Abstract
Opioid peptides can be converted by tyrosinase into melanin-like compounds, in which the peptide moiety is retained. Such pigments, named opio-melanins, exhibit a characteristic absorption spectrum with a maximum at about 330 nm and a different solubility behaviour with respect to dopa-melanin, being completely soluble in hydrophylic solvents at neutral and basic pH. Opio-melanins precipitate in aqueous solutions below pH 5.0, and show apparent pKa values of 3.1, 3.6 and 4.4 for Tyr-Gly-melanin, Tyr-Gly-Gly-melanin and leuenk-melanin, respectively. The concomitant oxidation of dopa and opioid peptides by tyrosinase produces mixed polymers, showing the distinctive absorption peak at 330 nm. In the dark, in the pH range 5.5-7.0 the pigments are completely stable, whereas H2O2 addition provokes a slight degradation. At higher pH values or under simulated solar illumination with or without hydrogen peroxide, bleaching occurs more rapidly than in dopa-melanin. Upon photoirradiation the absorption spectrum of opio-melanins undergoes a marked variation, the peak at 330 nm being replaced by a broad shoulder in the range 280-350 nm. The absorption spectra of native and bleached pigments and the extent of opio-melanins degradation by bleaching agents, confirm the hypothesis that the different initial structure of the precursors accounts for a final diverse polymeric architecture of these pigments with respect to dopa-melanin.
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