Stability and Optical Absorption of a Comprehensive Virtual Library of Minimal Eumelanin Oligomer Models**

Abstract

Eumelanin is responsible for photoprotection in living organisms. It is made of 5,6-dihydroxyindole (DHI) oligomers. However, lack of detailed structural knowledge limits understanding its function and exploiting its potential in material science. To uncover the relationship between structural stability and optical properties, we have studied a virtual library of 830 DHI dimers. We find a preference for oxidized, polycyclic structures which speaks in favor of graphite-like structures for the larger oligomers, and propose an electrocyclic formation mechanism. Besides widely considered quinone oxidation patterns, also structures with interfragment double bonds and zwitterionic resonance structures are stable. Future theoretical melanine models will have to cover this diversity, and we introduce a new representative set of 49 stable dimers. Some stable oxidized dimers have absorption energies as low as 1.3 eV. They may be present as substructures in the naturally found oligomers and contribute to the absorption spectrum of the biopolymer.