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

A fundamental unsettled issue concerning eumelanins, the functional biopolymers of human skin and hair, is why they are black. The exptl. difficulty lies in the virtual insoly. of these pigments, causing marked scattering effects and hindering characterization of the intrinsic absorption properties of the heterogeneous species produced by oxidative polymn. of 5,6-dihydroxyindole (DHI) and related monomer precursors. The synthesis of spectrally robust, water-sol. 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-sol. DHI polymer produced by oxidn. 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 redn. 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 diln. 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 oxidn. state- and aggregation-dependent interchromophoric interactions causing perturbations of the heterogeneous ensemble of .pi.-electron systems and overall spectral broadening.