Involvement of Electron Transfer in the Radical-Scavenging Reaction of Melatonin in a Protic Medium

The antioxidative activity of melatonin (N-acetyl-5-methoxytryptamine), a neurohormone secreted by the pineal gland, has attracted much attention in terms of the prevention of diseases associated with oxidative stress as well as radiation damage. However, there has been a controversy about the radical-scavenging reactivity of melatonin. In this study, we investigated the reaction of melatonin with galvinoxyl (GO) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals.Upon addition of GO radical to a methanol (MeOH) solution of melatonin at 298 K, the absorption band at 428 nm due to GO radical was gradually decreased. When MeOH was replaced by acetonitrile, no consumption of GO radical was observed. From the decay of the absorbance at 428 nm, the second-order rate constant (k) was determined to be 6.7 x 10(-2) M(-1) s(-1) in MeOH. When Mg(ClO(4))(2) (0.1 M) was added to the melatonin-GO radical system, the k value was increased significantly (k = 1.5 x 10(-1) M(-1) s(-1)). Such an acceleration by magnesium ion (Mg(2+)) indicates that the GO radical-scavenging reaction of melatonin proceeds via an electron transfer from melatonin to GO radical followed by proton transfer, since redox-inactive metal ions are known to promote electron-transfer reactions. Similar results were obtained, when GO radical was replaced by DPPH radical. The kinetic data together with the thermodynamic parameters calculated by the density functional theory provide valuable and fundamental information about the radical-scavenging reactivity of melatonin.The 16th Biennial Meeting for the Society for Free Radical Research International (SFRRI 2012