Electron transfer properties of melanin. Optimum conditions and the effects of various chemical treatments.

Abstract

The optimum conditions for three oxidation–reduction reactions of melanin, viz. the oxidation of NADH, the reduction of ferricyanide, and a coupled reaction involving the oxidation NADH and the reduction of ferricyanide, have been studied. Experiments in which the pH of the reaction medium was varied showed that the rate of oxidation of NADH slightly increased when the pH was varied from 6.05 to 8.5, the reduction of ferricyanide markedly increased when the pH was increased from 5.5 to 8.5, and the oxidation of NADH in the coupled reaction increased when the pH was increased from 5.5 to 8.5 and the reduction of ferricyanide increased from pH 5.5 to 7.2. Experiments using varying temperatures showed that the rates of all the above reactions increased to varying degrees when the temperature was increased from 23 to 45 °C; the activation energies for the reactions were calculated. The effects of various treatments of melanin upon the above reactions were studied. Treatment of melanin with oxidizing agents such as KMnO4, benzoquinone, iodine, FeCl3, etc. increased the oxidation of NADH by melanin. Treatment of melanin with the reducing agent ascorbate decreased the oxidation of NADH by melanin, and treatment of melanin with KMnO4 decreased the reduction of ferricyanide by melanin. These results support the possibility that the oxidized and reduced forms of melanin may be participating in the reducing and oxidizing activities of melanin.