Thomas M. Schultz

Structural modifications in biosynthetic melanins induced by metal ions

A number of transition metal ions with a wide distribution in biological systems, e.g., Cu2+, Co2+ and Zn2+, are shown to affect markedly the chemical properties of melanins formed by the tyrosinase-catalysed oxidation of dopa. Acid decarboxylation and permanganate degradation provide evidence that melanins prepared in the presence of metal ions contain a high content of carboxyl groups arising from the incorporation of 5,6-dihydroxyindole-2-carboxylic acid (DICA) into the pigment polymer. Naturally occurring melanins from cephalopod ink and B16 mouse melanoma were found to be much more similar to melanins prepared in the presence of metal ions than to standard melanins prepared in the absence of metal ions. These results suggest that the presence of carboxylated indole units in natural melanins is probably due to the intervention in the biochemical pathway of metal ions which, as recently shown, catalyse the formation of DICA versus 5,6-dihydroxyindole in the rearrangement of dopachrome.

Oxidation of 4-, 6- and 7-hydroxyindoles.

Abstract Oxidation of 4-, 6- and 7-hydroxyindoles with sodium periodate in phosphate buffer at pH 7.0 leads to complex mixtures of oligomeric products, the majority of which have beenisolated and characterised as the O-acetyl derivatives.7-Hydroxyindole (6) gives predominantly the dimers 9 and 10 as well as the trimer 11 and the tetramer 12 in smaller amounts. The 4- and 6-hydroxy isomers 7 and 8 follow less clear-cutreaction paths, characterised by the formation of the oligomers13-16 and 17-19 respectively, along with polymeric materials. The observed mode of polymerisation of hydroxyindoles 6-8 isapparently consistent with a mechanism involving nucleophilicaddition of the starting indoles to the electrophilic positions of transient quinonimine or phenoxonium-like intermediates.