Unraveling the Photoprotecting Mechanism of Melanin - From a Library of Fragments to Simulation of Spectra and Function

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Melanin is the deeply coloured, insoluble and heterogenous biopolymer of animal skin, hair and eyes responsible for protection against harmful light action. It also has a pathogenic role as a generator of reactive oxygen species and possibly through chemiexcitation, and its polydopamine synthetic analogue has raised great interest in the materials community. In spite of these multiple points of interest, the details of melanin structure and its main photoprotecting function are still largely unknown, mostly due to the heterogeneous polymer structure. Closing this gap is an urgent challenge, and our proposal aims at doing that with the methods of computational chemistry. Based on the widely accepted hierarchical structure model for melanin, in WP1 we will generate a library of aggregates made of 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) oligomers with different connectivity and redox state (diol, semiquinone and quinone forms). In WP2 we will simulate the absorption spectra of the aggregates and compare them with the experimental melanin spectrum, which starts at the near IR and increases monotonically into the UV. In WP3 we will simulate the excited-state decay mechanisms for the different aggregates in order to explain melanin's lack of fluorescence, which is indicative of fast decay responsible for its photoprotecting function. The results of our project will provide a new view on the photoprotecting function of melanin based on a systematic consideration of its heterogeneous structure, and they will allow us to assess what specific aggregates are responsible for the different functional characteristics. They will help us to assess the possible pathogenic role of melanin chemiexcitation, and provide new principles for design of PDA-based functional materials.