Identification of partially degraded oligomers of 5,6-dihydroxyindole-2-carboxylic acid in Sepia melanin by matrix-assisted laser desorption/ionization mass spectrometry

Despite extensive efforts over more than half a century, the direct structural investigation of natural melanins has, so far, been largely unsuccessful, because of the adverse physical and chemical properties of these pigments. In the present study, we applied matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to the direct analysis of fresh melanin samples from the cuttlefish Sepia officinalis, and succeeded in identifying clearly distinct patterns of low molecular weight ionic species, ranging from 450 to 1200 Da. Detailed analysis of the molecular weights and mass differences between the major species, aided by comparison with MALDI-MS spectra of synthetic melanins, allowed formulation of the major components as oligomers of 5,6-dihydroxyindole-2-carboxylic acid partially degraded by peroxidative fission of the o-diphenol moieties with concomitant decarboxylation and oxygenation reactions. These results provide the first direct insight into the structure of Sepia melanin and confirm the unique value of MALDI-MS techniques for the investigation of highly complex and heterogeneous polymers such as melanins. (C) 1997 by John Wiley & Sons, Ltd.