Department of Chemistry, Inha University, Incheon 402-751, Korea.
Org Biomol Chem. 2013 Feb 28;11(8):1407-13. doi: 10.1039/c2ob26602d.
The tautomerization process of glycine between the neutral (NE) and zwitterionic (ZW) forms in aqueous solution was explored theoretically using the conductor-like polarizable continuum model (CPCM) by adopting the PAULING cavity model at the B3LYP, MP2 and CCSD levels with the 6-311+G(d,p) basis set. The tautomerization of glycine is unable to be predicted satisfactorily within the equilibrated framework of the CPCM method. Instead, in this study, three plausible non-equilibrated solvation situations were assumed: (S-1) one water molecule attached to the transferring proton in the ZW moves together with the transferring proton; (S-2) one water molecule attached to the transferring proton in the ZW remains motionless at a fixed position near the NH(2) fragment at the TS structure; and (S-3) proton transfer occurs without changing the position of the surrounding water molecules from their initial state, the ZW form, in the eight water clusters. Although the calculation of (S-3) failed, the Gibbs free energies of activation for tautomerization from the ZW to NE, ΔG(≠)(ZW → NE), was well consistent with the experimental findings in the hypothetical non-equilibrated solvation states of (S-1) and (S-2). This suggests that non-equilibrium solvation is essential to explain the observed experimental data.
在水溶液中，甘氨酸从中性（NE）到两性离子（ZW）的互变异构过程，采用 PAULING 空穴模型，在 B3LYP、MP2 和 CCSD 水平下，用 6-311+G(d,p)基组，通过导体类似极化连续模型（CPCM）进行了理论探讨。在 CPCM 方法的平衡框架内，甘氨酸的互变异构无法得到满意的预测。相反，在本研究中，假设了三种合理的非平衡溶剂化情况：（S-1）ZW 中转移质子上连接的一个水分子与转移质子一起移动；（S-2）ZW 中连接转移质子的一个水分子在 TS 结构中 NH2 片段附近的固定位置保持不动；（S-3）质子转移不改变其初始状态（ZW 形式）时周围水分子的位置，即在 8 个水分子簇中。尽管（S-3）的计算失败，但从 ZW 到 NE 的互变异构的吉布斯自由能活化能，ΔG(≠)(ZW → NE)，与（S-1）和（S-2）的假设非平衡溶剂化状态下的实验结果非常一致。这表明非平衡溶剂化对于解释观察到的实验数据是必不可少的。
