Characterization of the Fe(III)-binding site in Sepia eumelanin by resonance Raman confocal microspectroscopy

The resonance Raman spectrum of Sepia eumelanin is discussed by analogy to model compounds containing catechol (CAT)-like structural units. These data are then compared with the analogous data on Fe(III)-enriched Sepia eumelanin. In contrast to the natural eumelanin, the Fe(III)-enriched samples exhibit absorption features in the visible and near-IR spectral regions, which are attributed to ligand-to-metal charge-transfer (LMCT) bands. Resonance Raman spectra collected by exciting these LMCT bands reveal bands at 580 and 1470 cm^-1; the intensity of these features increases with increasing Fe(III) content. The 580 and 1470 cm^-1 bands are assigned to Fe-OR stretching and ring deformation modes, respectively. These data further substantiate that the Fe(III)-melanin-binding site in melanin is composed of CAT-like structural units.