Reduction of Substituted p-Benzoquinones by FeII Near Neutral pH

Abstract

The oxidation of dihydroxyaromatics to benzoquinones by FeIII (hydr)oxides is important in respiratory electron shuttling by microorganisms and has been extensively studied. Prior publications have noted that the Gibbs Free Energy (ΔG) for the forward reaction is sensitive to dihydroxyaromatic structure, pH, and concentrations of reactants and products. Here, we address the back reaction, benzoquinone reduction by FeII. Rates markedly increase with increasing pH, in accord with increases in ΔG. Ring substituents that raise the potential of the p-benzoquinone/hydroquinone half reaction raise reaction rates: –OCH3 < –CH3 < –C6H5 < –H < –Cl. p-Naphthoquinone, with a reduction potential lower than the five substituted p-benzoquinones just listed, yields the lowest reaction rates. The complexity of the reaction is reflected in lag periods and less-pronounced S-shaped time course curves. Benzoquinone reduction by FeII may be an important link in networks of electron transport taking place in suboxic and anoxic environments.