Electron transfer and photoprotective properties of melanins in solution.

The polyquinoid nature of eumelanin(s) enables them to couple oxidation of electron donors with the reduction of electron acceptors. We have studied the ability of synthetic (Sigma) and "biological" (cuttlefish sepia) melanins to mediate electron transfer between hydroxybenzene donors (tyrosine, dopa, chemical depigmenters) and model acceptors (ferricyanide, tyrosinase). 1) Depending on the reductant, melanin either 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 accelerating 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 UV-induced damage to acid-soluble collagen, as judged by irreversible loss of intrinsic collagen fluorescence. Photoprotection against this type of damage appears primarily to involve optical absorption/scattering by the pigment.