Chiral recognition, and

Kuhn, R., Emi, F., Bereuter, T., and Hausler, J., Chiral recognition and enantiomeric resolution based on host-guest complexation with crown ethers in capillary zone electrophoresis, Anal. Chem., 64, 2815, 1992. 

A particularly elegant example of cluster formation involving chiral recognition and retention of chirality through an increasingly complex hierarchical series of clusters is that of rubrene on Au l 1 1 [9] illustrated in Figure 1.5... 

One may use the stronger term chirality discrimination when a substantial suppression of one intermolecular diastereomer with respect to the other occurs. This requires multiple strong interactions between the two molecular units and therefore more than simple monofunctional alcohols. Some examples where one of the molecules involved is a chiral alkanol are reported in Refs. 112 and 119 121. Pronounced cases of higher-order chirality discrimination have been observed in clusters of hydroxy esters such as methyl lactate tetramers [122] and in protonated serine octamers [15,123,124]. The presence of an alcohol functionality appears to be favorable for accentuated chirality discrimination phenomena even in these complex systems [113,123,125,126]. Because the border between chirality recognition and discrimination is quite undefined, it is suggested that the two may be used synonymously whenever both molecular partners are permanently chiral [127]. 

The fundamental behaviour of stationary phase materials is related to their solubility-interaction properties. A hydrophobic phase acts as a partner to a hydrophobic interaction. An ionic phase acts as a partner for ion-ion interactions, and surface metal ions as a partner for ligand complex formation. A chiral phase partners chiral recognition, and specific three-dimensional phases partner affinity interactions. 

Figure 21, Proposed model of adsorbed chiral selector (A-alkylproline)- Cu(U)-[free amino acid] mixed chelate complex, The lipophilized proline selector is held in position via intercalation of the alkyl chain. Case A the alkyl part of the mixed chelate complex is fixed by hydrophobic interactions with stationary phase (RP-J). Case B the complex formation is stabilized by other types of hydrophobic attraction. Chiral recognition and elution order is therefore not only dependent on the simple and isolatedly viewed chelate complex stability. In general, retention and chiral recognition in chiral LC is based on mixed-mode adsorption/dcsorption processes which act synergisticallv and also antagonistically with respect to the observed chiral resolution and intermolecular complex formation.

The presence of H- -it bonding can relate to apparently anomalous reactivities. These interactions are of importance in chiral recognition and in other intermolecular and intramolecular interactions143. 

The host is achiral. Incorporation of chirality would give access to studies on chiral recognition and asymmetric catalysis. 

Conversion of Racemic Cyanohydrin into One Optically Acrtive Iosmer in the Presence of Brucine, Chem. Lett., 661-664. b) Tanaka, K. and Toda, F. (1987) Chiral Recognition and Optical Resolution of Cyanohydrin by Complexation with Brucine, Nippon Kagaku Kaishi, 456-459. 

Most of the enzymes show extremely strict chiral recognitions, and only one of the enantiomers can be the substrate of the enzyme. For example, chymotrypsin incorporates L-peptides only to the enzyme-substrate binding site to form enzyme-substrate complex, so it shows very high enantioselectivity (Figure 3 (a)). Oxidoreductases also form the enzyme-substrate complex of only one enantiomer, so enantioselectivities are high when isolated enzymes are used for reactions instead of whole cells containing both (R)- and (.S )-specific enzymes, which leads to overall low enantioselectivities. 

Marchelli used the copper(II) complex of histamine-functionalized P-cy-clodextrin for chiral recognition and separation of amino acids [27]. The best results were obtained for aromatic amino acids (Trp). Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent P-cyclodextrin has been recently published by the same author [28, 29]. The host containing a metal-binding site and a dansyl fluorophore was shown to form copper(II) complexes with fluorescence quenching. Addition of d- or L-amino acids induced a switch on of the fluorescence, which was enantioselective for Pro, Phe, and Trp. This effect was used for the determination of the optical purity of proline. 

W.A. Tao et al., Copper(II)-assisted enantiomeric analysis of D, L-amino acids using the kinetic method Chiral recognition and quantification in the gas phase. J. Am. Chem. Soc. 122, 10598-10609 (2000)... 

Pirkle37 has observed that three simultaneous interactions between the chiral stationary phase and the analytes must occur for chiral recognition and separation. Typical interactions are those discussed in Chapter 3 and include hydrogen bonding, dipole-dipole interactions, and dipole-induced dipole interactions. The geometric arrangement of the chiral phase... 

Most chromatographic systems employ process control of operating parameters. These may well be built into the instrument. Temperature control is particularly important, especially for contemporary techniques such as chiral recognition and protein interaction.23 In liquid chromatography, for instance, temperature directly effects retention, separation efficiency, and selectivity. Stability of temperature is thus extremely important, since variations of more than 1°C can lead to noticeable effects.24... 

Not all separations can be accomplished with simple buffers. For example, chiral recognition and separation of neutral molecules always require additives. Even charged-molecule separations can often be improved via secondary equi-... 

There have also been examples of ligand-exchange CSPs. Schmid et al. [159] used a ligand-exchange monomer as a chiral selector. The chiral selector, monomer, cross-linker, and charged monomer were polymerized to produce monolithic capillaries capable of chiral recognition and generation of EOF. The separation is achieved due to the differences in the stability between the ternary mixed copper complexes formed by the enantiomers and the CSP. 

Fig. 4.32. Chiral recognition and induction of molecular memory in Ce(IV) complexes with a substituted porphyrin. Redrawn from S. Shinkai et al., J. Chem. Soc., Perkin Trans. I 3259, 1999.

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