Analysis of the ESR spectrum of synthetic dopa melanin

The 35 GHz ESR spectrum of frozen aqueous suspensions of synthetic melanin from autoxidation of dopa is asymmetric in the pH range 3-12. This asymmetry increases with increasing pH. A detailed computer analysis of second-derivative spectra suggests that the asymmetry is a result of two factors: approximately axial anisotropy of the g-tensor of the radical species; superposition of ESR spectra arising from a total of four radical species. The relative amounts of these individual spectra vary in a pH-dependent manner. Anisotropy of spectra varies with pH, probably due to pH-induced changes in the delocalization of the unpaired electrons. Thus the species present at high pH are suggested to be relatively localized radical anions, while the species detected at low pH are suggested to be protonated forms of the high pH species in which the unpaired electron is more extensively delocalized.