Effect of alkylimidazolium substituents on enantioseparation ability of single-isomer alkylimidazolium-β-cyclodextrin derivatives in capillary electrophoresis

A family of 6-mono(3-alkylimidazolium)-β-cyclodextrins with one primary hydroxyl group replaced by an alkylimidazolium cation has been developed. The effect of alkyl substitutents on the enantioresolution ability of these single-isomer cyclodextrins towards dansyl amino acids has been studied by capillary electrophoresis. Systematical investigations on the effect of buffer pH and selector concentration on the enatioseparation show that chiral selectors with a shorter alkyl chain (R = CnH2n+1, n ≤ 4) presented more powerful chiral recognition ability. These newly introduced single-isomer β-cyclodextrin derivatives proved to be effective chiral selectors for most selected dansyl amino acids at low buffer pH (e.g. pH 5.0) with selector concentration no less than 3 mM. The apparent complex stability constants between alkylimidazolium β-CDs and dansyl amino acids were also theoretically determined by using the mobility difference model proposed by Wren and Rowe. The side alkyl chains from both dansyl amino acids and alkylimidazolium β-CDs displayed significant effect on the apparent complex stability constants. Both the optimum selector concentrations calculated according to the model, however, were much lower than the experimental values giving the maximum chiral resolution of enantiomers.