Insight into the Final Step of the Supramolecular Buildup of Eumelanin

Abstract

The final step in the supramolecular buildup of eumelanin particles is investigated regarding the involved species and mechanism. Time-resolved in situ light scattering and scanning electron microscopy reveal an aggregation of particles with a narrow size distribution around 40 nm, previously only observed as substructures. These form larger particles with again very uniform size and diameters around 200 nm. Aggregation of each single particle takes only a few minutes to complete, whereas the entire process goes on for at least 3 h, partly due to the kinetics of the precursors. The individual particles also undergo an additional consolidation step toward their final form, which takes up to 24 h. Atomic force microscopy shows that the size before consolidation is around twice the size of the final particles, due to free space between the substructures. Light scattering also reveals that the aggregation is random with respect to where the particles attach, as the shape of aggregates changes from sphere to coil, before it returns to a spherical shape at the end. Application of enzyme mediated autodeposition finally shows the potential to stop the supramolecular buildup at each level, and therefore enables isolation of the respective eumelanin particles at will. This may enable the full potential for melanin materials in nanotechnology deriving from its unique (for biological polymers) properties like paramagnetism, electrical conductivity, and many more.

Dark- or Light-Colored Melanins: Generating Pigments Using Fe2+ and H2O2

“…Although for many reaction mixtures no visible precipitations were observed and the aliquots of the reaction mixtures could be analyzed using SEC, we can not exclude the possibility that the pigment materials generated through the oxidation reactions may exist as aggregates of finely-dispersed small molecules as is discussed for MN pigments. [23][24][25] Fig.4, panel C, correlates the extent of the reactivity of the compound and the intensity of the color of the reaction mixture as evaluated through absorbance readings at 400nm. For all compounds, with the exception of (8), the absorbance increased with declining % of compound remaining up to the point when about 0% of the compound remained; typically when 0.075% v/v H 2 O 2 was present.…”

Eumelanin for nature‐inspired UV‐absorption enhancement of plastics

“…DHI and DHICA form oligomers differing by number of units (up to tens), DHI/DHICA ratio and polymerization sites . Such oligomers stack via π – π interactions, forming so‐called protomolecules, which, in turn, assemble into a complex supramolecular structure . Eumelanin is widely investigated by both physicists and engineers .…”

“…fleece from alpaca) has been demonstrated . In addition, as eumelanin is formed of protomolecules, it would be less likely to leach from a matrix polymer than small‐molecule additives …”

“…6 Such oligomers stack via -interactions, forming so-called protomolecules, which, in turn, assemble into a complex supramolecular structure. 7 Eumelanin is widely investigated by both physicists and engineers. 8 The biopigment, indeed, features physicochemical properties such as radical scavenging, redox activity, metal ion chelation and hydration-dependent electrical conduction, all of interest for energy storage as well as bio and green electronics.…”

“…fleece from alpaca) has been demonstrated. 22 In addition, as eumelanin is formed of protomolecules, 7 it would be less likely to leach from a matrix polymer than small-molecule additives. 19,23 Natural eumelanin (sepia melanin, extracted from the ink sac of cuttlefish), fungal melanin and polydopamine (a eumelanin-like material) 24 have been assessed as additives for improving the UV-absorption properties of polyimide (PI), 25 poly(lactic acid) 26 and poly(vinyl alcohol) (PVA).…”