A Planar Catechin Analogue Having a More Negative Oxidation Potential than (+)-Catechin as an Electron Transfer Antioxidant against a Peroxyl Radical

The hydrogen transfer reaction of antioxidative polyphenol with reactive oxygen species has proved to be the main mechanism for radical scavenging. The planar catechin (P1H₂), in which the catechol and chroman structure in (+)-catechin (1H₂) are constrained to be planar, undergoes efficient hydrogen atom transfer toward galvinoxyol radical, showing an enhanced protective effect against the oxidative DNA damage induced by the Fenton reaction. The present studies were undertaken to further characterize the radical scavenging ability of P1H₂ in the reaction with cumylperoxyl radical, which is a model radical of lipid peroxyl radical for lipid peroxidation. The kinetics of hydrogen transfer from catechins to cumylperoxyl radical has been examined in propionitrile at low temperature with use of ESR, showing that the rate of hydrogen transfer from P1H₂ is significantly faster than that from 1H₂. The rate was also accelerated by the presence of Sc(OSO₂CF₃)₃. Such an acceleration effect of metal ion indicates that the hydrogen transfer reaction proceeds via metal ion-promoted electron transfer from P1H₂ to oxyl radical followed by proton transfer rather than via a one-step hydrogen atom transfer. The electrochemical ease of P1H₂ for the one-electron oxidation investigated by second-harmonic alternating current voltammetry strongly supports the two step mechanism for hydrogen transfer, resulting in the enhanced radical scavenging ability.