Effects of Metal Ions Distinguishing between One-Step Hydrogen- and Electron-Transfer Mechanisms for the Radical-Scavenging Reaction of (+)-Catechin

Abstract

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(ClO4)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 Mg2+. 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 = OSO2CF3)]. 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.

A Planar Catechin Analogue as a New Type of Synthetic Antioxidant

“…Therefore, the eŠects of CA, PCA, and quercetin on hydroxyl radical-mediated DNA breakage were investigated (14). DNA-strand scission in supercoiled pBR322DNA was induced by a hydroxyl In agreement with previously published results, 15 the measured pro-oxidant eŠects of CA and quercetin may be attributed to autoxidation of the antioxidant in the presence of transition metal, leading to the generation of primary radicals such as the hydroxyl radical. In contrast to the pro-oxidant eŠects of CA and quercetin, the addition of PCA protected DNA from Fenton reaction-mediated damage at all concentrations tested.…”

“…We have recently reported that the hydrogen transfer from CA to a galvinoxyl or cumylperoxyl radical proceeds via electron transfer from CA to galvinoxyl or cumylperoxyl radical (15). This transfer is accelerated in the presence of metal ions such as Mg 2＋ or Sc 3＋ , followed by proton transfer.…”

Theoretical Elucidation of DPPH Radical-Scavenging Activity Difference of Antioxidant Xanthones

“…1) dominates in nonpolar solvents [7,8] and can be characterized by the X À H bond dissociation enthalpy (BDE) of the antioxidant [9 -11], whereas the second way (Eq. 2) is preferred in polar solvents [12,13] and can be measured by the ionization potential (IP) of the antioxidant or anions derived thereof [14,15]. Although low X-H BDE or low IP values are beneficial to enhance the Hatom-donating or electron-transfer power of the antioxidants, a extremely low IP will enhance the danger of generating a superoxide anion radical through transfer of the electron directly to surrounding O 2 [14].…”

Toward the prediction of the activity of antioxidants: Experimental and theoretical study of the gas-phase acidities of flavonoids

“…This loss can be visualized as the simultaneous release of a proton and of an electron [7,8]. The weaker the O™H bond dissociation energy the higher will be the antioxidant activity of the flavonoid.…”