Reexamination of the structure of eumelanin

The generally accepted concept that the black melanin eumelanin is made mostly from 5,6-dihydroxyindole but not from 5,6-dihydroxyindole-2-carboxylic acid (DHIC) was reexamined by comparison of synthetic and natural eumelanins. The analytical methods used were elemental analysis and determination of the carboxyl group by acid treatment to yield CO2 and by permanganate oxidation to yield pyrrole-2,3,5-tricarboxylic acid. It was found that DHIC-derived monomer units comprise only approx. 10% of enzymically prepared dopa-melanins but as much as a half of intact, natural eumelanins. The results also show that dopa-melanins prepared at higher pH retain higher percentages of the carboxyl group of dopa and contain higher percentages of pyrrole units, and that melanins are decomposed to a significant extent on acid treatment, the method commonly used to isolate melanins from natural sources.