“Invisible” ligands stabilize colloidal melanin particles – the case of L-DOPA

This report details observations that the in vitro synthesis of L-DOPA-based melanin leads to a two-component material: dark-colored aggregates stabilized by colorless ligands. Despite the appearance of dissolution, the melanin materials generated behave as dispersed, colloidal particles. All dispersible materials can, in part, be aggregated and precipitated through the addition of mono- or multivalent cations but in a non-linear, concentration-dependent fashion. The addition of cationic species, at sufficiently high concentrations, fractionates the colloids into a dark precipitate and a dispersible, colorless fraction. Precipitated aggregates could be redispersed in water through the addition of sufficiently high concentrations of KCl; an additional indicator of their colloidal nature. The dispersible fraction exhibits absorbance in the UVA and UVB range of the electromagnetic spectrum, but little to no absorbance in the visible range. The fractionated melanins were characterized using liquid chromatography, UV-Vis and FT-IR spectroscopy. Based upon the results presented we suggest a model for synthetic melanins as colloidal particles built from a dark-colored core aggregate stabilized by a set of colorless ligands.