Phenolcarboxylic Acids in the Reaction with Electrogenerated Hydroperoxyl Radical
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olcarboxylic Acids in the Reaction with Electrogenerated Hydroperoxyl Radical N. I. Belayaa,*, A. V. Belyia, O. M. Zarechnayab, I. N. Shcherbakovc, and V. S. Doroshkevicha a
b
Donetsk National University, Donetsk, 83001 Ukraine L.M. Litvinenko Institute of Physical and Organic Chemistry and Coal Chemistry, Donetsk, 83114 Ukraine c Southern Federal University, Rostov-on-Don, 344006 Russia *e-mail: [email protected] Received May 21, 2020; revised May 21, 2020; accepted June 1, 2020
Abstract—Using the chronovoltammetric method, it has been shown that the reduction of oxygen on a mercury electrode in the presence of phenolcarboxylic acids in an acidic medium occurs as reversible electrode process followed by rate-limiting chemical reaction involving the hydroperoxyl radical. The rate constants of the phenolcarboxylic acids reaction with the radical have been calculated using the theory of electrode process followed by chemical reaction. The suggested reaction mechanism (electron transfer from the acid molecular form to the radical) has been confirmed by the correlation between the experimental rate constants and the ionization potentials of phenolcarboxylic acids simulated using the DFT method. Keywords: phenolcarboxylic acid, hydroperoxyl radical, antioxidant, ionization potential
DOI: 10.1134/S1070363220100023 Phenolcarboxylic acids form one the most numerous groups of plant phenolic compounds. Their key structural element is the phenolic OH group which determines their antiradical and antioxidant activity [1, 2]. Due to the presence of the carboxylic group COOH ionized at low рН value, phenolcarboxylic acids can react with radicals in acidic media in the form of carboxylate ions, more reactive than the neutral molecules [3, 4]. The ability of phenolcarboxylic acids to deactivate the oxygen active forms (О2•–, НОО•, НО•, etc.) is the most important manifestation of their diverse biological activity [5–7]. The active oxygen forms are involved in the crucial physiological processes yet can be harmful to human organism at high concentration. The protective mechanism which controls such negative effects utilized the properties of natural antioxidants including phenolcarboxylic acids and their derivatives. The study of kinetics and mechanism of radical oxidation of the latter and elucidation of molecular descriptors determining the process rate are promising fields of research in chemistry, biochemistry, medicine, and pharmaceutics. We investigated the antiradical activity of phenolcarboxylic acids (НООСArОН) and their esters (ROOCArOH) in the reaction with electrogenerated
hydroperoxyl radical НОО• by means of chronovoltammetry and quantum-chemical simulation. The reaction occurred during electroreduction of О2 into Н2О2 on a mercury film electrode at рН = 2. The considered compounds included the mono- (1‒6) and dihydroxybenzoic acids and their esters (7‒14), representatives of plant phenolcarboxylic compounds [8] (Scheme 1). The protonated form of the superoxide anion-radical O2•– (the hydroperoxyl radical HOO•
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