A 13C solid-state NMR study of the structure and auto-oxidation process of natural and synthetic melanins.
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.
Hervé
M
M
Laboratoire de RMN et de Modélisation Moléculaire, UMR 50 CNRS/Bruker/Université L. Pasteur, Strasbourg, France.
Hirschinger
J
J
Granger
P
P
Gilard
P
P
Deflandre
A
A
Goetz
N
N
D000818:Animals; D049832:Decapodiformes; D009682:Magnetic Resonance Spectroscopy; D008543:Melanins / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction
D008543:Melanins