Electron spin resonance study of melanin treated with reducing agents.

The electron spin resonances (ESR) of several native and modified melanins have been determined. Melanins isolated from black wool and synthesized from 3,4-dihydroxyl-L-phenylalanine (L-DOPA) and tyrosine all show similar ESR signals. Modification of the isolated melanins by treatment with reducing agents causes some lightening in color and slight changes in the ESR spectra. Lithium and liquid ammonia (Birch) reduction applied to melanins from wool and L-DOPA gave very different results, as reflected by ESR spectra, but in both cases the changes were much greater than those produced by other treatments. In general, reductive treatments in nonaqueous media in the presence of metals increase the free radical content and line width, whereas treatment in aqueous media resulted in decreased free radical content. These observations are consistent with a melanin pigment which is an irregular polymer and has unpaired electrons localized on different but similar monomer units.