Unravelling eumelanin structure

Elucidation of the fundamental structural features and supramolecular organization of eumelanins is an essential prerequisite for an understanding of their unique physicochemical properties and biological role beyond the mere provision of a pigment. Recently, the results of an integrated chemical, mass spectrometric and pulse radiolytic, bottom-up approach to the mode and degree of polymerization of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) have disclosed several aspects of critical relevance to the structure of synthetic eumelanins. The synthesis of the first water soluble DHI-based polymer has yielded moreover unprecedented insights into the origin of eumelanin “black chromophore” and the underlying broadband UV-visible absorption, an issue of central relevance to skin photoprotection. Overall, the results emerging from these studies indicated that DHI polymers are highly heterogeneous mixtures of species with a maximum detectable length of ca. 30 units and diverse levels of disorder. Within these species, oxidized and reduced substructures coexist and the degree of electron delocalization is dictated by the mode of coupling of the monomer units. Hindered rotation around the interring bond limits in particular coplanarity and visible chromophore development in DHICA-derived polymers. The new structural data that will be presented provide an explanation to the different properties of DHI and DHICA polymers and may guide future work aimed at unravelling the structure and mechanism of build-up of natural eumelanins.