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

Background: Gallic acid (3,4,5-trihydroxybenzoic acid) is found in a wide variety of plants; it is extensively used in tanning, ink dyes, as well as in the manufacturing of paper. The gallate moiety is a key component of many functional phytochemicals. In this work electron paramagnetic spectroscopy (EPR) was used to detect the free radicals generated by the air-oxidation of gallic acid. Results: 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 (a(H) = 1.00 G, a(H) = 0.23 G, a(H) = 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 (a(H) (2) = 1.07 G). Using D2O as a solvent, we demonstrate that the third hydrogen (i.e. a(H) = 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 pK(a) of this proton has been determined to be 10. Conclusions: 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.