EPR OF MELANIN AT W BAND

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Various types of melanin (DOPA, 5-S-cystienylDOPA, dopamine, DHI) at a range of sample pH values have been studied by W-band EPR. The greatly enhanced resolution at high frequency allows for the unique g-anisotropies of different melanins to be studied as a function of pH. Melanins which have essentially identical spectra at X-band can be distinguished at W-band. Predictive MO theory helps one to choose between postulated structures for these species. Different semiquinone anion radicals have been identified in different melanins under alkaline conditions. These semiquinone radicals are primarily due to dihydroxyindole groups that are well hydrated. Melanin also contains a set of free radicals which are not semiquinone or semiquinone imines and vary little with pH. These radicals are associated with the less solubilized or condensed portion of the polymer.