Porras Simo P, Jussila Matti
Department of Chemistry, University of Helsinki, Finland.
Electrophoresis. 2007 Oct;28(20):3590-9. doi: 10.1002/elps.200700291.
ACN is an extremely poor hydrogen bond donor and therefore the anions dissolved in it are solvated mainly by other hydrogen bond donors (e.g. uncharged acids) possibly present in the solution. Under properly selected experimental conditions stabilization via hydrogen bonding can be used for separation in CE as has been demonstrated for uncharged acids by several authors. Electromigration based on heteroconjugation can be of importance, e.g. when aqueous separation medium cannot be used due to stability reasons. It also allows CE to be used as a tool for solution chemistry measurements, if the required physicochemical properties of the studied system are known or they can be predicted with sufficient accuracy by existing theories. In the present work we showed that also an uncharged base can stabilize an anion via hydrogen bonding in ACN. In the setup imidazole was chosen as a model base and acetate ion as complexing anion in equimolar acetic acid-acetate buffer. The resulted hydrogen-bonded imidazole-acetate complex (i.e. heteroconjugate) possesses a charge and can thus migrate in CE. It was shown that the studied complexation in ACN is sensitive to competition by other hydrogen bond donors such as water and methanol. On the other hand, acetone, which is a poor hydrogen bond donor, did not have much effect on the complexation. To take the effect of ionic strength on mobility into account, mobilities of the imidazole-acetate complex measured at various ionic strengths were corrected to zero ionic strength by the aid of conductivity equation. A fit of the 1:1 binding isotherm to the ionic strength corrected mobility versus acetate concentration data led to rather good correlation. However, x-reciprocal linear transformation of the binding isotherm showed nonlinearity, which could be partly explained by homoconjugation of acetic acid and acetate ion. Since the homoconjugation constant for acetic acid under present experimental conditions was not available, theoretical simulations were used to demonstrate the effects of homoconjugation. The possibility of multiple complexation of imidazole was discussed as well.
乙腈是一种极弱的氢键供体，因此溶解在其中的阴离子主要被溶液中可能存在的其他氢键供体（如不带电荷的酸）溶剂化。在适当选择的实验条件下，通过氢键进行的稳定作用可用于毛细管电泳分离，几位作者已针对不带电荷的酸证明了这一点。基于异共轭的电迁移可能很重要，例如由于稳定性原因无法使用水性分离介质时。如果已知所研究体系所需的物理化学性质，或者可以通过现有理论以足够的精度进行预测，那么它还可使毛细管电泳用作溶液化学测量的工具。在本工作中，我们表明不带电荷的碱也可通过在乙腈中的氢键作用使阴离子稳定。在该装置中，选择咪唑作为模型碱，在等摩尔乙酸 - 乙酸盐缓冲液中选择乙酸根离子作为络合阴离子。所得的氢键结合的咪唑 - 乙酸盐络合物（即异共轭体）带有电荷，因此可在毛细管电泳中迁移。结果表明，在乙腈中研究的络合作用对水和甲醇等其他氢键供体的竞争很敏感。另一方面，作为弱氢键供体的丙酮对络合作用影响不大。为了考虑离子强度对迁移率的影响，借助电导率方程将在不同离子强度下测得的咪唑 - 乙酸盐络合物的迁移率校正到零离子强度。将1:1结合等温线与离子强度校正后的迁移率对乙酸盐浓度数据进行拟合，得到了较好的相关性。然而，结合等温线的x倒数线性变换显示出非线性，这部分可以用乙酸和乙酸根离子的同共轭作用来解释。由于在当前实验条件下乙酸的同共轭常数不可得，因此使用理论模拟来证明同共轭作用的影响。还讨论了咪唑多重络合的可能性。
