Kubica Dominika, Gryff-Keller Adam
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland.
J Phys Chem B. 2015 May 7;119(18):5832-8. doi: 10.1021/acs.jpcb.5b02410. Epub 2015 Apr 28.
Orotic acid, a biologically important compound, can exist in aqueous solutions in several ionic and tautomeric forms. Interpretation of the experimental (13)C NMR chemical shifts of this compound based on the results of energy and magnetic shielding calculations performed by DFT B3LYP/6-311++G(2d,p)/PCM method has shown that in water solutions the diketo tautomers are the dominant structural forms of this acid and its anions. For neutral molecules the anti conformation is preferable, monoanion occurs as the conventional carboxylate anion, whereas the orotic dianion exists in two tautomeric forms in the proportion of ca. 4:1. It has been found that the experimental (13)C NMR chemical shifts can be reproduced well by the results of DFT calculations, although for the orotate monoanion some small but characteristic divergences can be noticed. Similar divergences have been also observed for a few other aromatic carboxylates. It seems that the specific solute-solvent interactions occurring in our systems can be the cause of this inconsistency. To support this hypothesis, a simple and effective method of including the specific hydration into the theoretical calculations has been proposed.
乳清酸是一种具有重要生物学意义的化合物，它在水溶液中可以以几种离子和互变异构形式存在。基于采用密度泛函理论（DFT）B3LYP/6 - 311++G(2d,p)/极化连续介质模型（PCM）方法进行的能量和磁屏蔽计算结果，对该化合物的实验（13）C核磁共振化学位移进行解释表明，在水溶液中，二酮互变异构体是该酸及其阴离子的主要结构形式。对于中性分子，反式构象更占优势，单阴离子以常规羧酸根阴离子形式存在，而乳清酸二阴离子以两种互变异构形式存在，比例约为4:1。研究发现，尽管对于乳清酸单阴离子可以注意到一些小的但具有特征性的差异，但DFT计算结果能够很好地重现实验（13）C核磁共振化学位移。在其他一些芳香族羧酸盐中也观察到了类似的差异。似乎我们体系中发生的特定溶质 - 溶剂相互作用可能是这种不一致的原因。为了支持这一假设，已经提出了一种将特定水合作用纳入理论计算的简单有效方法。
