NEW PYRROLE ACIDS BY OXIDATIVE-DEGRADATION OF EUMELANINS WITH HYDROGEN-PEROXIDE - FURTHER HINTS TO THE MECHANISM OF PIGMENT BREAKDOWN

Oxidative degradation of natural and synthetic eumelanins with alkalin e H2O2 afforded a complex mixture of low molecular weight products whi ch comprised, besides 1 and 2, three novel pyrrole acids. These were i solated and identified as 2-carboxyhydroxymethylpyrrole-3,5-dicarboxyl ic acid (3), 2-carboxymethylpyrrole-3,5-dicarboxylic acid (4) and 2-hy droxymethylpyrrole-3,5-dicarboxylic acid (5). Investigation of the oxi dative degradation of the eumelanin precursors 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid, as well as of the oligomers 6- 10 showed that pyrrole acids 3-5 originate by peroxidative disruption of both carboxylated and non-carboxylated indole units, and that pyrro le 4 arises mainly from indole units not substituted at the 7-position . None of the new pyrroles was converted to 1 by treatment with alkali ne H2O2. suggesting that they are formed by different degradation rout es. These results can be accommodated into an improved mechanistic sch eme for rationalising the origin of pyrrole acids by H2O2 degradation of eumelanins. Copyright (C) 1996 Elsevier Science Ltd