Disentangling Eumelanin “Black Chromophore”: Visible Absorption Changes As Signatures of Oxidation State- and Aggregation-Dependent Dynamic Interactions in a Model Water-Soluble 5,6-Dihydroxyindole Polymer

Abstract

A fundamental unsettled issue concerning eumelanins, the functional biopolymers of human skin and hair, is why they are black. The experimental difficulty lies in the virtual insolubility of these pigments, causing marked scattering effects and hindering characterization of the intrinsic absorption properties of the heterogeneous species produced by oxidative polymerization of 5,6-dihydroxyindole (DHI) and related monomer precursors. The synthesis of spectrally robust, water-soluble DHI polymers is therefore an important goal in the prospects of disentangling intrinsic absorption properties of eumelanin components by circumventing scattering effects. Reported herein is the first water-soluble DHI polymer produced by oxidation of ad hoc designed 5,6-dihydroxy-3-indolyl-1-thio-beta-D-galactopyranoside (1). The dark brown polymer exhibited a distinct band at 314 nm and a broad visible absorption, resembling that of natural eumelanins. Main isolable oligomer intermediates including 2,7'- and 2,4'-biindolyls 2 and 3, attest the close resemblance to the mode of coupling of the parent DHI. Sodium borohydride reduction caused decoloration and a marked absorbance decrease in the visible region around 550 nm, but did not affect the UV band at 314 nm. Measurements of absorbance variations with dilution indicated a linear response at 314 nm, but a significant deviation from linearity in the visible region, with the largest decrease around 500 nm. It is argued that eumelanin black color is not only intrinsically defined by the overlap of pi-electron conjugated chromophores within the individual polymer components, as commonly believed, but also by oxidation state- and aggregation-dependent interchromophoric interactions causing perturbations of the heterogeneous ensemble of pi-electron systems and overall spectral broadening.

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

“…We note that several groups have considered the effect of dynamical disorder 48,49 , where short-range interactions among nearby molecules affect the molecular geometry dynamically. However, as in the case of chemical (static) disorder model, the excitons are still restricted within one (segment of a) molecule since their interactions are neglected in these studies.…”

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

“…As compared to the reduced pigment, solubilized oxidized synthetic eumelanin shows a red-shift in absorption. 7,8 The co-existence of both oxidized and reduced states in the full pigment is hence to some extent responsible for the broad featureless absorption. The oxidation state also likely affects stacking behavior.…”

“…In addition to the highly heterogeneous structure, probably also the co-existence of both reduced and oxidized polymer chain domains and aggregation of polymer chains contribute to the featureless absorption. 7,8 The peculiar absorption of eumelanin and pheomelanin might allow protecting the skin against UV-induced damage, although the functionality of epidermal pigments is so far not at all understood. There are indications that eumelanin indeed may be photoprotective, while pheomelanin is thought to be phototoxic.…”

Superior Photoprotective Motifs and Mechanisms in Eumelanins Uncovered

“…The resulting DHICA eumelanin consists of both reduced and oxidized domains (such as in natural eumelanin) in an approximately 1:1 ratio, with the first being predominantly excited at 267−280 nm. 10 For the fluorescence measurements the samples were diluted and absorption spectra were measured. The concentration was set with an absorbance at the excitation wavelength of about 0.2 in a 2 mm cell.…”

“…6−9 The coexistence of both reduced and oxidized domains favoring aggregation of polymer chains and intermolecular chromophore perturbation has also been implicated to account for the featureless visible absorption spectrum of eumelanins. 10,11 Eumelanin is known to possess very efficient (99.9%) dissipation channels of absorbed UV energy into heat, but despite many spectroscopic studies the mechanisms of this very efficient photoprotection are not known. 12−17 In particular, the minimum functional units and active photochemical processes associated with the efficient UV energy dissipation are unknown.…”