Evaluation of the chiral separation ability of single-isomer cationic β-cyclodextrins in capillary electrophoresis

Eight single-isomer ammonium--cyclodextrin derivatives with different side chains were successfully developed as chiral selectors for the chiral separation of selected racemates in capillary electrophoresis. The number of substituted groups at N-atom as well as the alkyl chain length greatly influenced the chiral separation. With the numbers of hydroxylalkyl groups at N-atom growing, the aqueous solubility of resolving agents were distinctly decreased and chiral separation ability was also significantly reduced. The apparent complex stability constants between CDs and analytes were further determined. The best enantioseparations of hydroxyl acids was achieved with the use of mono-6(A)-(3-hydroxypropyl)-1-ammonium--cyclodextrin chloride and mono-6(A)-(3-methoxypropyl)-1-ammonium--cyclodextrin chloride. The nuclear magnetic resonance experiments were carried out using them with mandelic acid as guest molecules, revealing the inclusion pattern as well as electrostatic interactions and hydrogen bonding interactions as additional chiral driving force. The contribution of potential interaction sites in the sidearm could enhance the enantioseparations.