Use of negatively charged sulfobutyl ether-?-cyclodextrin for enantiomeric separation by capillary electrophoresis

A newly modified charged ?-cyclodextrin (sulfobutyl ether-?-cyclodextrin) was investigated as a chiral selector in capillary electrophoresis in a study of the enantiomeric separation of a variety of underivatized anionic and cationic compounds of pharmaceutical interest and uncharged phenyl alcohols and dansyl-amino acids. Owing to the presence of four sulfonic groups, the chiral selector is negatively charged at all pH values used (2.5-9) and the complexation caused an increase in migration time for each compound studied. At a relatively low pH (2.5) the chiral selector could only be used at low concentration (0.1-0.5 mg/ml) for basic compounds, whereas at higher pH (6-9) the modified cyclodextrin in the concentration range 0-20 mg/ml was used. The concentration of the chiral selector, the distance from the aromatic group of the asymmetric centre of the analytes and the chemical composition and pH of the background electrolyte influenced the complexation, selectivity and resolution. Good enantiomeric separation was obtained for terbutaline at all pH values studied, whereas for other racemic compounds, warfarin, acenocoumarol, promethazine, bupivacaine and some dansyl-amino acids and phenyl alcohols, the pH range 6-9 was effective for optimizing the chiral resolution. Non-polar substituent groups on the asymmetric carbon of the analytes seem to enhance the complexation and the stereoselectivity