Fluorescence properties of melanins from opioid peptides

Recently our group synthesized a new class of melanins obtained by the tyro sinase-catalyzed oxidation of opioid peptides (opiomelanins). Owing to the presence of the peptide moiety such pigments exhibit high solubility in hyd rophilic solvents, which allows spectroscopic investigations. In particular , the absence of solid-state quenching effects enables the study of melanin fluorescence properties, till now poorly investigated due to the complete insolubility of melanins produced from tyrosine or Dopa. Opiomelanins disso lved in acqueous medium show a characteristic emission peaked at 440 and 52 0 nm when excited around 330 nm, where a maximum is observed in the absorpt ion spectrum. Kinetic measurements performed on the tyrosinase-catalyzed ox idation of opioid peptides show that the 440-nm fluorescence band arises in the early stages of peptide oxidation, whereas the 520-nm band appears in later stages of oxidation, i.e., during the polymerization of indole-quinon e units. Moreover, molecular sieve fractionation shows that in the opiomela nin fraction with a molecular weight lower than 10 kDa the 440-nm band is d ominant in the fluorescence spectrum. The breakdown of the polymer induced by hydrogen peroxide and light (i.e., the photobleaching of melanin pigment s) produces a marked enhancement of the 440-nm fluorescence band while the 520-nm band disappears. Hence, our findings suggest that the observed fluor escence contains contributions from both oligomeric units (440-nm band) and high-molecular-weight polymers (520-nm band). (C) 1999 Academic Press.