Antioxidant properties of melatonin: a pulse radiolysis study

Abstract

Various one-electron oxidants such as OH , tert-BuO , CCl 3OO , Br 2 − and N 3 , generated pulse radiolytically in aqueous solutions at pH 7, were scavenged by melatonin to form two main absorption bands with λ max = 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 × 10 9 dm 3 mo l −1 s −1. These transients decayed by second order, as observed in the case of Br 2 − and N 3 radical reactions. Both the NO 2 and NO radicals react with the substrate with k = 0.37 × 10 7 and 3 × 10 7 dm 3 mol −1 s −1, respectively. At pH ∼2.5, the protonated form of the transient is formed due to the reaction of Br 2 − radical with melatonin, pK a ( MelH ⇌ 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 E 1 7 = 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 e aq − and H atoms with melatonin have been shown to occur at near diffusion rates.