Single or double hydrogen atom transfer in the reaction of metal - Associated phenolic acids with •OH radical: DFT study

Results of quantum chemical calculations based on B3LYP/6-31+G* are reported for 3,4-dihydroxyphenylpyruvic acid (3,4-DHPPA), 3,4-dihydroxycinnamic acid (3,4-DHCA) and 3,4-dihydroxybenzoic acid (3,4-DHBA) chelated with Cu2+, Mg2+ and Ca2+. These results are discussed in regard to the capacity of these phenolic acids (PAs) to scavenge free radicals in the presence or the absence of a metal ion. The OH bond dissociation enthalpies, the HOMO eigenvalues, the NBO charges and the structural changes are parameters that seem to be the best indicators of the antiradical property of these PAs. These parameters indicate that the antioxidant activity of the PAs are ordered as follows: 3,4-DHPPA > 3,4-DHCA > 3,4-DHBA. The C7C8 double bond, and the O8H8···O9 intramolecular hydrogen bond present in the side chain are influential in determining this activity. We also showed that the chelation with a metal ion led to the oxidation of the PAs and the reduction of the metal ion, a behaviour evidenced by charge transfer from the PAs to the ion. This tendency is more pronounced for the copper ion for which we registered a charge transfer of roughly 1.02e. Chelation with a metal ion led to important structural changes at the site of chelation and contributed to the decrease of the Bond Dissociation Enthalpy (BDE). It greatly facilitated the hydrogen atom transfer and consequently enhanced the antioxidant activity of the PAs.