Free radicals produced by the oxidation of gallic acid: An electron paramagnetic resonance study

We found that gallic acid produces two different radicals as a function of pH. In the pH range between 7-10, the spectrum of the gallate free radical is a doublet of triplets (aH = 1.00 G, aH = 0.23 G, aH = 0.28 G). This is consistent with three hydrogens providing hyperfine splitting. However, in a more alkaline environment, pH >10, the hyperfine splitting pattern transforms into a 1:2:1 pattern (aH (2) = 1.07 G). Using D2O as a solvent, we demonstrate that the third hydrogen (i.e. aH = 0.28 G) at lower pH is a slowly exchanging hydron, participating in hydrogen bonding with two oxygens in ortho position on the gallate ring. The pKa of this proton has been determined to be 10.This simple and novel approach permitted the understanding of the prototropic equilibrium of the semiquinone radicals generated by gallic acid, a ubiquitous compound, allowing new insights into its oxidation and subsequent reactions.Gallic acid (3,4,5-trihydroxybenzoic acid), found in a variety of plants, is extensively used in tanning, ink dyes, as well as in the manufacturing of paper [1]. In addition, the gallate moiety is a key component of many foods and drinks, e.g. there are two gallate moieties in the important polyphenol, (-)-epi-gallocatechin-3-gallate (EGCG); this and related polyphenols are responsible for the antioxidant, anticarcinogenic, and antiviral properties of some of the most widely consumed beverages in the world, such as green tea [2,3]. The three aromatic phenoxyl groups of gallic acid are prone to oxidation with the formation of hydrogen peroxide, quinones, and semiquinones [4]. We have observed the formation of two distinct semiquinones formed upon the oxidation of gallic acid. Here we have investigated the nature of these two different radicals.Gallic acid (3,4,5-trihydroxybenzoic acid, CAS No.: 149-91-7) was obtained from Sigma, USA; sodium hydroxide was from Fisher. All experiments were carried out in 100 mM potassium phosphate. The final concentration of gallic ac