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 st ructural investigation of natural melanins has, so far, been largely u nsuccessful, because of the adverse physical and chemical properties o f these pigments. In the present study, we applied matrix-assisted las er desorption/ionization mass spectrometry (MALDI-MS) to the direct an alysis of fresh melanin samples from the cuttlefish Sepia officinalis, and succeeded in identifying clearly distinct patterns of low molecul ar weight ionic species, ranging from 450 to 1200 Da. Detailed analysi s of the molecular weights and mass differences between the major spec ies, aided by comparison with MALDI-MS spectra of synthetic melanins, allowed formulation of the major components as oligomers of 5,6-dihydr oxyindole-2-carboxylic acid partially degraded by peroxidative fission of the o-diphenol moieties with concomitant decarboxylation and oxyge nation reactions. These results provide the first direct insight into the structure of Sepia melanin and confirm the unique value of MALDI-M S techniques for the investigation of highly complex and heterogeneous polymers such as melanins. (C) 1997 by John Wiley & Sons, Ltd.