Enantiomer separation by capillary electrophoresis using highly sulfated cyclodextrins as a chiral selector

Enantiomer separation by capillary electrophoresis (CE) has attracted consi derable attention among analytical chemists as a promising new technique, e specially in the fields of pharmaceutical, clinical, and agrochemical scien ces. Capillary zone electrophoresis (CZE) and electrokinetic chromatography (EKC), in which a chiral selector is simply added to the running buffer, a re most widely used for enantiomer separation among various CE modes. Among the chiral selectors employed in CE, cyclodextrins (CDs) are the most succ essful in enantiomer separation. Especially charged CDs have been found to have a relatively wide enantioselectivity. compared with electrically neutr al CDs. In this study, we investigated the enantiomer separation of drugs ( 28 compounds) by CE with sulfated CDs. This mode can be regarded as one typ e of EKC, because a charged pseudo-phase is added to the running buffer, an d electrically neutral analytes can be separated by this mode. Three types of highly sulfated CDs (HS-CDs) were employed as chiral selectors. Compared with the usual sulfated beta -CD (S-beta -CD), the degree of substitution (DS) of the HS-CDs is high (average 12) and its distribution range is nar r ow. By using one of the HS-CDs as a chiral selector, 75% of the compounds t ested were enantioseparated. The binding constants were also calculated fro m Lineweaver-Burk type plots. The values of the binding constants obtained for HS-CDs were 200 similar to 4000 M-1. These values were about ten-times larger than those in electrically neutral CDs. Therefore, the optimum conce ntration of the HS-CDs calculated from the equation derived from the Wren a nd Rowe model became very low. The electrostatic interaction will effective ly contribute to the enantiorecognition of drugs. Furthermore, the migratio n order of some enantiomers (for example, denopamine and trimetoquinol) was reversed between the CE with HS-beta -CD and that with HS-gamma -CD.