Capillary zone electrophoresis at subzero temperatures II: chiral separation of biogenic amines

The separation of enantiomer pairs of eight biogenic amines by capillary zone electrophoresis (CZE) was investigated with heptakis (2,6-di-O-methyl)-beta-cyclodextrin as the chiral selector at temperatures ranging from -20 degrees to 40 degrees C by using a commercial electrophoresis unit retrofitted with an external thermostated refrigerated circulating bath in order to assist the original cooling system. Sodium phosphate in both neat aqueous and methanolic media at pH 2.5, as measured by the glass pH electrode, were used with the fused silica capillary from 1.5 degrees to 40 degrees C and -20 degrees to 40 degrees C, respectively. The effect of temperature on enantioselectivity was found to depend on the number of phenolic hydroxyl groups in the molecule. Upon lowering the temperature from 40 degrees to -20 degrees C, the chiral selectivity of the system, as measured by the relative mobility difference, increased tenfold for the amines with two vicinal phenolic hydroxyls, whereas the increase was insignificant for those having no phenolic hydroxyl groups. The complex formation constants of three amines which have the same molecular structure but the number of phenolic hydroxyl groups were determined at different temperatures and the thermodynamic parameters as well as compensation temperatures for the process were evaluated. Whereas the compensation temperature was 690 K for the amine without phenolic hydroxyl group, it was < 400 K for the amines with one or two phenolic hydroxyl groups. The difference in the compensation temperatures indicates that the intrinsic mechanisms of their complexation with the chiral selector are not the same; this may account for the discrepancies observed in the temperature dependency of the chiral selectivity. The enthalpy change per phenolic hydroxyl group was 2.5 kcal mol(-1), which compares favorably with the typical value for a single hydrogen bond. Therefore, when the amines have phenolic hydroxyl groups, the strong increase in chiral selectivity with decreasing temperature may be due to enhanced H-bonding between the cyclodextrin and the phenolic hydroxyls under the conditions employed in this study.