Morphological and optical investigation of synthetic DOPA-melanin

Eumelanin is an important natural pigment, which is produced in epidermal melanocytes by tyrosinase-catalized oxidation of tyrosine. Such pigment, besides its important biological functions as the protection against the biochemical damages induced by solar exposure, has physical properties, like broad UV-visible absorption spectra, electrical conductivity and photoconductivity, which are interesting for a possible integration in photovoltaic and electronic devices. The physical properties are strongly dependent on the structural organization of the biopolymer. Although some recent studies have supported a structural model based on the aggregation of few stacked planar sheets of oligomeric structures, made of monomeric units consisting of few indolic compounds, to form eumelanin protomolecules, the detailed knowledge of eumelanin structure is not well documented. In fact, eumelanin protomolecules are difficult to image because of their tendency to form aggregates. Further, optical signals from single functional groups inside the eumelanin compound have been scarcely resolved in literature, because of broadening and overlapping effects. In particular, it is interesting to investigate structural modification occurring when such biopolymer is deposited onto a substrate, in view of the design and production of novel materials and devices based on eumelanin properties. In this work eumelanin biopolymer synthetized from L-DOPA has been characterized by Atomic Force Microscopy (AFM), Dynamic Light Scattering (DLS) and Surface Enhanced Raman Spectroscopy (SERS) techniques. AFM images have shown that eumelanin units self-organize on mica and glass substrates on which they are deposited according to filamentous structures about 200 nm width and 50 nm height, but the presence of particles characterized by few nanometers size suggests that the filamentous structures can be due to the aggregation on nanometer scale units. In fact, DLS measurements confirm that the size distribution of particles in solution, before deposition on the substrate, is centred on a modal value of about 3040 nm. SERS spectra confirm the presence of specific indolic compound, as 5,6-dihydroxyindole (DHI), 5,6-indolequinone (IQ) and semiquinone (SQ) units in synthetic eumelanin, as well as the presence of carboxylic acid and residual components from the synthesis process. On the whole, our results are consistent with indolic-based protomolecules formation in solution and subsequent self-assembly into fibril-like structures when deposition process of eumelanin onto a substrate occurs.