Oxidative degradation of natural and synthetic eumelanins with alkaline H2O2 afforded a complex mixture of low molecular weight products which comprised, besides 1 and 2, three novel pyrrole acids. These were isolated and identified as 2-carboxyhydroxymethylpyrrole-3,5-dicarboxylic acid (3), 2-carboxymethylpyrrole-3,5-dicarboxylic acid (4) and 2-hydroxymethylpyrrole-3,5-dicarboxylic acid (5). Investigation of the oxidative 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 pyrrole 4 arises mainly from indole units not substituted at the 7-position. None of the new pyrroles was converted to 1 by treatment with alkaline H2O2. suggesting that they are formed by different degradation routes. These results can be accommodated into an improved mechanistic scheme for rationalising the origin of pyrrole acids by H2O2 degradation of eumelanins.
New pyrrole acids by oxidative degradation of eumelanins with hydrogen peroxide. Further hints to the mechanism of pigment breakdown
M. D'ISCHIA;
1996
Abstract
Oxidative degradation of natural and synthetic eumelanins with alkaline H2O2 afforded a complex mixture of low molecular weight products which comprised, besides 1 and 2, three novel pyrrole acids. These were isolated and identified as 2-carboxyhydroxymethylpyrrole-3,5-dicarboxylic acid (3), 2-carboxymethylpyrrole-3,5-dicarboxylic acid (4) and 2-hydroxymethylpyrrole-3,5-dicarboxylic acid (5). Investigation of the oxidative 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 pyrrole 4 arises mainly from indole units not substituted at the 7-position. None of the new pyrroles was converted to 1 by treatment with alkaline H2O2. suggesting that they are formed by different degradation routes. These results can be accommodated into an improved mechanistic scheme for rationalising the origin of pyrrole acids by H2O2 degradation of eumelanins.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
