Reactivity of dpph• in the oxidation of catechol and catechin

A kinetic study of the reduction of pyrocatechol and catechin by dpph^? radical has been carried out in various ratios of CH₃OH/H₂O mixed solvent at pH 5.5–7.5, μ = 0.10 M (n‐Bu)₄N]ClO₄, and T = 25°C. The rate constants of oxidation in aqueous solvent, k, were obtained from the extrapolation of the linear plots of the specific rate constants k vs. % H₂O plots at each pH value. A linear relationship between k and 1/H⁺] was observed for both flavonoids with k = k₁K_a1/H⁺], where K_a1 was the first acid dissociation constant on the catechol ring and k₁ is the rate constant of the oxidation of the mononegative species HX^?. The values of k₁ obtained from the slopes of the plots are (8.2 ± 0.2) × 10⁵ and (6.1 ± 0.1) × 10⁵ M^?1 s^?1 for pyrocatechol and catechin, respectively. The analysis of the reaction on the basis of Marcus theory for an outer‐sphere electron transfer reaction yielded a value of 3.7 × 10³ M^?1 s^?1 for the self‐exchange rate constant of dpph^?/dpphH couple. © 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 147–153, 2011