Decoding eumelanin's spin label signature: a comprehensive EPR analysis

Abstract

Eumelanin is a black-brown natural pigment found throughout nature. As a material class, it possesses unique physical and chemical properties that entice much attention from chemists, physicists, and material scientists...

Melanin: Nature's 4th bioorganic polymer

“…The origin of these radicals are believed to be in part due to unterminated radicals left over from synthesis, and in part due to the generation of radicals from the underlying quinone chemistry of the monomer units. 99–101 Their concentration according to some of the published estimates may reach more than 10 20 radicals per gram of material. 84,102 In general case concentration of radicals in PM seems to be higher than in EM.…”

“…There are a number of issues where the concerned reader may find a wide discussion and summary of the present level of understanding in a recent EM electron paramagnetic resonance (EPR) spectroscopy review. 100 However, we will highlight some key points in regards to the radicals in melanin.…”

“…It is still an outstanding question as to the exact nature of this radical species and whether there are even more, minor radical species present within EM. 100…”

“…Despite numerous attempts to model, its chemical nature remains unclear and is an ongoing topic of research. 100 However, what is relatively unambiguous is that CCR is remarkably stable and less reactive than the SQR.…”

Electrochemical Doping Effect on the Conductivity of Melanin-Inspired Materials

“…Melanin is a polyindolequinone system formed predominantly by two basic building blocks: 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole carboxylic acid (DHICA). ,, These monomers exist in various oxidative states ( o -hydroquinone, o -semiquinone, o -quinone imine, o -indole quinone), and they coexist within planar-like sheet structures due to covalent interactions. These sheets, in turn, self-assemble through the π–π stacking of nanoaggregates, contributing to the formation of the melanin macromolecular structure. − The interplanar distance between melanin sheets (3.2–4 Å) depends on the type of melanin and the degree of hydration of the melanin. − Even so, there are still some gaps in knowledge that need to be better understood to map their properties and functions. , Physical properties of eumelanins, such as protonic and electronic conductance, precise charge transport mechanism, solid-state doping behavior, solubility, structure of adsorbed water, and content of free radicals have been studied since the 1960s. − …”