Enantiorecognition mechanisms

More than 100 CSPs are commercially available nowadays, which should make the separation of any pair of enantiomers feasible. However, the enantiorecognition mechanisms involved in the chiral recognition between the analytes and the CSPs are complex and therefore the selection of the appropriate CSPs, depending on the structure of the analyte, is a difficult task. A common approach to develop a new enantioseparation is the stepwise trial-and-error approach based on detailed consideration of the enantiorecognition mechanisms between the chiral selector and the analyte, or on the analyst s experience, or on the consultation of literature or databases. However, this approach is time-consuming and often unsuccessful owing to the fact that achieving enantioresolution is often purely empirical... 

Alain Berthod received his PhD in 1979 from the University of Lyon. He took an assistant professor s position at the French National Center for Scientific Research (CNRS) working in electrochemistry. In 1983 he was promoted as associate professor and in 1993 as research director. He focused on the developing and the use of micellar solutions and microemulsions in chromatography. His interests lie in the separation of chiral molecules and enantiorecognition mechanisms. He has contributed to the development of the countercurrent chromatography technique that uses a sup-port-free liquid stationary phase. He was member of the editorial board of major analytical chemistry and chromatography journals. He is editor-in-chief of Separation Purification Reviews (Taylor Francis, Philadelphia, Pennsylvania). 

H. Ding, N. Grinberg, R. Thompson, and D. Ellison, Enantiorecognition mechanisms for derivatized cellulose under reversed phase conditions, J. Liq. Chrom. Relat. Technol. 23 (2000), 2641-2651. 

The last line of Table 5 shows that the normal-phase enantiorecognition mechanism of 5-methyl-5-phenyl hydantoin is completely different from that observed in reversed-phase mode. Especially, the AvV term is positive meaning that steric interactions contribute to enantiorecognition, a trend opposite to what was obtained in reversed-phase mode. The magnitude of the AsS term for dipolar interactions is also significant and completely different from the reversed-phase mode (Table 5, bottom). 

Figure 7 shows the separation of terbutaline on the V and T CSPs with the same mobile phase. The retention, selectivity, and resolution factors are very different for the same solute eluted on the two CSPs. The key point of the example is the reversal of elution order that is observed on the polarimeter trace (Fig. 7). Such reversal signs a different selector-selectand separation mechanism with the same mobile phase at least with one of the interactions needed for enantiorecognition [29],...

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... 

A chromatographic method for the calculation of the association constants CS-enantiomers, Kur/s, assuming 1 1 complexes, was described by the same authors [60]. Some modifications on the anilide ring of the CS had been introduced to further elucidate the mechanism of enantiorecognition. The studies corroborate the key role of the 7T-7T-interaction between the n-donor group of the CS and the K-acceptor group of the analyte in enantiorecognition. 

In the presence of water, the solvated molecules of the analyte will be forced to compete with those of water in the interaction with the solvated CS on the CSPs. When hydrogen bond interactions play a fundamental role in the recognition mechanism, this results in a decrease of enantiorecognition. 

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