Effect of temperature on the enantioselectivity of basic analytes using CE with sulfated beta-cyclodextrins

Twelve basic analytes, including ephedrine and its structurally related compounds, were used to study the influence of capillary temperature on enantioselectivity in CE enantioseparations under reversed polarity mode using sulfated beta-CD (S-beta-CD) as chiral selectors. All of the effective mobility changes of (+)-enantiomers between 35 and 20 degrees C were higher than those of (-)-enantiomers whosoever enantioselectivity increased or decreased with an increase in temperature. However, the unusual temperature effect that enantioselectivity was increased with an increase of temperature was observed for the compounds with hydroxyl substitution on phenyl ring and had relationship with the molecular structures. With performing NMR studies on the selected selector-analyte complexes, the results led to a deeper understanding of the chiral discrimination process. Inspection of the complexation-induced chemical shifts (CICS) of the enantiomers showed that the phenyl ring sits deeply and slantways in the cavity of S-beta-CD and alkyl chain pointed out of the wider rim of S-beta-CD with ion-ion and hydrogen bond interactions between analytes and S-beta-CD.