ELECTRON-TRANSFER AND PHOTOPROTECTIVE PROPERTIES OF MELANINS IN SOLUTION

The polyquinoid nature of eumelanin(s) enables them to couple oxidatio n of electron donors with the reduction of electron accepters. We have studied the ability of synthetic (Sigma) and ''biological'' (cuttlefi sh sepia) melanins to mediate electron transfer between hydroxybenzene donors (tyrosine, dopa, chemical depigmenters) and model accepters (f erricyanide, tyrosinase). I) Depending on the reductant, melanin eithe r retards or accelerates ferricyanide reduction. Reaction kinetics are consistent with a mechanism involving non-interactive binding of both hydroxybenzene and ferricyanide to melanin prior to coupled electron transfer 2) Melanins also act as an electron conduit in markedly accel erating the tyrosinase-catalyzed oxygenation of p-hydroxyanisole (MMEH ). The active species appears to be a complex between melanin and MMEH . The magnitude of both effects depend on the type of melanin as well as its oxidation state. Sepia (eu)melanin appears to protect against U V-induced damage to acid-soluble collagen, as judged by irreversible l oss of intrinsic collagen fluorescence, Photoprotection against this t ype of damage appears primarily to involve optical absorption/scatteri ng by the pigment.