Kinetics and Mechanism for the Scavenging Reaction of the 2,2-Diphenyl-1-picrylhydrazyl Radical by Synthetic Artepillin C Analogues
作者:Tomonori Kawashima、Sushma Manda、Yoshihiro Uto、Kei Ohkubo、Hitoshi Hori、Ken-ichiro Matsumoto、Kiyoshi Fukuhara、Nobuo Ikota、Shunichi Fukuzumi、Toshihiko Ozawa、Kazunori Anzai、Ikuo Nakanishi
DOI:10.1246/bcsj.20120005
日期:2012.8.15
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.
在脱气乙腈 (MeCN) 中检查了 2,2-二苯基-1-三硝基苯肼基 (DPPH•) 与阿替匹林 C 的反应动力学,阿替匹林 C 是巴西蜂胶中特有的一种异戊二烯化苯丙素及其类似物。酚类抗氧化剂自由基清除反应的机理以及构效关系。在所检查的类似物中,发现具有儿茶酚部分的化合物对于DPPH·清除反应具有最大的二级速率常数(k)。在 0.13 M CD3OD 或 CH3OH 存在的脱气 MeCN 中,298 K 下,观察到阿替匹林 C 的 DPPH·清除反应为 1.6 的氘动力学同位素效应。发现 log k 值与计算的能量差值线性相关(DHT,HT:氢转移)在artepillin C类似物和相应的苯氧基自由基之间,而log k值和计算的电离势(IP)、DHT - IP或实验单电子之间无法观察到这种线性相关性artepillin C 类似物的氧化电位。这些结果以及计算出的阿替林 C 类似物与 DPPH• 之间反应的过渡态结构表明,阿替林 C 类似物在脱气 MeCN 中的 DPPH• 清除反应是通过一步氢原子转移进行的。酚OH基团转移至DPPH•,而不是先进行电子转移,然后进行质子转移。