STRUCTURAL-ANALYSIS OF SYNTHETIC MELANINS FROM 5,6-DIHYDROXYINDOLE BYMATRIX-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 s till poorly defined, mainly because of the unfavourable properties of the materials, In this study, analysis of model pigments prepared by e nzymically- or chemically-induced oxidative polymerization of 5,6-dihy droxyindole (DHI), the ultimate biosynthetic precursor, was investigat ed by means of matrix-assisted laser desorption/ionization mass spectr ometry, All DHI melanins exhibited quite distinct pattern of ionic spe cies of low m/z ranging from 500 to 1500, none of which corresponded t o intact DHI oligomers, Analysis of the molecular weights and mass dif ferences between the oligomer species provided evidence for a signific ant breakdown of the pigment backbone by peroxidative fission of the i ndole 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 peroxi de, either added as the oxidant or slowly generated in the reaction me dium. In support of this, melanin samples prepared by tyrosinase oxida tion in the presence of catalase comprised, as major components, intac t DHI oligomers up to hexamers, Overall, these results offer a new pic ture of the structure of DHI melanins which may significantly contribu te to the understanding of the high degree of molecular heterogeneity of the natural pigments.