Ultrastructural Organization of Eumelanin from Sepia officinalis Measured by Atomic Force Microscopy

Abstract

Atomic force microscopy is used to investigate the structural organization of eumelanin isolated from the inks sacs of the cuttlefish Sepia officinalis. Deposits of eumelanin on mica reveal a range of structures. The most prevalent structure is an aggregate comprised of particles with diameters of 100−200 nm. This morphology is consistent with published SEM images of intact granules. Mechanical manipulation of these structures using the AFM tip show that these particles, while stable, are not a fundamental structural unit but are an aggregate of smaller constituents. Images of the bulk pigments also reveal the presence of filament structures that have an average height and width of ∼5 nm and tens of nanometers, respectively. Taken along with recent X-ray scattering and mass spectrometry experiments, the AFM data provides strong supporting evidence for the conclusion that eumelanin is comprised of small oligomeric units and that the structural morphology observed in imaging experiments reflects aggregation of these oligomeric molecules. On the basis of the types of structures observed in the AFM images, a model is proposed for the assembly of the macroscopic pigment. The diversity of functions attributed to melanin in the literature is proposed to result from the heterogeneity of aggregated structures.