Oxidative polymerisation of 5,6-dihydroxyindole-2-carboxylic acid to melanin: A new insight

Previous studies on the oxidative polymerisation of 5,6-dihydroxyindole-2-carboxylic acid, a key intermediate in the biosynthesis of eumelanins, had delineated a reaction pathway involving mainly repeated coupling of the indole units through the 4- and 7- positions. Using an improved HPLC methodology for the direct analysis of oligomer intermediates, we have now obtained evidence for a more complex mode of polymerisation involving formation, besides the 4,4' and 4,7' coupled dimers 5 and 6, of three new dimers, which have been isolated and identified as the 3,4'-, 3,7'- and 7,7'-biindolyls 7-9. The observed implication of the 3-position is unprecedented in the oxidative polymerisation of 5,6-dihydroxyindoles and yields important clues for future studies aimed at elucidating the chemical constitution of natural and synthetic eumelanins. Copyright (C) 1996 Elsevier Science Ltd