Free radical chemistry of flavan-3-ols:: Determination of thermodynamic parameters and of kinetic reactivity from short (ns) to long (ms) time scale

The physical chemistry and the free radical chemistry of the most abundant polyphenolic flavan-3-ols in food, catechin, its methylated metabolites, and several methylated analogues, have been investigated by laser flash photolysis and cyclic voltammetry studies. Two independent phenoxyl radicals formed upon oxidation of flavan-3-ols have been characterized and identified unambiguously: a short-lived resorcinol-like radical characterized by an absorption band at lambda = 495 nm and a long-lived catechol-like transient absorbing at A = 380 nm. The determination of all the thermodynamic constants of each phenolic function of flavan-3-ols, namely, redox potential (Edegrees(3)', = 0.13(5) V/SCE, E(4)degrees' = 0.11(0) V/SCE, E(5)degrees = 0.285 V/SCE) and microscopic dissociation constants (pK(a3)' = 9.02, pK(a4)' = 9.02, pK(a5) = 9.43, pK(a7) = 9.58) were performed. These values are discussed and compared to the prediction of the density functional theory calculations made on the different species catechin, catechin phenoxyl, and catechin phenolate for each phenolic site.