Structural analysis of synthetic melanins from 5,6-dihydroxyindole by Matrix-Assisted Laser Desorption/Ionization mass spectrometry.

Despite extensive investigation, the structure of melanins, the major determinants of skin colour differences in man and other mammals, is still poorly defined, mainly because of the unfavourable properties of the materials, In this study, analysis of model pigments prepared by enzymically- or chemically-induced oxidative polymerization of 5,6-dihydroxyindole (DHI), the ultimate biosynthetic precursor, was investigated by means of matrix-assisted laser desorption/ionization mass spectrometry, All DHI melanins exhibited quite distinct pattern of ionic species of low m/z ranging from 500 to 1500, none of which corresponded to intact DHI oligomers, Analysis of the molecular weights and mass differences between the oligomer species provided evidence for a significant breakdown of the pigment backbone by peroxidative fission of the indole units with concomitant decarboxyaltion and oxygenation reactions, These processes, which take place to a different extent depending on the preparation conditions; are possibly initiated by hydrogen peroxide, either added as the oxidant or slowly generated in the reaction medium. In support of this, melanin samples prepared by tyrosinase oxidation in the presence of catalase comprised, as major components, intact DHI oligomers up to hexamers, Overall, these results offer a new picture of the structure of DHI melanins which may significantly contribute to the understanding of the high degree of molecular heterogeneity of the natural pigments.