Oxygen adsorption and photoreduction on fractal melanin particles

The main putative functions of melanins in living cells. that is photoprotection and dark or light-dependent oxido-reductive activity, still requires an interpretation which takes into account the micro-mesoscopic structure of native melanin particles. It is indeed well established that a different chemical composition of melanins, even if derived from different biosynthetic pathways. has only a little influence on the biological and physical properties of the solid aggregates. the common form in which the pigment is found [P.R. Crippa et al., Chemistry of melanins, in: A. Brossi (Ed.) The Alkaloids, vol. 36, Academic Press. New York, 1989. pp. 253-323]. In the present work a model for interfacial electron transfer is proposed describing the process of light induced superoxide formation through a monoelectronic reduction of dioxygen adsorbed on melanin solid surface. This process is presumed to be dependent on the surface fractal characteristics. and its kinetics must be interpreted as a heterogeneous interfacial reaction involving light produced carriers and the adsorbed acceptor. like in colloidal inorganic semiconductors such as TiO2. (C) 2001 Elsevier Science B.V. All rights reserved.