Catechin antioxidant action at various pH studied by cyclic voltammetry and PM3 semi-empirical calculations

The mechanism of catechin electro-oxidation at various pH was studied using cyclic voltammetry (CV) on the glassy carbon (GC) electrode and PM3 semi-empirical calculations. The influence of activation of the surface of the GC electrode on CV results has been discussed. Mixed adsorption-diffusion control has been observed by applying mechanistic criteria of CV to the results obtained at the activated electrode. The calculated catechin diffusion coefficient D = 2.78 x 10(-6) cm(2) s(-1). A linear increase of the current peak has been observed with the increase of substrate concentration up to 40 mu mol dm(-3) (surface coverage Gamma similar to 10(-11) Mol cm(-2)). In the whole investigated pH range, the dE/dpH value is very close to the anticipated Nernstian dependence of -59 mV/pH indicating that the slope is not affected by the different sequences of e(-) and H+ transfer. Molecular modeling results show a decrease of approximate to 5 kcal mol(-1) in Delta HoF (between radical and parent molecule) and a decrease of approximate to 6 eV in IP (of the parent molecule) when the parent molecule is changed from neutral to monoanionic form of catechin showing that both processes - hydrogen and electron abstraction are facilitated by deprotonation. Electrochemical oxidation of catechin is known to proceed as a two step one-electron oxidation of the B-ring of o-phenolic groups. Upon an increase in the pH, the mechanistic pathway of catechin electro-oxidation in both oxidation steps changes from an eH to the He process. In the reaction with a free radical, this may induce the change from hydrogen to electron donation. (c) 2005 Elsevier B.V. All rights reserved.