title: A 13C solid-state NMR study of the structure and auto-oxidation process of natural and synthetic melanins.
abstract: This paper presents a 13C CP/MAS NMR study of the melanin pigments obtained through natural and synthetic origins: sepia-melanin from squid ink and three synthetic 5,6-dihydroxyindole-melanins prepared using different non-enzymatic oxidation pathways. The synthetic pigments can be distinguished from natural melanin by the absence of aliphatic carbons, thereby confirming the unreacted 3,4-dihydroxyphenylalanine and the proteinaceous origins of the aliphatic resonances in natural eumelanin. The spectra of selected non-protonated carbon resonances and those with only protonated carbon signals led to a quantitative analysis. An auto-oxidative experiment using a synthetic melanin, over a period of 130 h, has shown an unusually slow disappearance of hydrogen peroxide formed in situ. The 13C-NMR spectrum of the insoluble oxidized synthetic melanin compared to that before auto-oxidation clearly demonstrates that the oxidation process is associated with chemical changes within the pigment; i.e., carbonyl functional group formation and an increase of the non-protonated carbons fraction.
authors.affiliation: Laboratoire de RMN et de Modélisation Moléculaire, UMR 50 CNRS/Bruker/Université L. Pasteur, Strasbourg, France.
authors.forename: M
authors.initials: M
authors.lastname: Hervé
authors.forename: J
authors.initials: J
authors.lastname: Hirschinger
authors.forename: P
authors.initials: P
authors.lastname: Granger
authors.forename: P
authors.initials: P
authors.lastname: Gilard
authors.forename: A
authors.initials: A
authors.lastname: Deflandre
authors.forename: N
authors.initials: N
authors.lastname: Goetz
mesh_terms: D000818:Animals; D049832:Decapodiformes; D009682:Magnetic Resonance Spectroscopy; D008543:Melanins / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D008543:Melanins