Free Energy and Stacking of Eumelanin Nanoaggregates

Melanin: Nature's 4th bioorganic polymer

The pigments known as the melanins are widely recognized for their responsibility in the coloration of human skin, eyes, hair, and minimising the harmful effects of solar ultraviolet radiation. But specialists are aware that the melanins are present in all living kingdoms, barring viruses, and have functionality that extends beyond neutralizing ionising radiation. The ubiquitous presence of melanin in almost all human organs, recognized in recent years, as well as the presence of melanin in organisms that are evolutionarily distant from each other, indicate the fundamental importance of this class of material for all life forms. In this review, we argue for the need to accept melanins as the fourth primordial class of biological polymers, along with nucleic acids, proteins and polysaccharides. We consistently compare the properties of these canonical biological polymers with the properties of melanin and highlight key features that fundamentally distinguish melanins, their function and its mysteries.

Electrochemical Doping Effect on the Conductivity of Melanin-Inspired Materials

Eumelanin is a natural pigment that can be particularly valuable for sustainable bioelectronic devices due to its inherent biocompatibility and hydration-dependent conductivity. However, the low conductivity of eumelanin limits its technological development. In this research, electrochemical doping was proposed as an alternative route to increase the electronic conductivity of synthetic eumelanin derivatives. Thin films of sulfonated eumelanin were deposited on platinum interdigitated electrodes and electrochemically treated by using cyclic voltammetry and chronoamperometry treatments. X-ray photoelectron spectroscopy analysis confirmed ion doping in sulfonated melanin. Current-voltage, current-time, and electrochemical impedance measurements were used to investigate the effect of different aqueous electrolytes (including KCl and LiClO4) treatments on the charge transport of sulfonated eumelanin. We show that the conductivity depends on the type and size of the anion used and can reach 10-3 S·cm-1. Additionally, depending on the electrolyte, there is a change in charge transport from mixed ionic/electronic to a predominantly electronic-only conduction. Our results show that the chemical nature of the ion plays an important role in the electrochemical doping and, consequently, in the charge transport of eumelanin. These insights serve as inspiration to explore the use of alternative electrolytes with different compositions further and develop eumelanin-based devices with tunable conductivities.

Molecular mechanisms underlying nanowire formation in pristine phthalocyanine

Understanding the molecular processes of nanowire self-assembly is crucial for designing and controlling nanoscale structures that could lead to breakthroughs in functional materials. In this work, we focus on pristine phthalocyanines as a representative example of mesogenic supramolecular assemblies and have analyzed the formation of nanowires using classical molecular dynamics simulations. In the simulations, the molecules spontaneously form multi-columnar structures resembling supramolecular polymers that subsequently grow into more ordered aggregates. These self-assemblies are concentration dependent, leading to the formation of multi-columnar, dynamic aggregates at higher concentrations and nanowires at lower concentrations. The multi-columnar assemblies on a whole are more disordered than the nanowires, but have locally ordered domains of parallel facing molecules that can fluctuate while maintaining their overall shape. The nanowire formation at lower concentrations involves the initial interaction and clustering of randomly oriented phthalocyanine molecules, followed by the merging of small clusters into elongated segments and the eventual formation of a stable nanowire. We observe three main conformers in these self-assemblies, the parallel, T-shaped and edge-to-edge stacking of the phthalocyanine dimers. We calculate the underlying free energy landscape and show that the parallel conformers form the most stable configuration which is followed by the T-shaped and edge-to-edge dimer configurations. The findings provide insights into the mechanisms and pathways of nanowire formation and a step towards the understanding of self-assembly processes in supramolecular mesogens.

Melanin: insights into structure, analysis, and biological activities for future development

Melanin, a widely distributed pigment found in various organisms, possesses distinct structures that can be classified into five main types: eumelanin (found in animals and plants), pheomelanin (found in animals and plants), allomelanin (found in plants), neuromelanin (found in animals), and pyomelanin (found in fungi and bacteria). In this review, we present an overview of the structure and composition of melanin, as well as the various spectroscopic identification methods that can be used, such as Fourier transform infrared (FTIR) spectroscopy, electron spin resonance (ESR) spectroscopy, and thermogravimetric analysis (TGA). We also provide a summary of the extraction methods of melanin and its diverse biological activities, including antibacterial properties, anti-radiation effects, and photothermal effects. The current state of research on natural melanin and its potential for further development is discussed. In particular, the review provides a comprehensive summary of the analysis methods used to determine melanin species, offering valuable insights and references for future research. Overall, this review aims to provide a thorough understanding of the concept and classification of melanin, its structure, physicochemical properties, and structural identification methods, as well as its various applications in the field of biology.

Brilliant whiteness in shrimp from ultra-thin layers of birefringent nanospheres

A fundamental question regarding light scattering is how whiteness, generated from multiple scattering, can be obtained from thin layers of materials. This challenge arises from the phenomenon of optical crowding, whereby, for scatterers packed with filling fractions higher than ~30%, reflectance is drastically reduced due to near-field coupling between the scatterers. Here we show that the extreme birefringence of isoxanthopterin nanospheres overcomes optical crowding effects, enabling multiple scattering and brilliant whiteness from ultra-thin chromatophore cells in shrimp. Strikingly, numerical simulations reveal that birefringence, originating from the spherulitic arrangement of isoxanthopterin molecules, enables intense broadband scattering almost up to the maximal packing for random spheres. This reduces the thickness of material required to produce brilliant whiteness, resulting in a photonic system that is more efficient than other biogenic or biomimetic white materials which operate in the lower refractive index medium of air. These results highlight the importance of birefringence as a structural variable to enhance the performance of such materials and could contribute to the design of biologically inspired replacements for artificial scatterers like titanium dioxide.

Structural Investigation of DHICA Eumelanin Using Density Functional Theory and Classical Molecular Dynamics Simulations

Eumelanin is an important pigment, for example, in skin, hair, eyes, and the inner ear. It is a highly heterogeneous polymer with 5,6-dihydroxyindole-2-carboxylic acid (DHICA) and 5,6-dihydroxyindole (DHI) building blocks, of which DHICA is reported as the more abundant in natural eumelanin. The DHICA-eumelanin protomolecule consists of three building blocks, indole-2-carboxylic acid-5,6-quinone (ICAQ), DHICA and pyrrole-2,3,5-tricarboxylic acid (PTCA). Here, we focus on the self-assembly of DHICA-eumelanin using multi-microsecond molecular dynamics (MD) simulations at various concentrations in aqueous solutions. The molecule was first parameterized using density functional theory (DFT) calculations. Three types of systems were studied: (1) uncharged DHICA-eumelanin, (2) charged DHICA-eumelanin corresponding to physiological pH, and (3) a binary mixture of both of the above protomolecules. In the case of uncharged DHICA-eumelanin, spontaneous aggregation occurred and water molecules were present inside the aggregates. In the systems corresponding to physiological pH, all the carboxyl groups are negatively charged and the DHICA-eumelanin model has a net charge of -4. The effect of K+ ions as counterions was investigated. The results show high probability of binding to the deprotonated oxygens of the carboxylate anions in the PTCA moiety. Furthermore, the K+ counterions increased the solubility of DHICA-eumelanin in its charged form. A possible explanation is that the charged protomolecules favor binding to the K+ ions rather than aggregating and binding to other protomolecules. The binary mixtures show aggregation of uncharged DHICA-eumelanins; unlike the charged systems with no aggregation, a few charged DHICA-eumelanins are present on the surface of the uncharged aggregation, binding to the K+ ions.