Onset of the Electronic Absorption Spectra of Isolated and π-Stacked Oligomers of 5,6-Dihydroxyindole: An Ab Initio Study of the Building Blocks of Eumelanin

Abstract

Eumelanin is a naturally occurring skin pigment which is responsible for developing a suntan. The complex structure of eumelanin consists of π-stacked oligomers of various indole derivatives, such as the monomeric building block 5,6-dihydroxyindole (DHI). In this work, we present an ab initio wave-function study of the absorption behavior of DHI oligomers and of doubly and triply π-stacked species of these oligomers. We have simulated the onset of the electronic absorption spectra by employing the MP2 and the linear-response CC2 methods. Our results demonstrate the effect of an increasing degree of oligomerization of DHI and of an increasing degree of π-stacking of DHI oligomers on the onset of the absorption spectra and on the degree of red-shift toward the visible region of the spectrum. We find that π-stacking of DHI and its oligomers substantially red-shifts the onset of the absorption spectra. Our results also suggest that the optical properties of biological eumelanin cannot be simulated by considering the DHI building blocks alone, but instead the building blocks indole-semiquinone and indole-quinone have to be considered as well. This study contributes to advancing the understanding of the complex photophysics of the eumelanin biopolymer.

Control of heteropolymeric to oligomeric character in electrospray deposited melanin films

“…, the spectra are characterized by smooth, structureless, wide band behaviour, with exponential absorption at lower wavelenghts and optical density with a steep increase in the visible range but without a clearcut absorption edge. Such spectral behaviour is consistent with the semiconductor character of the material; in fact the monotonic exponential broad band absorption coefficient results from the ‘chemical disorder’ whereby several chromophores, with their distinct and broadened inhomogeneous spectral features, overlap each other . Therefore the optical measurements of the eumelanin films suggest a better organization for the indolic/phenolic components without affecting their distribution and show an increased degree of π ‐stacked oligomers .…”

Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents

“…14 shows selected chemical filter molecules currently used in commercial sunscreens. 185 These broadly partition into seven classes,…”

The modulation of melanin‐like materials: methods, characterization and applications

“…Tuna et al indicated that the -stacking of DHI and its oligomers substantially red-shifts the onset of the absorption spectrum. 98 Equilibrium structures of different intermediates such as dimers, 97,99,100 tetramers, 44,101 hexamers 89 and other oligomers 91 have been examined by evaluating their energy. Supramolecular interactions 96 and aggregated structures 22,90 of melanin-like compounds have also been investigated by using simulation methods.…”

“…In addition, there are a growing number of studies which have reported investigations of the structures and properties of melanin‐like materials by using theoretical calculations . These computations and other experimental research complement each other in deepening our understanding of synthetic melanins.…”

“…For example, theoretical calculations revealed that the geometric order and disorder of melanin's aggregated structure and chemical disorder are main factors which dominate the broad absorption spectra. Tuna et al indicated that the π‐stacking of DHI and its oligomers substantially red‐shifts the onset of the absorption spectrum . Equilibrium structures of different intermediates such as dimers, tetramers, hexamers and other oligomers have been examined by evaluating their energy.…”