NEW INTERMEDIATES IN THE OXIDATIVE POLYMERIZATION OF 5,6-DIHYDROXYINDOLE TO MELANIN PROMOTED BY THE PEROXIDASE/H2O2 SYSTEM

According to the generally held view, the biosynthesis of melanins, the major determinants of skin colour differences in man, involves ultimately the oxidative polymerisation of 5,6-dihydroxyindole (1) promoted by the enzyme tyrosinase. We have now found that such a process is brought about more efficiently by the peroxidase/H2O2 system at physiological pHs, under which conditions the oxidation reaction leads in the early stages to a distinct pattern of oligomer products. These were isolated after acetylation and identified as the tetraacetoxybiindolyls 2 and 3 and the related trimers 4 and 5. The observed mode of polymerisation of 1 seemingly involves the attack of the nucleophilic 2-position of the indole to the electron deficient C-4 and C-7 sites of 5,6-indolequinone arising from further oxidation or disproportionation of peroxidase generated phenoxyl radicals. © 1990.