Leszek J. Wolfram

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.

Chapter 6 Chemistry of Melanins

Publisher Summary This chapter provides a concise, yet comprehensive and up-to-date overview of the field of melanins, with particular emphasis on the chemical, physical, and biosynthetic aspects of these important and ubiquitous pigments. Tyrosinase is known to catalyze the biosynthesis of not only black but also brown, yellow, reddish brown, and carrot-red pigments. Melanin pigmentation is mainly determined by two chemically distinct but biogenetically related types of pigments. One of them is the dark, insoluble eumelanins that are produced from the tyrosinase-catalyzed oxidation of tyrosine, and the other is the alkalisoluble phaeomelanins that originate from an altered eumelanin pathway through the intervention of cysteine. Evidence suggests that certain heavy metal ions, commonly found in pigmented tissues, play an important role in melanogenesis. Of particular interest is the finding that copper and, to a lesser degree, zinc, cobalt, and iron have the ability to catalyze the rearrangement of dopachrome to 5,6-dihydroxyindole(s), which is a key regulatory step in the biosynthesis of eumelanins.