Isabelle Weissbuch

Interplay Between Malaria, Crystalline Hemozoin Formation, and Antimalarial Drug Action and Design

This review on the interplay between malaria, antimalarial drugs, and hemozoin crystals is directed, on one hand, to biologists and pharmaceutical chemists who wish to have a broader knowledge of the crystalline state related to malaria and, on the other hand, to crystallographers and physical chemists who wish to have a simple introduction to the disease and drugs employed against it. The call for this review is dictated by various considerations, not the least being the need for new antimalarial drugs, in view of developing parasitic resistance to the commonly used ones. The increasing spread of malaria is also due to several other contributing factors; besides climatic and environmental factors, the Anopheles mosquito has become increasingly resistant to insecticides and has adapted so as to avoid * To whom correspondence should be addressed. E-mail: (I.W.) isabelle.weissbuch@weizmann.ac.il or (L.L.) leslie.leiserowitz@weizmann.ac.il. The review represents the fruits, the offshoot, one may say, that may be reaped from the experience and knowledge gleaned from a collaborative effort, extending for more than a quarter of a century, on the design and use of auxiliaries for the control of crystal nucleation, morphology, and polymorphism, the structure determination of monoand multilayer films of amphiphilic molecules by grazing incidence synchrotron X-ray diffraction at the air-liquid interface, and computational studies of molecular interactions at interfaces and in the crystal bulk. These studies have been conducted in close collaboration with Meir Lahav and with other colleagues. Chem. Rev. 2008, 108, 4899–4914 4899

Racemic β-Sheets as Templates of Relevance to the Origin of Homochirality of Peptides: Lessons from Crystal Chemistry

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 acids--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-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and MS/MS with monomers enantioselectively tagged with deuterium. The reaction performed in aqueous solutions encompasses the following sequential steps: (i) formation of a library of short racemic peptides enriched with isotactic diastereoisomers during the early stages of the polymerization, and (ii) self-assembly of oligopeptides into racemic beta-sheet colloidal-like aggregates that are delineated by enantiotopic sites or rims; these operate as templates (nuclei) for regio-enantioselective growth in the ensuing steps of chain elongation. Desymmetrization of the racemic mixtures of peptides was achieved with enantiopure alpha-amino acid esters as initiators. The enantiomeric excess of the isotactic peptides, not including the initiator, varies with chain length, the result of a cross-enantiomeric impeding mechanism. Our results suggest a feasible scenario in which primitive homochiral peptides might have emerged early in the prebiotic world.

Manipulating Crystal Orientation in Nanoscale Cylindrical Pores by Stereochemical Inhibition

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 [010] 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 [010] 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.

Parity Violating Energetic Difference and Enantiomorphous Crystalsp-Caveats; Reinvestigation of Tyrosine Crystallization

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 Shinitzkys 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.

ASPECTS OF SPONTANEOUS SEPARATION OF ENANTIOMERS IN TWO- AND THREE-DIMENSIONAL CRYSTALS

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 herring-bone 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. Chirality 10:415–424, 1998.

Oriented Crystalline Monolayers and Bilayers of 2×2 Silver(I) Grid Architectures at the Air-Solution Interface: Their Assembly and Crystal Structure Elucidation

Oriented crystalline monolayers, approximately 14 A thick, of a 2 x 2 Ag+ grid complex, self-assembled at the air-solution interface starting from an water-insoluble 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 counter-ions, 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 self-assembled 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.

'Tailormade' auxiliaries for nucleation, growth and dissolution of organic crystals

A new stereochemical approach for the controlled nucleation, growth and dissolution of organic crystals with well designed auxiliaries is described. The method is conprised 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 accorrplished with the assistance of designed Langrruir 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.

Assignment of Absolute Structure of Polar Crystals Using Tailor-Made Additives. Solvent Surface Interactions on the Polar Crystals of α-Resorcinol, (R, S) Alanine and Y-Glycine

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.

MORPHOLOGY ENGINEERING OF ORGANIC CRYSTALS WITH THE ASSISTANCE OF “TAILOR-MADE” GROWTH INHIBITORS

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

Racemic β-Sheets as Templates for the Generation of Homochiral (Isotactic) Peptides from Aqueous Solutions of (RS)-Valine or -Leucine N-Carboxy- anhydrides: Relevance to Biochirogenesis

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 MALDI-TOF 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 alpha-amino acids as initiators.