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 approximately 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.

The Ultraviolet Absorption Coefficient of Melanosomes Decreases with Increasing Pheomelanin Content

“…19 This is common to this class of organelle and has been widely observed. [28][29][30] Third, the outermost pigment surface of these melanosomes is pure eumelanin, despite the significant difference in their overall pigment composition, consistent with a "casing" model of mixed melanogenesis (the pheomelanin present in the melanosome is encased by eumelanin). 5,6,[31][32][33][34] Therefore, with increased pheomelanin content, the outer eumelanin coating will be thinner.…”

Photoprotection of human retinal pigment epithelium cells against blue light-induced apoptosis by melanin free radicals from Sepia officinalis

“…These -stacked aggregates of indolequinone-like radicals may be ideal for free radical migration (14). Atomic force microscopy and scanning electron microscopy have been successfully used to visualize melanin ultrastructure, revealing that aggregates of native RPE eumelanin are composed of regular 150-nm substructures, which are themselves composed of Ϸ10-nm constituent aggregates (28)(29)(30). These studies also demonstrated that Sepia officinalis melanin is more similar to native RPE eumelanin than other melanins (28)(29)(30).…”

“…Conductivity studies indicate that some melanin species are likely to be negatively charged (24,25). Melanin monomers are cross-linked into planar oligomers of Ϸ4-6 monomers, and several planar oligomers are assembled into successively larger aggregates by -stacking and van der Waals interactions (26)(27)(28)(29). These -stacked aggregates of indolequinone-like radicals may be ideal for free radical migration (14).…”

Optical and Paramagnetic Properties of Size-Controlled Ink Particles Isolated fromSepia officinalis

“…This structural characteristic is consistent with that reported in previous AFM studies. 8,9) The huge aggregated structure was observed irrespective of whether an ultrasonic procedure was performed on the loading solution. We inferred that the aggregation might have been induced by evaporation of the solvent.…”