CAPILLARY ZONE ELECTROPHORESIS AT SUBZERO TEMPERATURES .2. CHIRAL SEPARATION OF BIOGENIC-AMINES

The separation of enantiomer pairs of eight biogenic amines by capilla ry zone electrophoresis (CZE) was investigated with heptakis (2,6-di-O -methyl)-beta-cyclodextrin as the chiral selector at temperatures rang ing from -20 degrees to 40 degrees C by using a commercial electrophor esis unit retrofitted with an external thermo-stated refrigerated circ ulating bath in order to assist the original cooling system. Sodium ph osphate in both neat aqueous and methanolic media at pH 2.5, as measur ed by the glass pH electrode, were used with the fused silica capillar y from 1.5 degrees to 40 degrees C and -20 degrees to 40 degrees C, re spectively. The effect of temperature on enantioselectivity was found to depend on the number of phenolic hydroxyl groups in the molecule. U pon lowering the temperature from 40 degrees to -20 degrees C, the chi ral selectivity of the system, as measured by the relative mobility di fference, increased tenfold for the amines with two vicinal phenolic h ydroxyls, whereas the increase was insignificant for those having no p henolic hydroxyl groups. The complex formation constants of three amin es which have the same molecular structure but the number of phenolic hydroxyl groups were determined at different temperatures and the ther modynamic parameters as well as compensation temperatures for the proc ess were evaluated. Whereas the compensation temperature was 690 K for the amine without phenolic hydroxyl group, it was < 400 K for the ami nes with one or two phenolic hydroxyl groups. The difference in the co mpensation temperatures indicates that the intrinsic mechanisms of the ir complexation with the chiral selector are not the same; this may ac count for the discrepancies observed in the temperature dependency of the chiral selectivity. The enthalpy change per phenolic hydroxyl grou p was 2.5 kcal mol(-1), which compares favorably with the typical valu e for a single hydrogen bond. Therefore, when the amines have phenolic hydroxyl groups, the strong increase in chiral selectivity with decre asing temperature may be due to enhanced H-bonding between the cyclode xtrin and the phenolic hydroxyls under the conditions employed in this study.