EPR AND SPIN-TRAPPING INVESTIGATION OF FREE-RADICALS FROM THE REACTION OF 4-METHOXYBENZENEDIAZONIUM TETRAFLUOROBORATE WITH MELANIN AND MELANIN PRECURSORS

The interaction of synthetic DOPA melanin (DM) and its precursors (cat echols and phenols) with 4-methoxybenzenediazonium tetrafluoroborate ( 4-MeO-PhN2BF4) has been studied using EPR spectroscopy and the spin-tr apping technique. We found that DM, catechol, 3,4-dihydroxybenzoic aci d, 3,4-dihydroxyhydrocinnamic acid, 3,4-dihydroxyphenylalanine, and 6- hydroxydopamine all react with 4-MeO-PhN2+ through a one-electron-tran sfer process which gives rise to an aryl radical (4-MeO-Ph.) derived f rom the diazonium compound and to radicals from melanin and from the c atechol(amine)s. The formation of aryl radicals is an autocatalytic pr ocess. To explain the autocatalysis, we postulate a mechanism in which the key step is the formation of o-quinones. In aerated solutions the aryl radicals react with oxygen, which leads to oxygen consumption. T he reaction was found to be order 1, 0.5, and 0.35 with respect to 4-M eO-PhN2+, catechol, and oxygen concentration, respectively. Phenol, 4- hydroxyanisole, and tyrosine do not reduce 4-MeO-PhN2+ unless they are activated by the enzyme tyrosinase. In the presence of tyrosinase, ty rosine produces the most efficient reducing agent. This indicates that the conversion of phenols to o-dihydroxybenzene derivatives by tyrosi nase is essential for aryl radical formation from 4-MeO-PhN2+. These o bservations substantiate the ability of hydroquinones and semiquinone radicals to promote the homolysis of diazonium salts to generate aryl radicals. Such reductive activation of diazonium compounds may be pert inent to their biological, mutagenic, and carcinogenic action.