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Article

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

1
Center for Photonics and 2D Materials, Moscow Institute of Physics and Technology, Institute Lane 9, 141701 Dolgoprudny, Russia
2
Department of Chemistry, Swansea University, Singleton Park, Swansea SA2 8PP, UK
*
Author to whom correspondence should be addressed.
Academic Editor: Adriana Kovalcik
Polymers 2021, 13(24), 4403; https://doi.org/10.3390/polym13244403
Received: 29 October 2021 / Revised: 29 November 2021 / Accepted: 9 December 2021 / Published: 15 December 2021
(This article belongs to the Section Polymer Chemistry)
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” C𝌁O bonds, typical for o-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. View Full-Text
Keywords: melanin; FTIR spectroscopy; water; comproportionation melanin; FTIR spectroscopy; water; comproportionation
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MDPI and ACS Style

Bedran, Z.V.; Zhukov, S.S.; Abramov, P.A.; Tyurenkov, I.O.; Gorshunov, B.P.; Mostert, A.B.; Motovilov, K.A. Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy. Polymers 2021, 13, 4403. https://doi.org/10.3390/polym13244403

AMA Style

Bedran ZV, Zhukov SS, Abramov PA, Tyurenkov IO, Gorshunov BP, Mostert AB, Motovilov KA. Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy. Polymers. 2021; 13(24):4403. https://doi.org/10.3390/polym13244403

Chicago/Turabian Style

Bedran, Zakhar V., Sergey S. Zhukov, Pavel A. Abramov, Ilya O. Tyurenkov, Boris P. Gorshunov, A. B. Mostert, and Konstantin A. Motovilov. 2021. "Water-Activated Semiquinone Formation and Carboxylic Acid Dissociation in Melanin Revealed by Infrared Spectroscopy" Polymers 13, no. 24: 4403. https://doi.org/10.3390/polym13244403

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