Hydration-Controlled X‑Band EPR Spectroscopy: A Tool for Unravelling the Complexities of the Solid-State Free Radical in Eumelanin

• Published in: The journal of physical chemistry. B Vol. 117; no. 17; pp. 4965 - 4972
• Main Authors: Mostert, A. Bernardus, Hanson, Graeme R, Sarna, Tadeusz, Gentle, Ian R, Powell, Benjamin J, Meredith, Paul
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02.05.2013
• Subjects: Biological and medical sciences; Density; Electron Spin Resonance Spectroscopy; Electronics; Electrons; Free radicals; Free Radicals - chemistry; Fundamental and applied biological sciences. Psychology; Hydrogen-Ion Concentration; Indoles - chemistry; Melanin; Melanins - chemistry; Melanins - metabolism; Molecular biophysics; Obstacles; Physico-chemical properties of biomolecules; Radicals; Water - chemistry; X-band; Continuous wave; pH effect; Electron paramagnetic resonance; Hydration; Melanin; Free radical
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp401615e

Melanin, the human skin pigment, is found everywhere in nature. Recently it has gained significant attention for its potential bioelectronic properties. However, there remain significant obstacles in realizing its electronic potential, in particular, the identity of the solid-state free radical in eumelanin, which has been implicated in charge transport. We have therefore undertaken a hydration-controlled continuous-wave electron paramagnetic resonance study on solid-state eumelanin. Herein we show that the EPR signal from solid-state eumelanin arises predominantly from a carbon-centered radical but with an additional semiquinone free radical component. Furthermore, the spin densities of both of these radicals can be manipulated using water and pH. In the case of the semiquinone radical, the comproportionation reaction governs the pH- and hydration-dependent behavior. In contrast, the mechanism underlying the carbon-centered radical’s pH- and hydration-dependent behavior is not clear; consequently, we have proposed a new destacking model in which the intermolecular structure of melanin is disordered due to π–π destacking, brought about by the addition of water or increased pH, which increases the proportion of semiquinone radicals via the comproportionation reaction.