Spectroscopic features of native and bleached opio-melanins.
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.
Rosei
M A
MA
Department of Biochemical Sciences, University of Rome, La Sapienza, Italy.
Mosca
L
L
De Marco
C
C
D000595:Amino Acid Sequence; D003624:Darkness; D006861:Hydrogen Peroxide / Q000494:pharmacology; D008543:Melanins / Q000737:chemistry* / Q000528:radiation effects; D008969:Molecular Sequence Data; D014442:Monophenol Monooxygenase / Q000378:metabolism; D018847:Opioid Peptides / Q000737:chemistry* / Q000187:drug effects / Q000528:radiation effects; D010782:Photolysis; D011108:Polymers / Q000737:chemistry / Q000528:radiation effects; D013053:Spectrophotometry; D013472:Sunlight
D008543:Melanins; D018847:Opioid Peptides; D011108:Polymers; C091643:opiomelanin; D006861:Hydrogen Peroxide; D014442:Monophenol Monooxygenase