AFM investigations of self-assembled DOPA–melanin nano-aggregates

The formation of longitudinal, fibril-like structures upon drop drying of the synthetic DOPA–melanin colloidal suspension was investigated. Synthetic DOPA–melanin comprises mainly molecular species derived from the coupling of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid monomers that aggregate forming granules. AFM images have shown that for different substrates (mica, glass, gold), melanin granules have a tendency to form aggregates on nanometer-size scale, which in turn self-assemble into fibril-like structures. The deposits are about 250 nm in width and 50 nm in height. The formation of observed deposits has been explained assuming Van der Waals interactions between stacked planar aromatic structures present in melanin protomolecules. These results can support the recent stacked oligomer model for eumelanin protomolecules and provide experimental evidence for formation of organic self-assembled nano-aggregates of melanin polymer.