Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems

The molecular mechanisms by which the black eumelanin biopolymers exert their photoprotective action on human skin and eyes are still poorly understood, owing to critical insolubility and structural heterogeneity issues hindering direct investigation of excitation and emission behavior. Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole (DHI)-based polymers as useful models for disentangling intrinsic photophysical properties of eumelanin components from aggregation and scattering effects. Herein, we report the absorption properties and ultrafast emission dynamics of two separate sets of DHI-based monomer-dimer-polymer systems which were made water-soluble by means of poly(vinyl alcohol) or by galactosyl-thio substitution. Data showed that dimerization and polymerization of DHI result in long-lived excited states with profoundly altered properties relative to the monomer and that glycosylation of DHI imparts monomer-like behavior to oligomers and polymers, due to steric effects hindering planar conformations and efficient interunit electron communication. The potential of S-glycation as an effective tool to probe and control emission characteristics of eumelanin-like polymers is disclosed.