Eumelanin Buildup at the Nanoscale: Aggregate Growth/Assembly and Visible Absorption Development in Biomimetic 5,6-Dihydroxyindole Polymerization.

Establishing structure-property relationships in the black insol. eumelanins, the key determinants of human pigmentation and skin photoprotective system, is a considerable conceptual and exptl. challenge in the current drive for elucidation of the biol. roles of these biopolymers and their application as advanced materials for organoelectronics. Herein, we report a new breakthrough toward this goal by the first detailed investigation at the nanoscale level of the oxidative polymn. of 5,6-dihydroxyindole (DHI), a model process of eumelanin synthesis. Based on a combined use of spectrophotometry, dynamic light scattering (DLS) and small angle neutron scattering (SANS) investigations, it was possible to unveil the dynamics of the aggregation process before pptn., the key relationships with visible light absorption and the shape of fundamental aggregates. The results indicated a polymn. mechanism of the type: Polymern+ DHIx = Polymern+x, where DHIx indicates monomer, dimer or low oligomers ( ). During polymn., visible absorption increases rapidly reaching a plateau. Particle growth, on the other hand, proceeds slowly, with formation of two-dimensional structures ∼ 55 nm thick, until pptn. occurs, i.e. when large aggregates with a max. hydrodynamic radius ( ) of ∼ 1200 nm are formed. Notably, markedly smaller values, up to ∼ 110 nm, were detd. in the presence of poly-(vinyl alc.) (PVA) that was shown to be an efficient aggregation-preventing agent for polymg. DHI ensuring water-solubilization. Finally, it is shown that DHI monomer can be efficiently and partly irreversibly depleted from aq. solns. by the addn. of eumelanin suspensions. This behavior is suggested to reflect oxidant-independent competing pathways of polymer synthesis and buildup via monomer conversion on the active aggregate surface contributing to particle growth. Besides filling crucial gaps in DHI polymn., these results support the attractive hypothesis that eumelanins may behave as a peculiar example of living biopolymers. The potential of PVA as a powerful tool for soln. chem.-based investigations of eumelanin supramol. organization and for technol. manipulation purposes is underscored.