Enantiomers recognition mechanisms

Some insights of the enantiomer recognition mechanism of vancomycin were previously studied in test tubes where the partition of the product is reproduced. These studies concluded that the recognition promoted by vancomycin is highly dependent on the solvent mixture where the association CS-enantiomers takes place, since slight variations in the composition of this mixture leads to a decrease or even the suppression of the enantiorecognition of the DNS-( )-Nle enantiomers. The use of an aromatic solvent, such as toluene, as a component of the solvent system... 

This observation is important in the study of the chiral recognition mechanism in this system. This may be a practical matter when determining the trace amount of one enantiomer in the presence of its dominant antipode. The smaller peak is always desired to be eluted first for best quantitation. 

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. 

Besides, information on intermolecular interactions has been derived in these studies from complexation-induced shifts (CIS). The chemical shift is an indicator for the shielding of a nucleus and thus for the electronic state of a specific proton. Since the electronic environment may change on complexation, CIS can be used to monitor where host-guest contacts may take place. If these interactions occur stereoselectively, the CIS will be different for the two guest enantiomers (AS distinct from 0) giving possibly some insight into the chiral recognition mechanism. 

While the distinct amino acid residues have mostly only a modulating effect (see Table 1.9) (e.g., FMOC-protected amino acids), the type of protection group or derivative formed decides on the molecular and chiral recognition mechanism and hence on the obtained elution order as well as the level of enantiomer recognition (i.e., magnitudes of a-values) that can be afforded. From a practical point of view, we may distinguish between two groups of IV-derivatives ... 

This reversal of elution order is due to the changed CIP priorities, but not a result of an altered binding and chiral recognition mechanism. Moreover, in yet another study, BOC and DNP-protected a-substituted proline derivatives have been resolved into enantiomers and elution orders were determined [48], The method allowed the sensitive and accurate analysis of samples with regards to their enantiomeric purities. 

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 inclusion of enantiomers into the chiral cavities of the network is supposed to be the main chiral recognition mechanism. Moreover, hydrogen bonding between polar groups of the solutes and the amide groups of the polymers are also assumed to participate in the chiral recognition process. Apolar mobile phases such as hexane-dioxane and toluene-dioxane mixtures are therefore commonly used with this type of CSPs. 

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