A model for interfacial electron transfer on colloidal melanin

Though melanins are involved in photochemical reactions (mainly of oxido-re ductive type) in vitro and this activity is supposed to have biological imp lications, no satisfactory model of the reaction kinetics has so far been p roposed. The main difficulty arises from the particulate structure of the i nsoluble melanins and the consequent necessity to describe their reactivity in the framework of heterogeneous chemistry, i.e., at the solid-liquid int erface. Our paper presents a simplified model of the monoelectronic reducti on reaction of dioxygen, based on well-established experimental facts and s ome reasonable assumptions: (1) surface adsorption of O-2 on colloidal mela nins can be described by a Langmuir isotherm; (2) the kinetics of superoxid e formation is photodependent and includes an interfacial electron-transfer process; (3) the photochemical behaviour of the single melanin granule can be described in terms of the electronic properties of amorphous semiconduc tor particles. Some satisfactory comparisons with experimental data and cal culated values of the kinetic constants for the process are presented and d iscussed. (C) 1999 Elsevier Science S.A. All rights reserved.