Chiral crystalline structures

Preferential stereoselective incorporation of an enantiomeric monomer or its elective polymerization from racemic mixtures were accomplished with O.A. chiral catalysts, and it was also discovered that chiral crystalline structures can transfer asymmetry to a polymerization mechanism. 

Liquid Crystalline Structures. In certain ceUular organeUes, deoxyribonucleic acid (DNA) occurs in a concentrated form. Striking similarities between the optical properties derived from the underlying supramolecular organization of the concentrated DNA phases and those observed in chiral nematic textures have been described (36). Concentrated aqueous solutions of nucleic acids exhibit a chiral nematic texture in vitro (29,37). 

Clathrate formation is very attractive for exploitation in solid-state chemistry. It allows one to modify in a simple way the environment of the guest molecule, to place this molecule in a crystalline phase with a structure different from its own (one structure may be chiral, and the other not), and even to achieve a stable crystalline structure at a temperature above the melting point of the pure guest. Some of the variety available for a single compound, acetic acid, is... 

Several prominent types of host molecule, such as the steroidal bile acids and the cyclodextrins, are chiral natural products that are available as pure enantiomers. Chemical modification of these parent compounds provides an easy route to the preparation of large numbers of further homochiral substances. Since all these materials are present as one pure enantiomer, it automatically follows that their crystalline inclusion compounds must have chiral lattice structures. It is not currently possible to investigate racemic versions of these compounds, but the examples discussed previously in this chapter indicate that very different behaviour could result. 

Toda, F., Tanaka, K., Matsumoto, T., Nakai, T., Miyahara, I., and Hirotsu, K. (2000) A New Host 2,3,6,7,10,11-Hexahydroxytriphenylene Which Forms Chiral Inclusion Crystalline Lattice X-ray Structural Study of the Chiral Crystalline Lattice, J. Phys. Org. Chem., 13, 39-45. 

Determination of lateral periodicities in the self-assembled layer is an important goal in surface analysis. 2D surface crystal structures are best studied with low energy electrons, since their escape depth, contrary to X-rays, is basically limited to the top-most atomic layers. Consequently, LEED has become the most important method in surface monolayer crystallography. However, single-crystalline substrates are required. Via this technique, 2D supramolecular chiral lattice structures on single crystal surfaces had already been observed in 1978 [19]. 

Cholesterol esters form crystalline structures that are similar to those formed by other lipids, consisting of alternating infinite lamellae, so that the hydrocarbon chains form close-packed sheets segregated from layers of cholesterol skeletons. There are three t) s of such structures [6]. One such can be represented by the chiral molecule cholesterol oleate, where pairs of cholesterol skeletons are arranged in an antiparallel packing in one layer, with the hydrocarbon chains in the adjacent layer. The cross-sectional area of the cholesterol molecule is about 40 A2 (derived from pressure-area monolayer curves), corresponding to the cross-sectional area of two hydrocarbon chains. The chains therefore form an interpenetrating layer. 

Several types of photochemical reactions have been observed in chiral crystalline cyanoethyl cobaloximes in which there is sufficient space for an inversion of configuration. In one type, crystals contain one molecule per asymmetric unit, and 50% of the cyanoethyl groups are rotated during the photochemical reaction. Loss of crystallinity is not observed because the space occupied by the rotated cyanoethyl group is similar to that occupied by it in the original structure. Even the unit cell dimensions of... 

Abstract Enantioselective heterogeneous catalysis requires surfaces with structures that are chiral at the atomic level. It is possible to obtain naturally chiral surfaces from crystalline inorganic materials with chiral bulk structures. It is also possible to create naturally chiral surfaces from achiral materials by exposing surfaces that have atomic stractures with no mirror symmetry planes oriented perpendicular to the surface. Over the past decade there have been a number of experimental and theoretical demonstrations of the enantiospecific physical phenomena and surface chemistry that arise from the adsorption of chiral organic compounds on the naturally chiral, high Miller index places of metals. 

Naturally chiral surfaces can be created from achiral crystalline materials. The bulk structures of many crystalline materials such as metals are highly symmetric, contain one or more mirror symmetry elements and thus, are not chiral. Although it may seem counterintuitive, such achiral bulk structures can, nonetheless, expose surfaces with chiral atomic structures. These are planes whose normals do not lie in one of the bulk mirror planes. The classification of the symmetry of surfaces of a variety of bulk crystal structures has recently been reviewed by Jenkins et al. and they have identified all planes in those crystal structures that are chiral [9,10]. As a simple example consider the two surfaces illustrated in Fig. 4.1. These are the two enantiomers of the (643) surfaces of a face centered cubic lattice. 

At high shear rates the chiral nematic structure changes to a flow-induced nematic phase. However, the shear-oriented phase is easy to disrupt after removing the shear force. This is because of the driving force for the liquid crystalline solution to form the more thermodynamically stable chiral nematic struc-ture. Relaxation in a pseudo-nematic lyotropic... 

A similar approach has been used to produce materials with a chiral (cholesteric) structure by performing the experiments described above in the presence of a low molecular weight chiral liquid crystalline material (Figure 9.6). The chiral material is not covalently attached to the network and can be removed subsequently to produce an imprinted chiral structure. As before, the polymer displays a nematic mesophase between the glass transition (Tg 33°C) and the transition to an isotropic fluid (rN, 128°C). 

When the molecules that form a liquid-crystalline phase are chiral, the structure of these mesophases can have an additional property. In the chiral nematic phase (N ) the director precesses about an axis perpendicular to the director and describes in this way a helix (Figure 2.7). The pitch of a chiral nematic phase is the distance along the helix over which the director rotates over 360°. The chiral nematic phase is sometimes... 

Cellulose Triacetate. Cellulose triacetate (triethylcellulose) can interact stereoselectively with enantiomers of chiral drugs [48-50]. It has regions of crystallinity, which allow for enantioselective inclusion of drugs (solutes), especially those having substituent-free phenyl groups. However, it loses its enantioselectivity when solubilized and reprecipitated owing to the breakdown of the crystalline structure. 

R.-H. Jin et al. reported the preparation of silica on chiral crystalline catalytic template (polyethyleneimine and d-, l-, or Roc-tartaric acid) which has fibrous and very thin nanoribbon structures where chirality... 

Fig.21 Structure of biotinylated lipid 35 [165] schematic representation of the helical crystallization of streptavidin on a chiral tubular structure, a Formation of a chiral tubule functionalized with biotin b helical crystallization of streptavidin on the preexisting chiral tubes c secondary binding of biotinylated macromolecules on the remaining binding sites of streptavidin. Right TEM image of a helical array of RNA polymerase on a functionalized ceramide tubule (positive surface charge). The diffraction pattern below, with visible peaks to 1/38 A illustrates the crystalline nature of the helices. Photographs reprinted with permission from [166]. Copyright 1998 National Academy of Sciences USA...

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