Chiral domain

Several groups have studied the structure of chiral phases illustrated in Fig. IV-15 [167,168]. These shapes can be understood in terms of an anisotropic line tension arising from the molecular symmetry. The addition of small amounts of cholesterol reduces X and produces thinner domains. Several studies have sought an understanding of the influence of cholesterol on lipid domain shapes [168,196]. 

Today, the use of CHIRBASE as a tool in aiding the chemist in the identification of appropriate CSPs has produced impressive and valuable results. Although recent developments diminish the need for domain expertise, today the user must possess a certain level of knowledge of analytical chemistry and chiral chromatography. Nevertheless, further refinements will notably reduce this required level of expertise. Part of this effort will include the design of an expert system which will provide rule sets for each CSP in a given sample search context. The expert system will also be able to query the user about the specific requisites for each sample (scale, solubility, etc.) and generate rules which will indicate a ranked list of CSPs as well their most suitable experimental conditions (mobile phase, temperature, pH, etc.). 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

In Section 1.2.1, we discussed the phenomenon of adsorbate-induced chirality whereby the adsorption of achiral species (e.g., glycine) results in the formation of two mirror equivalent domains on the surface. It has recently been shown that the presence of relatively small mole fractions of chiral dopants can result in the exclusive... 

A very complex phase diagram was predicted in the seventies by Villain for quasi ID helimagnets. Configurations with opposite chirality are degenerate and in the paramagnetic phase the chains are composed by domains with opposite chirality separated by a domain wall (DW). On lowering the temperature a 3D chiral spin liquid phase was predicted, which can be schematized as a 3D ensemble of helices all with the same spin chirality but with random phase between different chains, as shown in Figure 4.6b. 

Unlike electron and scanning tunneling microscopy, the use of fluorescent dyes in monolayers at the air-water interface allows the use of contrast imaging to view the monolayer in situ during compression and expansion of the film. Under ideal circumstances, one may observe the changes in monolayer phase and the formation of specific aggregate domains as the film is compressed. This technique has been used to visualize phase changes in monolayers of chiral phospholipids (McConnell et al, 1984, 1986 Weis and McConnell, 1984 Keller et al., 1986 McConnell and Moy, 1988) and achiral fatty acids (Moore et al., 1986). 

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

In the first half of the nineteenth century, it was known that certain minerals, the prime example being quartz, formed chiral crystals. Often, it was seen that rocks could be composed of a physical mixture of small but macroscopic right-handed and left-handed crystals. This kind of mixture, composed of macroscopic chiral domains (crystals) occurring in both enantiomeric forms, was termed a conglomerate. 

Spontaneous reflection symmetry breaking in achiral LCs is also well known, driven by specific boundary conditions. A very simple example of this type of chiral domain formation is illustrated in Figure 8.11. Suppose we start with two uniaxial solid substrates, which provide strong azimuthal anchoring ... 

The starting system is achiral (plates at 90° with isotropic fluid between), but leads to the formation of a chiral TN structure when the fluid becomes nematic. In this case, enantiomeric domains must be formed with equal likelihood and this is precisely what happens. The size of these domains is determined by the geometry and physics of the system, but they are macroscopic. Though the output polarization is identical for a pair of heterochiral domains, domain walls between them can be easily observed by polarized light microscopy. This system represents a type of spontaneous reflection symmetry breaking, leading to formation of a conglomerate of chiral domains. 

Since the walls between heterochiral domains are unacceptable defects in an LC display, enantiomericafly enriched dopants are added to the LC to favor one sign of twist over the other in actual devices, providing a monodomain in the TN cell. It should be noted, however, that the chirality of the structure derives from the interaction of the LC director with the surfaces the molecular chirality serving simply to break the degeneracy between mirror image domains to favor one over the other. 

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