Spectroscopic Study and Simulation from Recent Structural Models for Eumelanin: II. Oligomers

Spectroscopic simulations of a leading structural model for melanin, which is the pigment responsible for coloration and photo protection in humans and animals, were conducted. In direct continuation of an earlier study on possible monomer and dimer subunits of eumelanin, we have performed density functional theory (DFT) calculations on a recent structural model for eumelanin based on higher oligomers of neutral 5,6-indolequinone. This paper further reports on our semiempirical spectroscopic simulations for the higher oligomer (tetramers through hexamers) energy-minimized structures. A linear combination of the oligomeric spectra reproduces several features of the experimental spectrum. This result strongly supports the assumption that melanin is constituted of substructures such as those considered in this study.