A model for interfacial electron transfer on colloidal melanin

Though melanins are involved in photochemical reactions (mainly of oxido-reductive type) in vitro and this activity is supposed to have biological implications, no satisfactory model of the reaction kinetics has so far been proposed. The main difficulty arises from the particulate structure of the insoluble melanins and the consequent necessity to describe their reactivity in the framework of heterogeneous chemistry, i.e., at the solid-liquid interface. Our paper presents a simplified model of the monoelectronic reduction reaction of dioxygen, based on well-established experimental facts and some reasonable assumptions: (1) surface adsorption of O-2 on colloidal melanins can be described by a Langmuir isotherm; (2) the kinetics of superoxide 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 semiconductor particles. Some satisfactory comparisons with experimental data and calculated values of the kinetic constants for the process are presented and discussed. (C) 1999 Elsevier Science S.A. All rights reserved.