Chiral recognition factors

Although the chiral recognition factor in such systems is relatively weak, there is no question that it is measurable and provides a useful approach to elucidating intermolecular interactions between nonreacting molecules. 

The extreme values of Ccyde/ non-cyde derived from (5)-(64) and (5)-(65) varied from >130 to >1170 for reaction with the L-ester salts of alanine and leucine respectively, demonstrating stabilization of a transition state by complexation with the macrocycle. Massive concentrations of acted as a competitive and dominant binder of (64), and eliminated the acceleration caused by the structured complexation. The preference of (5)-(64) over its enantiomer (i )-(64) to react with the L-amino-ester salts and the relative values of these chiral recognition factors has been rationalized on the basis of the more stable (5)-(64) to L-co. ifiguration for complexation in the rate-limiting transition state (66). Comparisons were made between these reactions and those catalysed by enzymes. 

ABSTRACT. The chiral recognition properties of chiral 18-crown-6 ethers with phenyl, l naphthoxymethyl, l naphthylmethyl, or 2,3,5,6-tetra-methylphenylmethyl substituents for a-phenylglycine methyl ester and 1-phenylethylamine perchlorate were investigated by a standard extraction procedure. The chiral recognition factor of 1-naphthoxymethyl substi-tuted crown ether is 2.0 for the former salt but near 1.0 for the latter, whereas that of the other crown ethers is not so dependent on the structure of salts, which indicates the importance of the mutual relation of the structure of host and guest molecules. 

This is because the increased turbulence from higher flow rates decreases the possibility for inclusion complexation, a necessary event for chiral recognition in reversed phase. Some effect has also been observed in the new polar organic mode when (capacity factor) is small (< 1). Flow rate has no effect on selectivity in the typic normal-phase system, even at flow rates up to 3 inL miir (see Fig. 2-11). 

In another study, the authors reported a comparative study of the enantiomeric resolution of miconazole and the other two chiral drugs by high performance liquid chromatography on various cellulose chiral columns in the normal phase mode [79], The chiral resolution of the three drugs on the columns containing different cellulose derivatives namely Chiralcel OD, OJ, OB, OK, OC, and OE in normal phase mode was described. The mobile phase used was hexane-isopropanol-diethylamine (425 74 1). The flow rates of the mobile phase used were 0.5, 1, and 1.5 mL/min. The values of the separation factor (a) of the resolved enantiomers of econazole, miconazole, and sulconazole on chiral phases were ranged from 1.07 to 2.5 while the values of resolution factors (Rs) varied from 0.17 to 3.9. The chiral recognition mechanisms between the analytes and the chiral selectors are discussed. 

The chiral recognition ability of a CSP is quantitatively evaluated from the results of chromatographic separation of enantiomers. Figure 3.4 shows a chromatogram of the resolution of benzoin (19) on cellulose tris(3,5-dimethylphenylcarbamate). The (+)-isomer elutes first followed by the (—)-isomer complete baseline separation is achieved. The results of the separation can be expressed by three parameters—capacity factors (k1), separation factor (a), and resolution factor (Rs)—defined as follows ... 

Both steric and electronic factors are used for chiral recognition of saccharides by the R and S forms of S-3. A difference in PET efficiency is created by the asymmetric immobilization of the amine groups relative to the binaphthyl moiety upon 1 1 complexation of saccharides by d- or L-isomers. For instance, D-fructose is recognized by the R form of S-2 with a large fluorescence enhancement. 

Several CD derivatives (charged and uncharged) are available which should allow the separation of most chiral molecules with at least one of them. However, due to the complexity of chiral recognition mechanisms, the determination of the best selector based on the analyte structure is challenging. Eurthermore, separations using CDs are influenced by numerous factors, so that no general rule can be applied for the successful resolution of enantiomers. ... 

The chiral recognition of enantiomers can be of three types (i) desionoselective, (ii) ionoselective, or (iii) duoselective, in which only the non-dissociated, the dissociated or both forms (charged and uncharged), respectively, of the enantiomers selectively interact with the chiral selector. In the case of ionoselective and duoselective interactions, a reversal of the migration order of the enantiomers is theoretically possible by the appropriate selection of CD concentration and the pH of the BGE. The addition of organic modifier to the BGE can also change selectivity by modifying the solubility of the chiral selector and/or of the solute, the complex equilibrium, the conductivity of the BGE and the electroendos-motic flow (EOE) level. Several other factors, such as the temperature, the type and the concentration of the BGE, and the level of the EOE can influence the separation. 

Possible explanation for Increased chiral recognition from experiments with solvent order and Irradiation. The necessity of both macro-solvent order and interconversion via an excited state of BN In order to observe the largest atroplsomerlc excesses Indicates that a combination of factors, working In concert, may be responsible. In toto, these must result in a greater Interaction energy (and, therefore specificity of Interaction) between solvent molecules and excited BN. 

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