Could linear solvation energy relationships give insights into chiral recognition mechanisms?: 2. Characterization of macrocyclic glycopeptide stationary phases

Five parameter linear solvation energy relationships (LSER) are known to have little or no shape recognition ability. However, it is proposed to use LSER studies to get insights into chiral recognition mechanisms. Since the two enantiomers have exactly the same five A–V solute descriptors being still separated by chiral stationary phases (CSPs), it can be considered that they form two different transient diastereoisomers with the CSP. It is then possible to perform LSER studies on the enantioselectivity factors taken as the two enantiomer retention factor ratios. In a first step, the five a  –vv system parameters of four CSPs of the macrocyclic glycopeptide types were determined using a set of test solutes with known A–V descriptors, both in the reversed phase and the normal phase modes. In a second step, the A–V descriptors of 18 enantiomeric pairs were tentatively established using five achiral columns with known a  –vv parameters. This was successful for the five molecular enantiomers only. It was found that the predicted retention factor for the molecular enantiomers separated on a given CSP corresponded either to retention factor of the first experimentally eluted enantiomer or to the second one or to none of them. Using the enantioselectivity factors it was possible to obtain the Δa  –Δv parameters corresponding to the difference in CSP properties seen by the two enantiomers. For the five molecular enantiomeric pairs in the reversed phase mode with a teicoplanin CSP, it was found that there was an elevated contribution by the e   coefficient that we interpret as a possible interaction between surface charges on the teicoplanin CSP and solute induced dipoles. Steric effects, seen on the vv parameter, are second in magnitude followed by H-bond and polar interactions. Only one solute could be studied in the normal phase mode showing a different mechanism with polar and steric major interactions.