ABSTRACT
Three charged substituted beta-cyclodextrins (beta-CDs), sulfobutylether-beta-(SBE-beta-CD), degree of substitution (DS) 4 and 7), and sulfated-beta-(S-beta-CD) cyclodextrins, were compared as chiral additives in capillary electrophoresis for the enantiomeric separation of basic spirobenzopyran derivatives (pKa 9.9) which differ from each other by an N-alkyl group. The number of sulfobutylether groups attached to the cyclodextrin moiety significantly influences the enantioseparation of the basic drugs. SBE-beta-CD (DS 7) which is more strongly bound to cationic analyte than SBE-beta-CD (DS 4.6), requires smaller concentrations to achieve the same resolution. Besides, better enantioresolutions were obtained with S-beta-CD rather than with SBE-beta-CDs though higher concentrations are required, which led to high current values. However, both pairs of enantiomers cannot be resolved using S-beta-CD while SBE-beta-CDs make it possible to resolve simultaneous enantioseparation of such solutes slightly differing in hydrophobicity. This supports the hypothesis that hydrophobic interactions (outside of the CD cavity) between the butyl group attached to SBE-beta-CD and the N-alkyl group of spirobenzopyran play a role in the enantioseparation. On the other hand, the sulfate group of S-beta-CD was directly attached to the CD moiety which means that the S-beta-CD-drug complexation mechanism arises through the combination of electrostatic and hydrophobic (inside the CD cavity) interactions. Finally, enantiomers of spirobenzopyran drugs were satisfactorily resolved by CE using a 20 mg/mL S-beta-CD concentration (resolution 4.0), 7 mg/mL SBE-beta-CD DS 4 (resolution 1.3), or 5 mg/mL SBE-beta-CD DS 7 (resolution 3.3) added to the phosphate buffer (pH 2.6, 50 mM ionic strength).
