Synthetic pheomelanins from enzymic oxidation of the 3,4-dihydroxyphenylalanine (dopa) derivative 5-S-cysteinyldopa have been examined by ESR spectroscopy. These alkalisoluble polymers contain a novel kind of free radical that is spectroscopically distinct from that found in eumelanins. Delocalization of the unpaired electron onto a nitrogen 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.Melanin pigmentation ofhair, skin, and eye is thought to result from two separate but biogenetically interrelated classes ofpigments: eumelanins (black-brown, insoluble in dilute alkali) and pheomelanins (yellow to reddish-brown, alkali soluble) (1). Eumelanins are generally considered to be derived from the enzymic oxidation of tyrosine through 3,4-dihydroxyphenylalanine (dopa) (structure I) (2), whereas pheomelanins arise by a diversion of the eumelanin pathway through the intervention of cysteine or related sulfhydryl compounds (3). A major intermediate in the biosynthesis of pheomelanins is the dopa metabolite 5-S-cysteinyldopa (structure II).

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