A biosynthetic approach to the structure of eumelanins. The isolation of oligomers from 5,6-dihydroxy-1-methylindole.

Enzymic oxidation, or autooxidation at pH 9, or led to a mixture of fluorescent products the major of which could be isolated, as the acetyl derivative, and identified as 5,6,5',6'-tetraacetoxy-1,1'-dimethyl-2,4'-biindolyl (). In the presence of metal cations, e.g. Ni2+, autooxidation of , at pH 7.5, led to the isolation, besides the dimer in lower amounts, of two more oligomers which were formulated as 5,6,5',6'-tetracetoxy-1,1'-dimethyl-2,2'-biindolyl () and 5,6-diacetoxy-1-methyl-2,4-di-[5',6'-diacetoxy-1'-methy1-2'-indolyl]-indole (). The unexpected reactivity of to undergo oxidative coupling at 2- and 4- position rather than at 3- and 7- position, as previously believed, is discussed in relation to the current views on the structure and biosynthesis of eumelanins.