Chirality enantiomer association

FIGURE 4.18 Comportment of enantiomers in the presence of another chiral molecule. Only one enantiomer reacts with an enzyme. Enantiomers associate differently with another chiral molecule. 

The GRIND descriptors are insensitive to the chirality of the structures. This has the undesirable side effect of providing exactly the same description for the two enantiomers associated with any chiral center. Diastereomers might, on the contrary, produce different correlograms, due to the presence of differences in the internal geometry. 

A chiral catalyst associated with the growing end-group exhibits a bias for one of the two enantiomers. In such a case, the resulting polymer is optically active, provided that an optically active catalyst was used in the preparation, while the residual monomer becomes enriched in the other enantiomer. This is an example of stereo-electivity. A racemic mixture of chiral catalysts yields a racemic mixture of polymers, each being enriched in one of the enantiomers. 

The heart of the Watson-Crick model is the postulate that a molecule of DNA is a complementary double helix consisting of two antiparallel polynucleotide strands coiled in a right-handed manner about the same axis. As illustrated in the ribbon models in Figure 20.6, chirality is associated with a double helix Like enantiomers, left-handed and right-handed double helices are related by reflection. 

A geometric isomer contribution from the presence of double bonds does not qualify as a steieocenter. When double bonds affect stereochemistry, the enantiomers are due only to chiral differences associated with a stereocenter. The enantiomers are always paired with identical configurations about the double bond. Both isomers must be cis- or trans- to meet the mirror image requirement that defines an enantiomer pair. 

The Cahn-Ingold-Prelog (CIP) rules stand as the official way to specify chirahty of molecular structures [35, 36] (see also Section 2.8), but can we measure the chirality of a chiral molecule. Can one say that one structure is more chiral than another. These questions are associated in a chemist s mind with some of the experimentally observed properties of chiral compounds. For example, the racemic mixture of one pail of specific enantiomers may be more clearly separated in a given chiral chromatographic system than the racemic mixture of another compound. Or, the difference in pharmacological properties for a particular pair of enantiomers may be greater than for another pair. Or, one chiral compound may rotate the plane of polarized light more than another. Several theoretical quantitative measures of chirality have been developed and have been reviewed elsewhere [37-40]. 

A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two rings is inhibited because of torsional strain associated with twisting rotation about the central single bond. Interconversion of enantiomers is prevented (see Fig. 1.16). 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

An alternative model has been proposed in which the chiral mobile-phase additive is thought to modify the conventional, achiral stationary phase in situ thus, dynamically generating a chiral stationary phase. In this case, the enantioseparation is governed by the differences in the association between the enantiomers and the chiral selector in the stationary phase. 

All enantioselective separation techniques are based on submitting the enantiomeric mixture to be resolved to a chiral environment. This environment is usually created by the presence of a chiral selector able to interact with both enantiomers of the mixture, albeit with different affinities. These differences in the enantiomer-selector association will finally result in the separation that is sought. 

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