An experimental and theoretical model for solar UVA-irradiation of soluble eumelanin: towards modelling UVA-photoreactions in the melanosome?

A model is developed for the UVA-irradiation of soluble eumelanin exposed to levels of irradiation comparable to sunlight. Radical production was determined in soluble DL- and L-dopa melanins exposed to solar levels of UVA, using electron spin resonance spectroscopy and the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). Steady-state concentrations of DMPO–O2H(radical dot), which increased up to 0.3 mg/ml melanin, and then declined above 0.3 mg/ml, were detected at pH 4.5. The kinetic model incorporated the photosensitising and radical-scavenging reactions of eumelanin, and assumed semiquinone radical reduction of oxygen to be fast compared to disproportionation. The model is consistent with experimental data for melanin concentrations <0.1 mg/ml; but >0.1 mg/ml melanin is consistent only with data at raised oxygen tension. The rate-constant for reaction of the melanin semiquinone-radical and oxygen is estimated to be 10^3 mol^-1 dm^3 s^-1. In this model, where DMPO competes with melanin for OH2(radical dot), at ambient oxygen levels, eumelanin exposed to solar levels of UVA photosensitises superoxide at concentrations <0.3 mg/ml melanin, and is increasingly stable towards oxidation when >0.3 mg/ml concentration. Eumelanin could have a negligible screening effect <0.1 mg/ml and very strong screening >1 mg/ml. This model would be biologically relevant if soluble forms of eumelanin were shown to exist in vivo, and is potentially useful for studies of the photochemistry and photophysics of eumelanin and phaeomelanin and to explore the effects of metal-ions, proteins and lipids in a model system.