Kinetics and mechanism for the scavenging reaction of the 2,2-diphenyl-1-picrylhydrazyl radical by synthetic artepillin c analogues

The kinetics for the reaction of the 2,2-diphenyl-1-picrylhydrazyl radical (DPPḢ) 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 structureactivity relationship. Among the examined analogues, a compound having a catechol moiety is found to have the largest second-order rate constant (k) for the DPPḢ-scavenging reaction. The deuterium kinetic isotope effect of 1.6 was observed for the DPPḢ-scavenging reaction of artepillin C in the presence of 0.13M CD ₃OD or CH ₃OH 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 1 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 DPPḢ suggest that the DPPḢ- scavenging reaction of the artepillin C analogues in deaerated MeCN proceeds via a one-step hydrogen-atom transfer from the phenolic OH group to DPPḢ rather than an electron transfer followed by proton transfer.