A C-13 SOLID-STATE NMR-STUDY OF THE STRUCTURE AND AUTOXIDATION PROCESS OF NATURAL AND SYNTHETIC MELANINS

This paper presents a C-13 CP/MAS NMR study of the melanin pigments ob tained through natural and synthetic origins: sepia-melanin from squid ink and three synthetic 5,6-dihydroxyindole-melanins prepared using d ifferent non-enzymatic oxidation pathways. The synthetic pigments can be distinguished from natural melanin by the absence of aliphatic carb ons, thereby confirming the unreacted 3,4-dihydroxyphenylalanine and t he proteinaceous origins of the aliphatic resonances in natural eumela nin. The spectra of selected non-protonated carbon resonances and thos e 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 hydrogenperoxid e formed in situ. The C-13-NMR spectrum of the insoluble oxidized synt hetic melanin compared to that before auto-oxidation clearly demonstra tes that the oxidation process is associated with chemical changes wit hin the pigment; i.e., carbonyl functional group formation and an incr ease of the non-protonated carbons fraction.