Model of CE enantioseparation systems with a mixture of chiral selectors : Part II. Determination of thermodynamic parameters of the interconversion in chiral and achiral environments separately

The theoretical assumption that a multi-CS enantioseparation system, the model of which was described in Part I of this work, can be treated as a separation system with only one CS is confirmed by a set of experiments. The model assumes that each individual analyte–CS interaction is fast, fully independent on other interactions and the analyte: CS ratio is 1:1 and that the analyte is present in its concentration small enough not to considerably change the concentration of free CSs. An enantioselective environment in affinity capillary electrophoresis is created using a commercially available mixture of highly sulfated β-cyclodextrines as chiral selectors (CSs) and lorazepam as an analyte that undergoes interconversion during the separation process. Dependencies of the electrophoretic mobilities of the two enantiomers on concentration of the CSs mixture are proved to follow the proposed multi-CS model. Global rate constants of interconversion are determined at various temperatures and concentrations of the CSs mixture. In accord with the proposed theory, linear dependencies of the global rate constants on CSs concentration are achieved. Intercepts and slopes of these plots correspond to local rate constants of interconversion in achiral (without the mixture of CSs in background electrolyte (BGE)) and chiral (with the CSs mixture in BGE) environments, respectively. The experimentally obtained electropherograms show an excellent fit with those resulting from the computer simulation based on our model.