Optical Spectroscopy and Structural Properties of Synthetic and Natural Eumelanin

Optical properties of synthetic and natural eumelanin are presented and compared, in order to investigate the structural organization of eumelanin, which is related to the function of this biopolymer. Synthetic eumelanin is produced by oxidation of tyrosine with hydrogen peroxide, whereas natural eumelanin is extracted from Sepia Officinalis and from Rana Esculenta. Vibrational spectroscopy techniques (as Raman scattering and infrared absorption) show that both types of biopolymer include chemical functional groups characteristic of the monomeric units of eumelanin, although natural eumelanin includes also protein-related groups, proportionally to the protein content. X-ray diffraction spectra are in agreement with the hypothesis that eumelanin monomers assembly themselves and form protomolecules consisting of stacked layers (distant 3 – 4 Å each other) of indolic sheets. Absorption measurements, characterized by a monotonic increase of optical density from near-IR to UV range, support the model that eumelanin consists of a distribution of aggregates of oligomeric structures having different size and chemical composition. The estimated values of the optical gap indicate that the natural eumelanins are characterized by a larger structural disorder than the synthetic one. Fluorescence spectra confirm that the biopolymer consists of ensembles of chemically distinct oligomer systems, which can be selectively excited. This result is also supported by Dynamic Light Scattering measurements, which permit to visualize the distribution of particles size. In fact, the nanoaggregate systems of natural eumelanin have a larger size than those of synthetic eumelanin. This might be related to the biological functions of such a biopolymer, particularly as far as photoprotective action is concerned.