Chemical and Structural Diversity in Eumelanins: Unexplored Bio‐Optoelectronic Materials

Abstract

Eumelanins, the characteristic black insoluble and heterogeneous bio-polymers of human skin, hair and eyes, have intrigued and challenged generations of chemists, physicists and biologists because of their unique structural and optoelectronic properties. Recently, an organic chemistry approach has been combined with advanced spectroscopic and imaging techniques, theoretical calculations and methods of condensed matter physics to gradually force these materials to reveal their secrets. Here we review the latest advances in the field with a view to showing how the emerging knowledge is not only helping us explain eumelanin functionality, but may also be translated into effective strategies for exploiting their properties to create a new class of biologically inspired high tech materials.

Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems

“…11 As polymerization proceeds, aggregation processes and deposition of black insoluble particles within the melanosomes become significant and concur to determine the complex supramolecular organization of eumelanin biopolymers. 12 Similar phenomena occur also in model processes of eumelanin buildup involving biomimetic oxidation of DHI and related metabolites: in all cases dark insoluble polymers are produced which share free radical and optoelectronic characteristics in common with natural eumelanins despite different morphological and surface properties. 13 Although a huge body of epidemiological and biological data supports the superior adaptation of dark-skinned individuals to outdoor lifestyles in sunny areas, the precise mechanisms underlying eumelanin's photoprotective role, including the intrinsic structural factors that govern broad-band absorption features (the "black" chromophore) and relaxation pathways from the monomer to polymer range, remain poorly understood.…”

“…2 Although the UV−visible absorption characteristics of DHI and its oligomers have been investigated to some extent, 15,17−21 no clear-cut relationship between spectral properties of the oligomer structures and the cross sections of the monomeric building blocks has been defined, since the spectroscopic properties of the oligomers vary in a complex and not entirely predictable manner with the bonding patterns between units. 4,12 Investigation of eumelanin emissive properties is even more challenging due to strong fluorescence quenching. Main difficulties derive from the variable broad-band absorption profiles of eumelanins, affecting comparison of data from different samples, and from intra-and intermolecular coupling, introducing fast excited state deactivation channels amplified in the heterogeneous supramolecular systems.…”

Eumelanin thin films: solution-processing, growth, and charge transport properties

“…16,30 High-resolution AFM images indicate that islands are made-up of diskshaped nanoaggregates, with a lateral extension of 10-30 nm ( Fig. 3b; occasionally larger as for the drop cast lm, Fig.…”

Natural melanin pigments and their interfaces with metal ions and oxides: emerging concepts and technologies

“…The presence of two building blocks, different polymerization sites, and redox states [the reduced form is the hydroquinone (H2Q), the intermediate form is the semiquinone (SQ), and the oxidized form is the quinone Q and its tautomer (QI)] co-existing in the pigment result in the well-established chemical heterogeneity of eumelanin. [11] Hierarchical development characterizes the formation of eumelanin-based materials, generated by supramolecular aggregation. Eumelanin planar oligomers can form from four or more DHI monomers (protomolecules), [11] possibly stacking via π-π interactions (Fig.…”