OH Radical Induced One-Electron Oxidation of Serotonin and Tryptamine

The interactions of the one-electron oxidants, N 3 • and Br 2 •- radicals, with serotonin, tryptamine and their analogues 5-methoxytryptamine, N -methyltryptamine and N(1)-methyltryptophan were studied using the technique of pulse radiolysis with spectrophotometric detection. One-electron oxidation of serotonin results in the formation of an indoloxyl radical with a p K a value ≪3. With tryptamine, N -methyltryptamine and 5-methoxytryptamine, their one-electron oxidation gives indolyl radicals with p K a values of 4.2, 4.3 and 4.8, respectively. The reactions of OH radicals ( • OH) with serotonin and tryptamine lead to the formation of the respective • OH-adducts, which decay by acid catalyzed water elimination to give almost quantitatively the corresponding indoloxyl and indolyl radicals, respectively. The first-order rate constants determined for water elimination are pH dependent, suggesting that the dehydration reaction is acid and base catalyzed. The • OH- adduct of serotonin reacts with oxygen in competition with the dehydration reaction to yield a peroxyl radical adduct, which is tentatively suggested to eliminate HO 2 • . On the basis of the above findings, the mechanisms for the • OH -induced formation of the indoloxyl and indolyl radicals from serotonin and tryptamine respectively are proposed.