Abstract
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 pKa values of MeG were evaluated using the chemometric modeling method. The first acid dissociation constant (pKa1) for MeG was evaluated to be 4.20 ± 0.01, and the second one (pKa2) was 10.78 ± 0.06.
Highlights
The structure of plant tannins has a large number of phenolic groups that can be deprotonated and form complexes with metals, especially transition metals [1]
Coordination of tannins with metal ions has been suggested through the phenolic groups based on the metal coordination properties of the simpler phenolics, such as methyl gallate and catechin [3,4,5]
(I) whether the phenolic hydroxyls in the galloyl moiety deprotonated simultaneously? If not, which phenolic hydroxyl is much easier to be deprotonated during metal binding reactions? (II) Is there is an oxidization reaction happening during the deprotonation reaction? (III) Can the phenolic groups can be oxidized to be quinoid structures at the higher pH? In this paper, a simple phenolic compound methyl gallate, which could be regarded as the monomer of hydrolysable tannins, was used as a model compound to study the deprotonation mechanism of phenolic groups in methyl gallate
Summary
The structure of plant tannins has a large number of phenolic groups that can be deprotonated and form complexes with metals, especially transition metals [1]. A large number of studies [6,7,8,9] have shown that the chelation of metal ions by phenolic compounds preferentially occurs at the deprotonated catechol site. It is well known that the catechol moiety is a powerful complexing agent for some metal ions, especially for transition metal ions and, despite its extremely high acid dissociation constant (pKa ) values, this function is able to coordinate metal ions at very low pH with a complete deprotonation of the hydroxyl groups. There has been no detailed study on deprotonation reaction of phenolic groups in the molecular structure of polyphenols and tannins. If not, which phenolic hydroxyl is much easier to be deprotonated during metal binding reactions?
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
