A BIOSYNTHETIC APPROACH TO THE STRUCTURE OF EUMELANINS. THE ISOLATION OF OLIGOMERS FROM 5,6-DIHYDROXY-1-METHYLINDOLE.

Article abstract of DOI:10.1016/S0040-4020(01)87625-0

Enzymic oxidation, or autooxidation at pH 9, of 1 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 (6).In the presence of metal cations, e.g.Ni(2+), autooxidation of 1, at pH 7.5, led to the isolation, besides the dimer 6 in lower amounts, of two more oligomers which were formulated as 5,6,5',6'-tetracetoxy-1,1'-dimethyl-2,2'-biindolyl (7) and 5,6-diacetoxy-1-methyl-2,4-di-<5',6'-diacetoxy-1'-methyl-2'-indolyl>-indole (8).The unexpected reactivity of 1 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.