Deprotonation Mechanism of Methyl Gallate: UV Spectroscopic and Computational Studies

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 pK(a) values of MeG were evaluated using the chemometric modeling method. The first acid dissociation constant (pK(a1)) for MeG was evaluated to be 4.20 ± 0.01, and the second one (pK(a2)) was 10.78 ± 0.06.