Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing 210042, China.
Int J Mol Sci. 2018 Oct 11;19(10):3111. doi: 10.3390/ijms19103111.
In the present paper, methyl gallate (MeG), a simple polyphenol and also the monomer of hydrolysable tannins, was selected to study the deprotonation process for the hydroxyls of the galloyl group by the combined use of spectroscopic measurements and quantum chemical calculations. The results of quantum chemical calculations show that the deprotonated form of methyl gallate undergoes the para-quinoid localization in the benzene ring, compared with free methyl gallate. The predicted spectra obtained from the free and deprotonated methyl gallate models are in agreement with the experimental UV-visible (UV-vis) absorption spectra. In the same way, the vibrational spectra of the para-quinoid MeG models validate the proposed mechanism of the deprotonation of MeG molecule. The pH influence on the deprotonation reaction and oxidization of phenolic groups has been also investigated. The p values of MeG were evaluated using the chemometric modeling method. The first acid dissociation constant (p) for MeG was evaluated to be 4.20 ± 0.01, and the second one (p) was 10.78 ± 0.06.
在本论文中，我们选择没食子酸甲酯（MeG）作为研究对象，它是一种简单的多酚类化合物，也是缩合鞣质的单体。我们采用光谱测量和量子化学计算相结合的方法研究了没食子酰基上的酚羟基的去质子化过程。量子化学计算的结果表明，与游离的没食子酸甲酯相比，去质子化的没食子酸甲酯在苯环上经历了对醌型的局部化。从游离和去质子化的没食子酸甲酯模型获得的预测光谱与实验的紫外可见（UV-vis）吸收光谱一致。同样，对醌型 MeG 模型的振动光谱验证了 MeG 分子去质子化的提议机制。我们还研究了 pH 值对酚羟基去质子化反应和氧化的影响。我们使用化学计量建模方法评估了 MeG 的 p 值。MeG 的第一个酸离解常数（p）被评估为 4.20±0.01，第二个酸离解常数（p）为 10.78±0.06。
