Transient state of chiral recognition in a binary mixture of cyclodextrins in capillary electrophoresis

The transient state (as the defined point where no enantioseparation is obtained in a dual chiral selector system) of chiral recognition of aminoglutethimide in a binary mixture of neutral cyclodextrins (CDs) was studied by capillary electrophoresis (CE). The following three dual selector systems were used: α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD); α-CD and heptakis(di-O-methyl-β-cyclodextrin) (DM-β-CD); α-CD and heptakis(tri-O-methyl-β-cyclodextrin) (TM-β-CD). The S-(−) enantiomer of the analyte was more strongly retained in the presence of either α-CD or TM-β-CD at pH 2.5, 100 mM phosphate buffer, while the R-(+) enantiomer was more strongly retained in the presence of either β-CD or DM-β-CD. In the more simple case, the elution order is invariably kept if the enantiomers have the same elution order in either one of the two hosts of the binary mixture. In contrast, the elution order may be switched by varying the concentration ratio of two hosts that produce opposite elution order for this particular analyte. In such a dual selector system, the enantioselectivity will disappear at the transient state at a certain ratio of host1:host2. Moreover, the migration times of the two enantiomers with host1 alone (diluted in buffer) is approximately equal to the migration times at the corresponding concentration of host2 alone (diluted in buffer), where the ratio of concentrations of host1:host2 is the same as in the binary mixture at the transient state. As found by nuclear magnetic resonance experiments, the analyte is forming a 1:1 complex with either one of the CDs applied. From this finding, a theoretical model based on the mobility difference of the two enantiomers was derived that was used to simulate the transient state.