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

Abstract

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.

A quantum yield map for synthetic eumelanin

“…Within the chemical disorder model, this feature can be explained by selective excitation of different-sized oligomers. Simulations of larger oligomeric structures show that polymerization leads to progressive redshifting of the gap 19 and increased delocalization of the electronic wave functions. Stark et al in their latest paper have augmented these findings, demonstrating further redshifting with stacking.…”

Functionality of epidermal melanin pigments: current knowledge on UV-dissipative mechanisms and research perspectives

“…It is in general attributed to the contribution from a large variety of different HOMO-LUMO gaps due to the large heterogeneity in chemical composition, and discussed in more detail in Section 3. [3][4][5][6] The monotonically towards the UV increasing absorption can be described by a superposition of a large number of inhomogeneously broadened Gaussian transitions associated with the individual segments in the pigment. The transitions in the UV are likely due to S 0 -S 1 excitation of smaller units and S 0 -S 2 (or higher) excitation of larger oligomers, while transitions in the visible part of the spectrum are due to S 0 -S 1 excitation of the latter.…”

Excitonic effects from geometric order and disorder explain broadband optical absorption in eumelanin

“…Accordingly, eumelanin consists of many chemically distinct species and its broadband absorption spectrum is a result of averaging over the spectra of these species. The sharp peaks due to 5,6-dihydroxyindole (DHI) and its oligomers could be eliminated after this process 3,[11][12][13][14][15][16] . Even though the chemical disorder model combined with the superposition principle are able to reproduce broadband absorption spectra, they are, however, still insufficient to provide a full explanation of the optical properties of eumelanin.…”

“…However, due to computational limitations, the system sizes were limited to no more than three molecules 11,13,[14][15][16]19,20 . Such a setup is unable to capture the full picture of the optical properties of eumelanin since it neglects the geometric disorder characteristic.…”