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Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy

Zakhar V. Bedran, Sergey S. Zhukov, Pavel A. Abramov, Ilya O. Tyurenkov, Boris P. Gorshunov, Bernard Mostert Orcid Logo, Konstantin A. Motovilov

Polymers, Volume: 13, Issue: 24, Start page: 4403

Swansea University Author: Bernard Mostert Orcid Logo

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DOI (Published version): 10.3390/polym13244403

Abstract

Eumelanin is a widespread biomacromolecule pigment in the biosphere and has been widely investigated for numerous bioelectronics and energetic applications. Many of these applications depend on eumelanin's ability to conduct proton current at various levels of hydration. The origin of this beha...

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Published in: Polymers
ISSN: 2073-4360
Published: MDPI AG 2021
Online Access: Check full text

URI: https://cronfa.swan.ac.uk/Record/cronfa59165
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Abstract: Eumelanin is a widespread biomacromolecule pigment in the biosphere and has been widely investigated for numerous bioelectronics and energetic applications. Many of these applications depend on eumelanin's ability to conduct proton current at various levels of hydration. The origin of this behavior is connected to a comproportionation reaction between oxidized and reduced monomer moieties and water. A hydration-dependent FTIR spectroscopic study on eumelanin is presented herein, which allows for the first time tracking the comproportionation reaction via the gradual increase of the overall aromaticity of melanin monomers in the course of hydration. We identified spectral features associated with the presence of specific "one and a half" CO bonds, typical for -semiquinones. Signatures of semiquinone monomers with internal hydrogen bonds and that carboxylic groups, in contrast to semiquinones, begin to dissociate at the very beginning of melanin hydration were indicated. As such, we suggest a modification to the common hydration-dependent conductivity mechanism and propose that the conductivity at low hydration is dominated by carboxylic acid protons, whereas higher hydration levels manifest semiquinone protons.
Keywords: melanin; FTIR spectroscopy; water; comproportionation
College: Faculty of Science and Engineering
Funders: This work was supported by the Russian Science Foundation, Grant 19-73-10154. A.B.M. is a Sêr Cymru II fellow, and the results incorporated in this work are supported by the Welsh Government through the European Union’s Horizon 2020 Research and Innovation Program under Marie Skłodowska-Curie Grant Agreement No. 663830
Issue: 24
Start Page: 4403