EPR and spin-trapping investigation of free radicals from the reaction of 4-methoxybenzenediazonium tetrafluoroborate with melanin and melanin precursors

Abstract

The interaction of synthetic DOPA melanin (DM) and its precursors (catechols and phenols) with 4-methoxybenzenediazonium tetrafluoroborate (4-MeO-PhN 2 BF 4 ) has been studied using EPR spectroscopy and the spin-trapping technique. We found that DM, catechol, 3,4-dihydroxybenzoic acid, 3,4-dihydroxyhydrocinnamic acid, 3,4-dihydroxyphenylalanine, and 6-hydroxydopamine all react with 4-MeO-PhN 2 + through a one-electron-transfer process which gives rise to an aryl radical (4-MeO-Ph . ) derived from the diazonium compound and to radicals from melanin and from the catechol(amine)s. The formation of aryl radicals is an autocatalytic process. To explain the autocatalysis, we postulate a mechanism in which the key step is the formation of o-quinones