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A Reassessment of the Structure of 5,6‐Dihydroxyindole‐2‐carboxylic Acid Melanins by Matrix‐assisted Laser Desorption/Ionization Mass Spectrometry

Matrix-assisted laser desorption/ionization mass spectrometry was applied to the structural investigation of synthetic melanins prepared by enzymatic or chemical oxidation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA), a major biosynthetic precursor. Following optimization of the experimental conditions, spectra with a clearly distinct pattern of peaks could be obtained for all melanin samples. In contrast to the commonly held notion of melanins as high molecular weight polymers, all the pigments analyzed are mixtures of oligomer species with molecular weights in the range 500–1500 Da. Marked differences in the nature of the oligomer components were observed depending on the preparation conditions. Indeed, significant degradation of the pigment backbone, resulting probably by peroxidative fission at the catechol site of the DHICA units, was found in most of the melanin pigments, even those prepared under mild oxidation conditions. The relevance of these results to the current use of synthetic melanins as models of the natural pigments is discussed.