Publication
559
Chem.
Mater., 124 (47), 14027 -14038, 2002
DOI: 10.1021/ja0262434 S0002-7863(02)06243-1 |
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Free Radical Chemistry of Flavan-3-ols: Determination of Thermodynamic
Parameters and of Kinetic Reactivity from Short (ns) to
Long (ms) Time Scale
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Cécile
Cren-Olivé, Philippe Hapiot, Jean Pinson, and Christian
Rolando
Contribution from the Laboratoire
de Chimie Organique et Macromoléculaire, Equipe
Polyphénols, Université des Sciences
et Technologies de Lille, UMR CNRS 8009, Bâtiment
C4, 59655 Villeneuve d'Ascq Cedex, France, Laboratoire
d'Electrochimie Moléculaire et Macromoléculaire,
Synthèse et Electrosynthèse Organiques,
Université de Rennes 1, UMR CNRS 6510, Campus
de Beaulieu, Bat. 10 C, 35042 Rennes Cedex, France,
and Laboratoire d'Electrochimie Moléculaire,
Université Denis Diderot (Paris 7), UMR CNRS
7591, 2 Place Jussieu, 75251 Paris Cedex 05, France
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  =
495 nm and a long-lived catechol-like transient absorbing at = 380 nm. The determination of all the thermodynamic constants of each
phenolic function of flavan-3-ols, namely, redox potential (E°3' =
0.135 V/SCE, E°4' = 0.110 V/SCE, E°5 =
0.285 V/SCE) and microscopic dissociation constants (pKa3' =
9.02, pKa4' = 9.12, pKa5 = 9.43,
pKa7 = 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. |
