Oxidative polymerization of 5,6-dihydroxyindoles. Tracking the biosynthetic pathway to melanin pigments.

The importance of the 5,6-dihydroxyindoles as intermediates in the biosynthesis of melanins, the black-to-brown pigments of human skin, hair and eyes, has been recognized since the beginning of the century. However, it is only during the last decade that several fundamental issues concerning the biosynthesis and oxidative polymerization of these metabolites have been brought to focus and clarified. Besides unveiling hitherto unknown facets of the reactivity of the 5,6-dihydroxyindole system, these advances have yielded unprecedented insights into the origin, structure and properties of melanin pigments, and have thrown new light on the possible functional significance of the indole melanogens that are produced and excreted by active epidermal melanocytes. It is the aim of the present report to provide an overview of the most significant achievements on the oxidation chemistry of 5,6-dihydroxyindoles gained at Naples and other laboratories, with highlights on those aspects of closer relevance to the biosynthesis of melanins and related processes.