Ma S, Horváth C
Department of Chemical Engineering, Yale University, New Haven, CT 06520-8286, USA.
Electrophoresis. 1997 Jun;18(6):873-83. doi: 10.1002/elps.1150180605.
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.
采用七（2,6-二-O-甲基）-β-环糊精作为手性选择剂，通过配备外部恒温制冷循环浴的商用电泳装置对原冷却系统进行辅助，在-20℃至40℃的温度范围内，研究了毛细管区带电泳（CZE）对8种生物胺对映体的分离。在1.5℃至40℃和-20℃至40℃下，分别使用玻璃pH电极测得的pH为2.5的纯水溶液和甲醇介质中的磷酸钠与熔融石英毛细管。发现温度对映选择性的影响取决于分子中酚羟基的数量。当温度从40℃降至-20℃时，对于具有两个邻位酚羟基的胺，通过相对迁移率差异测量的系统手性选择性增加了10倍，而对于没有酚羟基的胺，增加不明显。在不同温度下测定了三种具有相同分子结构但酚羟基数量不同的胺的络合形成常数，并评估了该过程的热力学参数和补偿温度。对于没有酚羟基的胺，补偿温度为690K，而对于具有一个或两个酚羟基的胺，补偿温度<400K。补偿温度的差异表明它们与手性选择剂络合的内在机制不同；这可能解释了在手性选择性温度依赖性中观察到的差异。每个酚羟基的焓变为2.5 kcal mol(-1)，与单个氢键的典型值相当。因此，当胺具有酚羟基时，随着温度降低手性选择性的强烈增加可能是由于在本研究采用的条件下环糊精与酚羟基之间的氢键增强。
