@misc{oai:repo.qst.go.jp:00070445,
 author = {中西, 郁夫 and カビル, ゾーダー and 川島, 知憲 and 上林, 將人 and 松本, 謙一郎 and 大久保, 敬 and 福住, 俊一 and 安西, 和紀 and 小澤, 俊彦 and 中西 郁夫 and カビル ゾーダー and 川島 知憲 and 上林 將人 and 松本 謙一郎},
 month = {May},
 note = {Melatonin (N-acetyl-5-methoxytryptamine), a pineal hormone, is known to show efficient antioxidative and radioprotective activities.  Recently, we have reported that galvinoxyl raidcal (GO) used as a model of reactive oxygen radical was scavenged by melatonin in methanol (MeOH).  However, the detailed molecular mechanism for the radical-scavenging reaction of melatonin has yet to be fully clarified.  In this study, we explored the effect of a metal ion on the radical-scavenging rates of melatonin in order to clarify the involvement of the electron-transfer process, which is known to be accelerated in the presence of redox inactive metal ions.
Upon addition of GO to a MeOH solution of melatonin at 298 K, the absorption band at 428 nm due to Go gradually decreased.  The decay of the absorbance at 428 nm obeyed pseudo-first-order kinetics, when the concentration of melatonin ([melatonin]) was maintained at more than 10-fold excess of the GO concentration.  The observed pseudo-first-order rate constant (kobs) increases linearly with increasing the melatonin concentration.  From the slope of the linear plot of kobs vs [melatonin] is determined the second-order rate constant (k) for the GO-scavenging reaction of melatonin to be 6.4 x 10(-2) M(-1) s(-1) in MeOH at 298 K.  When Mg(ClO4)2 (0.1 M) was added to the melatonin-GO system in MeOH, the k value was significantly increased (k = 1.4 x 10(-1) M(-1) s(-1)).  Such an acceleration in the presence of the metal ion suggests that the GO-scavenging reaction of melatonin proceeds via  a  proton-coupled electron transfer rather than via a one-step hydrogen atom transfer.  Similar results were obtained when GO was replaced by 2,2-diphyenyl-1-picrylhydrazyl radical (DPPH).  No radical-scavenging was observed when melatonin was replaced by N-methylmelatonin, suggesting that the deprotonation of the N-H hydrogen may be responsible for the radical-scavenging activity of melatonin.  The kinetic data together with the one-electron oxidation potential determined by the second-harmonic alternating current voltammetry (SHACV) as well as calculated thermodynamic parameters by the density functional theory (DFT) obtained in this study provide a detailed mechanistic insight into the antioxidative and radioprotective activities of melatonin., 第21回金属の関与する生体関連反応シンポジウム},
 title = {金属イオン存在下におけるメラトニンのラジカル消去反応},
 year = {2011}
}