Mathematical approach by a selectivity model for rationalization of pH- and selector concentration-dependent reversal of the enantiomer migration order in capillary electrophoresis.

A separation selectivity model for capillary electrophoresis enantioseparations of weak bases in the presence of uncharged chiral selectors was described as a function of buffer pH and chiral selector concentration. On the basis of the selectivity at the extreme pH and selector concentration values, 15 principal cases could be distinguished describing the migration behavior of the analytes when increasing either pH or selector concentration. A pH-dependent reversal of the enantiomer migration order can be observed (1) when the complex mobility ratio is reversed due to an enantioselective complexation-induced pK(a)-shift, (2) in the case of an inversion of the ratio of the mobilities of the analyte-selector complex and the free analyte due to significantly weaker complexation of the neutral species, and (3) in the case of opposite chiral recognition of the protonated and uncharged species by the chiral selector. Reversal of enantiomer migration order as a function of the selector concentration at a fixed buffer pH is caused by the opposing effects of complexation constants and complex mobilities at the respective pH value. The model was applied to rationalize the enantioseparations of dipeptides, especially the pH-dependent reversal of the migration order of the LL- and DD-enantiomers of Ala-Tyr in the presence of heptakis-(2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector based on the complexation constants of the protonated and zwitterionic forms of the analytes.