2012 Volume 85 Issue 8 Pages 877-883
The kinetics for the reaction of the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) with artepillin C, a prenylated phenylpropanoid found specifically in Brazilian propolis, and its analogues was examined in deaerated acetonitrile (MeCN) to shed light on the mechanism for the radical-scavenging reaction of phenolic antioxidants as well as on the structure–activity relationship. Among the examined analogues, a compound having a catechol moiety is found to have the largest second-order rate constant (k) for the DPPH•-scavenging reaction. The deuterium kinetic isotope effect of 1.6 was observed for the DPPH•-scavenging reaction of artepillin C in the presence of 0.13 M CD3OD or CH3OH in deaerated MeCN at 298 K. The log k values were found to be linearly correlated with calculated energy difference values (DHT, HT: hydrogen transfer) between the artepillin C analogues and the corresponding phenoxyl radicals, while such a linear correlation cannot be observed between the log k values and calculated ionization potentials (IP), DHT − IP, or experimental one-electron-oxidation potentials of the artepillin C analogues. These results together with a calculated structure of the transition state for the reaction between the artepillin C analogue and DPPH• suggest that the DPPH•-scavenging reaction of the artepillin C analogues in deaerated MeCN proceeds via a one-step hydrogen-atom transfer from the phenolic OH group to DPPH• rather than an electron transfer followed by proton transfer.
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