Abstract
Bleaching of eumelanin has been studied in model systems consisting of synthetic dopa-melanin and various concentrations of hydrogen peroxide, molecular oxygen, and copper(II) ions at neutral and alkaline pH. The data show that at neutral pH, in the dark, metal-ion-free melanin is very resistant to oxidation by hydrogen peroxide. However, the rate of bleaching of melanin, induced by H2O2 is significantly accelerated by illumination from UVA-visible light. Bound-to-melanin copper(II) also accelerates the bleaching of melanin with the efficiency dependent on concentration of H2O2 and oxygen. It suggests possible involvement of melanin-copper complexes in Fenton-like processes. The formation of hydroxyl radicals during melanin bleaching has been concluded on the basis of the electrochemical detection of hydroxylation products of salicylate used as OH scavenger. Redox conversion of bound-to-melanin copper ions was monitored by EPR spectroscopy and direct measurement of melanin-Cu(II) complexes. It has been shown that melanin-copper(I) complexes were readily oxidized by either oxygen or hydrogen peroxide. The data indicate that bleaching of melanin is a complex process with two distinct stages, reversible oxidation of the hydroquinone moieties of melanin followed by irreversible reactions of the monomers that lead to degradation of the melanin polymer.
Highlights
Wisconsin 53226 and the Department of Biophysics, Bleaching of eumelanin has been studied in model systems consisting of synthetic dopa-melanin and various concentrations of hydrogen peroxide, molecular oxygen, and copper(I1) ions at neutral and alkaline pH
The formation of hydroxyl radicals during melanin bleaching has been concluded on the basis of the electrochemical detection of hydroxylation products of salicylate used as OH scavenger
It has been shown that melanincopper(I) complexes were readily oxidized by either oxygen or hydrogen peroxide
Summary
Wisconsin 53226 and the Department of Biophysics, Bleaching of eumelanin has been studied in model systems consisting of synthetic dopa-melanin and various concentrations of hydrogen peroxide, molecular oxygen, and copper(I1) ions at neutral and alkaline pH. The data indicate that bleaching of melanin is a complex process with two distinct stages, reversible oxidation of the hydroquinone moieties of melanin followed by irreversible reactions of the monomers that lead to degradation of the melanin polymer. It has been shown that melanin can efficiently interact with reactive species: hydroxyl radicals and hydrated electrons (lo), singlet oxygen [11, 12], and superoxide anion [13, 14] Since such species can, under certain conditions, be formed in photochemical processes, this melanin property is consistent with its photoprotective role.
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