Redox behavior of melanins: direct electrochemistry of dihydroxyindole-melanin and its Cu and Zn adducts

Synthetic melanin films, formed on electrode surfaces by oxidative polymerization of 5,6-dihydroxyindole solution, were used to directly measure the chromophore's redox reactivity. Films on optically transparent indium-tin oxide (ITO) electrodes allow correlation of spectral changes with electrochemical potential. Spectroelectrochemical titrations show an initial reversible transformation that is ascribed to formation of a unique quinone-imine chromophore. The apparent E(1/2) for maximum quinone-imine formation is approximately 125 mV (vs. Ag/AgCl) but at potentials higher than 100 mV, an irreversible bleaching is evident. Correlation of the current with the monomer concentration implies that only one in six monomers is oxidized to the quinone-imine before the irreversible bleaching occurs. Films pretreated with CuCl(2) and Zn(CH(3)COO)(2) show elevated quinone-imine absorbances, even under reducing conditions, indicating a preferential stabilization of this state by coordination to the metals.