Antioxidant properties of melatonin: a pulse radiolysis study

Various one-electron oxidants such as OH√, tert-BuO√, CCl3OO√, Br2√− and N3√, generated pulse radiolytically in aqueous solutions at pH 7, were scavenged by melatonin to form two main absorption bands with λmax = 335 nm and 500 nm. The assignment of the spectra and determination of extinction coefficients of the transients have been reported. Rate constants for the formation of these species ranged from 0.6-12.5 × 109 dm3 mo l−1 s−1. These transients decayed by second order, as observed in the case of Br2√− and N3√ radical reactions. Both the NO2√ and NO√ radicals react with the substrate with k = 0.37 × 107 and 3 × 107 dm3 mol−1 s−1, respectively. At pH not, vert, similar2.5, the protonated form of the transient is formed due to the reaction of Br2√− radical with melatonin, pKa ( MelH√ right harpoon over left Mel√ + H+) = 4.7 ± 0.1. Reduction potential of the couple (Mel√/MelH ), determined both by cyclic voltammetric and pulse-radiolytic techniques, gave a value E17 = 0.95 ± 0.02 V vs. NHE. Repair of guanosine radical and regeneration of melatonin radicals by ascorbate and urate ions at pH 7 have been reported. Reactions of the reducing radicals eaq− and H√ atoms with melatonin have been shown to occur at near diffusion rates.