Design, synthesis and characterization of the first water soluble 5,6-dihydroxyindole polymer as a tool to disentangle eumelanin “chromophore”

A fundamental unanswered question concerning eumelanins, the enigmatic biopolymers of human skin, is why they are black. The major gap is the lack of information about the intrinsic absorption properties of the species produced by oxidative polymerization of 5,6-dihydroxyindole (DHI) and related precursors, due to their marked insolubility. Herein, we report the first water-soluble DHI polymer by oxidation of ad hoc designed S-galactosyl 5,6-dihydroxy-3-mercaptoindole (S-GalDHI). The dark brown pigment exhibited a distinct band at 314 nm and a broad visible absorption, resembling natural eumelanins. Sodium borohydride reduction caused decoloration of the solution and a marked absorbance decrease in the visible region, but had no effect on the 314 nm band. Dilution experiments revealed a linear dependence of the absorbance at 314 nm on concentration, but a significant decrease with increasing dilution in the whole visible region, with the larger deviation from linearity around 500 nm. Overall, these data indicate that eumelanin black color is not only intrinsically defined by the degree of electron conjugation within the individual polymer components but also by oxidation state- and aggregation-dependent interchromophoric interactions causing perturbations of the heterogeneous ensemble of -electron systems and spectral broadening.