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

Article abstract of DOI:10.1021/ja00070a021

The interaction of synthetic DOPA melanin (DM) and its precursors (catechols and phenols) with 4-methoxybenzenediazonium tetrafluoroborate (4-MeO-PhN₂BF₄) 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-PhN₂⁺ 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. In aerated solutions the aryl radicals react with oxygen, which leads to oxygen consumption. The reaction was found to be order 1, 0.5, and 0.35 with respect to 4-MeO-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, tyrosine produces the most efficient reducing agent. This indicates that the conversion of phenols to o-dihydroxybenzene derivatives by tyrosinase is essential for aryl radical formation from 4-MeO-PhN₂⁺. These observations 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 pertinent to their biological, mutagenic, and carcinogenic action.