Antioxidant properties of melatonin: A pulse radiolysis study

Various one-electron oxidants such as OH., tert-BuO., CCl3OO., 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 lambda(max) = 335 nm and 500 nm. The assignment of the spectra and determination of exti nction coefficients of the transients have been reported. Rate constants fo r the formation of these species ranged from 0.6-12.5 x 10(9) dm(3) mol(-1) s(-1). These transients decayed by second order, as observed in the case o f Br-2(.-) and N-3(.) radical reactions. Both the NO2. and NO. radicals rea ct with the substrate with k = 0.37 x 10(7) and 3 x 10(7) dm(3) mol(-1) s(- 1), respectively, At pH similar to 2.5, the protonated form of the transien t is formed due to the reaction of Br-2(.-) radical with melatonin, pK(a) ( MelH(.) reversible arrow Mel(.) + H+) = 4.7 +/- 0.1. Reduction potential o f the couple (Mel(.)/MelH), determined both by cyclic voltammetric and puls e-radiolytic techniques, gave a value E-7(1) = 0.95 +/- 0.02 V vs. NHE. Rep air of guanosine radical and regeneration of melatonin radicals by ascorbat e and urate ions at pH 7 have been reported. Reactions of the reducing radi cals e(aq)(-) and H . atoms with melatonin have been shown to occur at near diffusion rates. (C) 1999 Elsevier Science Inc.