Ultrastructural organization of eumelanin from Sepia officinalis measured by atomic force microscopy

Atomic force microscopy is used to investigate the structural organization of eumelanin isolated from the inks sacs of the cuttlefish Sepia officinali s. Deposits of eumelanin on mica reveal a range of structures. The most pre valent structure is an aggregate comprised of particles with diameters of 1 00-200 nm. This morphology is consistent with published SEM images of intac t granules. Mechanical manipulation of these structures using the AFM tip s how that these particles, while stable, are not a fundamental structural un it but are an aggregate of smaller constituents. Images of the bulk pigment s also reveal the presence of filament structures that have an average heig ht and width of similar to5 nm and tens of nanometers, respectively. Taken along with recent X-ray scattering and mass spectrometry experiments, the A FM data provides strong supporting evidence for the conclusion that eumelan in is comprised of small oligomeric units and that the structural morpholog y observed in imaging experiments reflects aggregation of these oligomeric molecules. On the basis of the types of structures observed in the AFM imag es, a model is proposed for the assembly of the macroscopic pigment. The di versity of functions attributed to melanin in the literature is proposed to result from the heterogeneity of aggregated structures.