Enantiomer separation of acidic racemates by capillary electrophoresis using cationic and amphoretic β-cyclodextrins as chiral selectors

Abstract

Enantiomer separations of various acidic racemates were performed by capillary electrophoresis (CE) using a commercial cationic β-cyclodextrin (β-CD), quaternary ammonium β-CD (QA-β-CD), and a commercial amphoteric β-CD (AM-β-CD) as chiral selectors. Eleven acidic racemates were successfully separated using QA-β-CD by changing the CD concentration and the buffer pH. These enantiomer separations were compared with the results using five neutral CD derivatives. Although most racemates were separated with some of the neutral CDs, relatively high CD concentrations were required to obtain baseline separations. In contrast, when QA-β-CD was employed, the enantiomer separations were successful at low concentrations below 5 m M. Enantiomers of five acidic racemates and ten dansylated amino acids (Dns-amino acids) were separated using AM-β-CD. Although the baseline separation of racemic 4-chloromandelic acid was not achieved with either QA-β-CD or five neutral CDs, AM-β-CD showed complete resolution. Furthermore, the simultaneous enantiomer separation of eight Dns-amino acids was also achieved with AM-β-CD. Both QA-β-CD and AM-β-CD were analyzed by CE and mass spectrometry (MS) in order to identify their compositions because they consisted of a mixture having different degrees of substitution. QA-β-CD consisted of six components having from one to six quaternary ammonium groups. The composition of AM-β-CD, however, was very complicated and could not be identified.