of the three-dimensional solid, the molecules are aligned with their long molecular axis tilted by about 23 degrees from the normal to the water surface. In polymorph beta the long molecular axis is perpendicular to the water surface. S-5 self-ageregates at the water surface to form mixtures of monolayers and bilayers of the beta polymorph; S-6 forms primarily crystalline monolayers of both alpha and beta forms. The crystalline assemblies preserve their integrity during transfer from the water surface onto solid supports, The relevance of the present results for the understanding of the early stages of crystal nucleation is presented.

Studies are presented on the two-dimensional (2-D) crystalline packing arrangements of enantiomerically pure and racemic ol-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 = CnH2n+1-, n = 10, 12, 16) due to the tendency for herringbone chain arrangements via glide symmetry or homochiral (for R = CnH2n+1CONH(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 alpha-glycine by these monolayers. The relevance of the present results to the possibility of ordering and spontaneous segregation of racemates of the natural hydrophobic alpha-amino acids at the air-solution interface is discussed.

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.

A systematic analysis of grazing incidence synchrotron X-ray diffraction data of uncompressed amphiphilic alcohols CnH2n+1OH (n = 31, 30, 23, 20, 19, 18, 16, 14, 13) on a water subphase at 5 degrees C is presented. Pronounced structural changes were observed on reduction of chain length from n = 31 to 13. The relative amount of two dimensional (2-D crystalline material formed fell drastically; shorter crystalline coherence lengths were also observed. For n = 31-18 the molecules are arranged in a rectangular cell (a approximate to 5 Angstrom, b increases from ca. 7.4 to ca. 8.2 Angstrom) with plane symmetry p1g1. For n

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.

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.

This chapter contains sections titled:IntroductionGeneration and Amplification of Chirality in the Solid StateSelf‐Aggregation of Amphiphilic Molecules at Air‐Solution InterfacesMolecular Recognition During Crystal NucleationChiral Amplification in The System Glycine‐Soluble CY‐Amino AcidsConclusion and Outlook

The advent of well collimated, high intensity synchrotron X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air-liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves and surface fluorescence techniques, and review recent results obtained for Langmuir films from their use. The methods have been successfully applied for the elucidation of the structure of crystalline aggregates of amphiphilic molecules at the water surface such as alcohols, carboxylic acids and their salts, alpha-amino acids and phospholipids. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, via the surface X-ray methods, new light is shed on the structure of the underlying attached solvent or solute ionic layer. Examples are given where singly or doubly charged ions bound to the two-dimensional (2D) crystal form either an ordered or diffuse counter-ionic layer. Finally, the surface diffraction methods provide data on transfer of structural information from 2D clusters to 3D single crystals which had been successfully accomplished by epitaxial-like crystallization both in organic and inorganic crystals.

The spontaneous formation and coexistence of crystalline polymorphic trilayer domains in amphiphilic films at air-liquid interfaces is demonstrated by grazing incidence synchrotron x-ray diffraction. These polymorphic crystallites may serve as models for the early stages of crystal nucleation and growth, helping to elucidate the manner in which additives influence the progress of crystal nucleation, growth, and polymorphism and suggesting ways of selectively generating and controlling multilayers on liquid surfaces. Auxiliary molecules have been designed to selectively inhibit development of the polymorphs, leading primarily to a single phase monolayer.

The structure of an uncompressed monolayer of C31H63OH, which induces freezing of supercooled water drops at similar to-2 degrees C, was studied by grazing incidence X-ray diffraction in the temperature range +6 degrees C to freezing of the water subphase. As the water was cooled, the hydrocarbon chains underwent a change in tilt angle. The monolayer preserved its structure on the epitaxially-grown surface of hexagonal ice (after freezing) and, after the ice was melted, subsequent to the temperature being raised above 0 degrees C. The coherence length of the observed ice crystallites parallel to the interface was about 25 Angstrom, which is in agreement with the extent of match between the lattice of the monolayer and the ab lattice of ice. This yields some information on the critical size of the ice nucleus bound to the monolayer, just below 0 degrees C.

The two-dimensional (2D) crystal structures of self-aggregated clusters of amphiphilic alcohols (tJ„tl2ji+iOH, n =23, 30, 31, and Ci9H39C02(CH2),,0H, n = 9, 10) on pure water at a temperature of 5 ®C have been determined to near-atomic resolution in order to understand the relative abilities of these monolayers to induce ice nucleation. The structures were solved making use of grazing incidence synchrotron X-ray diffraction (GID) data, complemented by
lattice energy calculations. The GID data of the different monolayers within each of the two series (C„H2n+iOH, C19H39C02C„H2n0H) are very similar. The molecules pack in a rectangular unit cell of average dimensions a =5.0
A, b - 7.5 A for the normal alcohols C„H2n+iOH and a - 5.7 A, b =7.5 A for C^f^CC^CnH^OH. The plane group symmetry is essentially p\g\ for the normal alcohols C„H2,+iOH and essentially pi lg for the other group. The molecular chains are tilted from the vertical by an average angle of 9°, in the direction of the b axis, for C„H2n+iOH and by 29®, in the direction of the a axis, for the other molecular type. The molecular chains related by glide (g) symmetry are arranged in a herringbone pattern. The fit to the Bragg rod intensity data of C„H2n+iOH permitted a reliable estimate of 0.07 A* 12 for the molecular mean-squared motion parallel to the water surface. The absolute orientations of the molecules Ci9H39C02CnH2«0H were determined by lattice energy calculations. The anisotropic coherence lengths of the crystallites of Cigl^CChCnH^OH derived from the widths of the two observed Bragg peaks have been correlated with the binding energies of molecules in different directions.

Two approaches are used to study suggested mechanisms for heterogeneous ice nucleation. In one, an epitaxial-like mechanism is studied by using Langmuir monolayers of aliphatic alcohols C n H2n+1OH (n=16 to 31) spread on supercooled water drops. The packing arrangement of several of these uncompressed monolayers on water, have been determined by grazing incidence x-ray diffraction (GID) and correlated with the structure of water molecules at the (001) face of hexagonal ice. The roles played by lattice match between the monolayers and ice, the coherence length of the amphiphile crystallites and the orientation of the hydroxyl groups at the interface in determining the nucleation efficiencies are specifically addressed. In another induced nucleation of ice by an electric field is studied using hydrophobic surfaces of chiral resolved and racemic α-amino acid crystals onto which water vapor has been condensed and nucleated into ice.

Tailor-made molecular auxiliaries for the control of nucleation and growth of molecular crystals may be classified into two broad categories: inhibitors and promoters. Tailor-made inhibitors of crystal growth can be used for a variety of purposes which include morphological engineering, reduction of crystal symmetry, assignment of absolute structure of polar crystals, elucidation of the effect of solvent on crystal growth and crystallization of a desired polymorph. As for crystal growth promoters, Langmuir monolayers on water have been used to induce growth of 3D crystals at the monolayer-solution interface by means of structural match, molecular complementarity or an electrostatic interaction. The 2D crystalline structures of these monolayers have been studied by grazing-incidence x-ray diffraction (GIXD). It has become possible to monitor, by GIXD, the growth and dissolution of self-aggregated crystalline monolayers affected by the interaction of solvent molecules in the aqueous subphase with the monolayer headgroups.

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.

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.

The advent of well-collimated, high-intensity synchrotron X-ray sources and the consequent development of surface-specific X-ray diffraction and fluorescence techniques have recently revolutionized the study of Langmuir monolayers at the air-liquid interface. These methods allowed for the first time the determination of the in-plane and vertical structure of such monolayers with a resolution approaching the atomic level. We briefly describe these methods, including grazing incidence X-ray diffraction, specular reflectivity, Bragg rods, standing waves, and surface fluorescence techniques, and review recent results obtained 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, alpha-amino acids, and phospholipids at the water surface. In addition, it became possible to monitor by diffraction the growth and dissolution of the crystalline self-aggregates as well as structural changes occurring by phase transitions. Furthermore, the surface X-ray 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 two-dimensional (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 epitaxial-like crystallization both in organic and inorganic crystals.

Nucleation, growth, and dissolution of crystals have been studied by stereochemical approach involving molecular recognition at interfaces. A methodology is described for using "tailor-made" additives designed to interact stereospecifically with crystal surfaces during growth and dissolution. This procedure was instrumental in controlling crystal morphology and in revising the concept of the structure and symmetry of solid solutions. Consequently, it was applied to the transformation of centrosymmetric single crystals into solid solutions with polar arrangement displaying second-harmonic generation and to the performance of asymmetric synthesis of guest molecules inside centrosymmetric host crystals. The method has led to a discovery of a new "relay" mechanism explaining the effect of solvent on crystal growth. Finally, it allowed for the design of auxiliary molecules that act as promoters or inhibitors of crystal nucleation that can be used to resolve enantiomers and crystallize desired polymorphs.

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 approximately 1000 angstroms at 9-degrees-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.

The formation of two-dimensional ordered aggregates of water-soluble hydrophobic -amino acids at the air-solution interface has been demonstrated through the induced epitaxial nucleation of -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 -glycine, at the solution surface, whereas the second class, Ze/7-butylglycine, neopentylglycine, and hexafluorovaline, did not. The comparison between the packing arrangement of the cv-glycine crystalline face attached at the interface and that of various hydrophobic a-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.

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 “tailor-made" 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 (f?,S)-alanine and -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 “relay”-type mechanism of crystal growth.

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.

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.

Crystal nucleation of glycine and sodium chloride has been studied under floating Langmuir monolayers at air/solution interfaces, and at glass slides coated with Langmuir-Blodgett films. The packing arrangements of the polar head groups of the monolayers were varied in a controlled manner by introducing different groups in the hydrophobic tails. The structures of some of these monolayers were independently elucidated by grazing incidence X-ray diffraction and reflectivity measurements using synchrotron light. It was found that the crystals nucleated at the interface exposed a top layer of molecules with an arrangement similar or complementary to that of the polar head groups of the monolayer. These results imply that the packing of the polar head groups determines the morphology and the nucleation rate of the attached crystal.

Controlled reduction in crystal symmetry has been accomplished and exploited to perform an absolute asymmetric photoreaction. The principle is based on selective introduction of a guest molecule into a centrosymmetric host structure, thus reducing the symmetry of the mixed crystal. Crystallization of (£)-cinnamamide (space group P2{/c) in the presence of (£)-cinnamic acid results in a mixed crystal composed of two enantiomorphous halves each containing 0.5-1.0% of the acid. The replacement of N-H—O hydrogen bonds between host molecules by 0—0 lone-pair repulsion between host and guest at the chiral surfaces of the growing crystal is responsible for the stereoselective occlusion of the guest and the reduction in symmetry. Irradiation of each half separately yielded the optically active cyclobutane dimer la or lb in excess, with an enantiomeric yield in the range of 40-60%, varying from one single crystal to another. The absolute configuration of the chiral dimer la formed in excess at the +b pole (\R,2R,3R,4R) was independently assigned by the Bijvoet method, NMR, and crystal-etching experiments and is in agreement with our proposed mechanism of molecular recognition.

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

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.

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 air–water 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.

The structures of the partially reacted crystals of the two complexes (5:2) deoxycholic acid (DCA)-acetophenone and (8:3) DCA-p-fluoroacetophenone were determined by low-temperature (-170 °C) X-ray diffraction. The structure analyses indicated that both independent guest acetophenone molecules G' and Gin DCA-acetophenone react and that in DCA-p· fluoroacetophenone the guest molecule G' reacts whereas the other guest molecule G remains unreacted because of packing of G and G' molecules along the channel. The observed guest photoconversion in DCA-acetophenone is 40%; the maximum value according to packing considerations is 50%. In DCA-p-fluoroacetophenone the observed guest photoconversion is 35%, very close to the maximum theoretical yield of 33% based on packing considerations. The analysis demonstrated that during the course of photoconversion there is minimal motion of the (guest) phenyl ring, pronounced rotation of the acetyl group about the exocyclic C-C bond, and displacement of unreacted and reacted molecules. Atom-atom potential energy calculations showed that in DCA-acetophenone a guest ketyl radical with a pyramidal geometry may rotate by a full 180° without inducing prohibitively short intermolecular contacts.

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.

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.

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.

The crystalline host-guest channel inclusion complexes 5:2 (DCA) deoxycholic acid-acetophenone (C6H 5C'OCH 3) and 3:1 DCA-m-chloroacetophenone (CLC6H 4 C'OCH 3) each yield on UV irradiation a photoproduct via addition of guest to the steroid tertiary carbon atom CS with the formation of a new chiral carbon center C' (OH )(CH 3)( C 6H 5)(DC A) of S configuration. The crystal structures of the two host-guest complexes were determined by low-temperature (103 K) X-ray diffraction; a low-temperature (16 K) neutron study was made on DCA- C6H 5COC D•3 The inclusion compounds DCA¬ C6H5COCH3 and DCA-C LC6H 4COCH 3 each contain two crystallographically independent guest molecules G and G' arranged along the channel axes such that both G and G' should form the same diastereomeric product at CS. A comparison of the stereochemistry of each of the two isolated photoproducts and the host-guest arrangements at the reaction sites in each corresponding complex indicates that photoaddition of the guest molecule to C5 takes place with a net rotation of 180° by the guest acetyl group.

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 atom-atom potential energy calculations.

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 atom-atom 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 Cβ-CH3 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.

Monomeric dienes 1-7, 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 ("impurities") . 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 "tailor-made" impurities are discussed.

A method is described for direct determination of absolute configuration of chiral polar crystals, based on changes in crystal habit induced by tailor-made impurities. The method embodies the assignment of the direction of the chiral substrate molecule with respect to the crystal polar axis based on differences in the hemihedral faces of the substrate crystals as grown from solution in the presence and absence of impurities. The molecular packing requirements and the choice of impurities for application of this method in lysine and trans-cinnamoyl alanine are outlined.

The 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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An “absolute” 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 “crystal engineering”, 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.

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.

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.

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.