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Probing eumelanin photoprotection using a catechol:quinone heterodimer model system

Abstract

Eumelanin is a natural pigment with photoprotective and radical scavenging characteristics that are vital for a multitude of living organisms. However, the molecular mechanisms behind these functions remain obscure, in part because eumelanin is a heterogeneous polymer composed of a complex assortment of structural and chemical domains. Despite uncertainty about its precise structure, the functional units of eumelanin are thought to include quinones in various oxidation states. Here, we investigate the photochemistry of a catechol:o-quinone heterodimer as a model system for uncovering the photoprotective roots of eumelanin. Ultrafast transient absorption measurements in the UV to near-IR spectral regions are used to identify the photochemical processes that occur following selective excitation of the o-quinone in the heterodimer using 395 nm light. We find that both singlet and triplet o-quinone excited states induce hydrogen atom transfer from the catechol, forming semiquinone radical pairs that persist beyond 2.5 ns, the upper time limit accessible by our instrument. Furthermore, the hydrogen atom transfer reaction was found to occur 1000 times faster via the singlet channel. Excited state pathways such as these may be important in eumelanin where similar hydrogen-bonded interfaces may exist between catechol and o-quinone functional groups.

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Publication details

The article was received on 06 Dec 2018, accepted on 09 Jan 2019 and first published on 10 Jan 2019


Article type: Paper
DOI: 10.1039/C8FD00231B
Citation: Faraday Discuss., 2019, Accepted Manuscript
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    Probing eumelanin photoprotection using a catechol:quinone heterodimer model system

    C. Grieco, J. M. Empey, F. R. Kohl and B. Kohler, Faraday Discuss., 2019, Accepted Manuscript , DOI: 10.1039/C8FD00231B

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