Vibrational spectroscopy of synthetic and natural eumelanin

We report vibrational spectra of two types of eumelanin samples: a synthetic one, produced by oxidation of tyrosine with hydrogen peroxide, and a natural one, extracted fromSepia officinalis,with the aim of pointing out the differences between the main vibrational properties of such two pigments. Fourier transform infrared, Raman and surface- enhanced Raman scattering spectra allow the identification of several vibrational modes involving specific monomeric units. The main differences between the spectra of the two types of eumelanin are related to (i) a larger amount of carboxylic acid in the synthetic than in the natural sample; (ii) a shift of spectral position of corresponding peaks; and (iii) a contribution of residual proteins to the signals from the natural sample. Such results suggest that vibrational techniques may be helpful for non-destructively determining the composition of various eumelanin pigments, with potential applications in biomedical, electronic and cultural heritage fields.