First electrochemical investigation of the redox properties of DOPA-melanins by means of a carbon paste electrode

The electrochemical properties of eumelanins, a cutaneous pigment responsible for skin color, insoluble in any solvent, was studied by using a carbon paste electrode. Eumelanins synthesized by enzymatic oxidation of L-DOPA were included in the carbon paste. The concentration of melanins (10%) and the potential scan rate (10(-4) V s(-1)) were chosen in ranges for which there is a linear relationship between these parameters and the current peak intensity. In these thin-layer conditions, the voltammogram of eumelanins at pH 5.6 reveals two main peaks in oxidation at + 460 and + 525 mV versus SCE and in reduction at + 20 and - 355 mV versus SCE. No variation of peak potential was observed when the pH was varied in the range 2-8. An average of two electrons (three for the first oxidation scan) were exchanged in these redox processes whereas the number of subunits in one molecule of melanin was around 15 as found by MALDI mass spectroscopy experiments. These results support the assumption of an irregular organization of monomer units in the structure of melanins. (C) 2000 Elsevier Science Ltd. All rights reserved.