Novel free radicals in synthetic and natural pheomelanins: distinction between dopa melanins and cysteinyldopa melanins by ESR spectroscopy.

Abstract

Synthetic pheomelanins from enzymic oxidation of the dopa derivative 5-S-cysteinyldopa were examined by ESR spectroscopy. These alkali-soluble polymers contain a novel kind of free radical that is spectroscopically distinct from that found in eumelanins. Delocalization of the unpaired electron onto a N atom and the ability of the radical to chelate complexing metal ions strongly suggest an o-semiquinonimine structure. The synthetic pheomelanin was compared with natural red pigments extracted from human red hair and from red chicken feathers. Spectroscopically, the chicken feather pheomelanin is almost identical to synthetic cysteinyldopa pheomelanin. In contrast, the pigment from red hair has a major spectral component very similar to that found in dopa melanin, with a smaller component corresponding to that found in cysteinyldopa melanin.