Effects of Metal Ions Distinguishing between One-Step Hydrogen- and Electron-Transfer Mechanisms for the Radical-Scavenging Reaction of (+)-Catechin Ikuo Nakanishi,* ,† Kentaro Miyazaki, ‡,§ Tomokazu Shimada, ‡,§ Kei Ohkubo, ⊥ Shiro Urano, § Nobuo Ikota, † Toshihiko Ozawa, † Shunichi Fukuzumi,* ,⊥ and Kiyoshi Fukuhara* ,‡ Redox Regulation Research Group, Research Center for Radiation Safety, National Institute of Radiological Sciences, Inage-ku, Chiba 263-8555, Japan, DiVision of Organic Chemistry, National Institute of Health Sciences, Setagaya-ku, Tokyo 158-8501, Japan, Department of Applied Chemistry, Shibaura Institute of Technology, Minato-ku, Tokyo 108-8548, Japan, Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST, Japan Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan ReceiVed: May 28, 2002; In Final Form: September 10, 2002 A kinetic study of a hydrogen-transfer reaction from (+)-catechin (1) to galvinoxyl radical (G • ) has been performed using UV-vis spectroscopy in the presence of Mg(ClO 4 ) 2 in deaerated acetonitrile (MeCN). The rate constants of hydrogen transfer from 1 to G • determined from the decay of the absorbance at 428 nm due to G • increase significantly with an increase in the concentration of Mg 2+ . The kinetics of hydrogen transfer from 1 to cumylperoxyl radical has also been examined in propionitrile (EtCN) at low temperature with use of ESR. The decay rate of cumylperoxyl radical in the presence of 1 was also accelerated by the presence of scandium triflate [Sc(OTf) 3 (OTf ) OSO 2 CF 3 )]. These results indicate that the hydrogen-transfer reaction of (+)-catechin proceeds via electron transfer from 1 to oxyl radicals followed by proton transfer rather than via a one-step hydrogen atom transfer. The coordination of metal ions to the one-electron reduced anions may stabilize the product, resulting in the acceleration of electron transfer. Introduction Catechins contained in green tea are a class of bioflavonoids that show significant antioxidative activity. 1-9 It has been suggested that catechins trap radical species by hydrogen atom transfer from its phenolic OH group on the B ring. 10,11 However, little is known about the elementary steps of hydrogen-transfer reactions from catechins to radical species. There are two possibilities in the mechanism of hydrogen-transfer reactions from phenolic anitioxidants to radical species, i.e., a one-step hydrogen atom transfer or electron transfer followed by proton transfer. 12 Kusu et al. have recently reported the quantitative relationship between the antioxidative activity of catechins and their oxidation potentials determined by flow-through column electrolysis. 13 The galloylated catechins having more negative oxidation potentials than those of nongalloylated catechins show more efficient antioxidative activity on NADPH-dependent microsomal lipid peroxidation than the nongalloylated cat- echins. 13 On the other hand, based on the gas-phase bond dissociation enthalpy and ionization potential calculated by density functional method, Wright et al. have concluded that in most hydrogen-transfer reactions of phenolic antioxidants, hydrogen-atom transfer will be dominant rather than electron transfer. 12 Thus, it is still not clear whether the hydrogen-transfer reaction of catechins proceeds via a one-step hydrogen atom transfer or electron transfer followed by proton transfer. It has previously been demonstrated that the effect of Mg 2+ on the hydrogen-transfer rates from NADH (dihydronicotinamide adenine dinucleotide) analogues to aminoxyl or nitrogen radicals provides a reliable criterion for distinguishing between the one- step hydrogen atom transfer and the electron-transfer mecha- nisms. 14 We report herein the effect of metal ions, such as Mg 2+ and Sc 3+ , on the rates of hydrogen transfer from (+)-catechin (1) to oxyl radical species, such as galvinoxyl and cumylperoxyl radicals. Cumylperoxyl radical, which is much less reactive than alkoxyl radicals, is known to follow the same pattern of relative reactivity with a variety of substrates. 15-17 The detailed kinetic studies provide valuable mechanistic insight into the mechanism of the antioxidative reactions of 1: whether the reaction between 1 and oxyl radical species proceeds via one-step hydrogen atom transfer or via electron transfer. Experimental Section Materials. (+)-Catechin (1) was purchased from Sigma. Galvinoxyl radical (G • ) was obtained commercially from Aldrich. Mg(ClO 4 ) 2 and acetonitrile (MeCN; spectral grade) were purchased from Nacalai Tesque, Inc., Japan, and used as received. Di-tert-butyl peroxide was obtained from Nacalai Tesque Co., Ltd., and purified by chromatography through alumina which removes traces of the hydroperoxide. Cumene was purchased from Wako Pure Chemical Ind. Ltd., Japan. Scandium trifluoromethanesulfonate, Sc(OTf) 3 (OTf ) OSO 2 - CF 3 , 99%) was obtained from Pacific Metals Co., Ltd. (Taiheiyo Kinzoku). Propionitrile (EtCN) used as solvent was purified and dried by the standard procedure. 18 * Corresponding authors. † National Institute of Radiological Sciences, Japan. ‡ National Institute of Health Sciences, Japan. § Shibaura Institute of Technology, Japan. ⊥ Osaka University. 11123 J. Phys. Chem. A 2002, 106, 11123-11126 10.1021/jp026190c CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002