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Proton-coupled electron transfer of tyrosine oxidation: buffer dependence and parallel mechanisms
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science.
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2007 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 129, no 50, 15462-15464 p.Article in journal (Refereed) Published
Abstract [en]

The proton-coupled electron transfer (PCET) from tyrosine covalently linked to a metal complex has been studied. The reaction was induced by laser flash excitation of the metal complex, and PCET was bidirectional, with electron transfer to the excited or flash-quenched oxidized metal complex and proton transfer to water or added buffers in the solution. We found a competition between three different PCET mechanisms: (1) A concerted PCET with water as the proton acceptor, which indeed shows a pH-dependence as earlier reported (Sjödin, M.; Styring, S.; Åkermark, B.; Sun, L.; Hammarström, L. J. Am. Chem. Soc. 2000, 122, 3932); (2) a stepwise electron transfer-proton transfer (ETPT) that is pH-independent; (3) a buffer-assisted concerted PCET. The relative importance of reaction 2 increases with oxidant strength, while that of reaction 1 increases with pH. At higher buffer concentrations reaction 3 becomes important, and the rate follows the expected first-order dependence on the concentration of the buffer base. Most importantly, the pH-dependence of reaction 1, with a slope of 0.4-0.5 in a plot of log k vs pH, is independent of buffer and cannot be explained by reaction schemes with simple first-order dependencies on [OH-], [H3O+], or buffer species.

Place, publisher, year, edition, pages
2007. Vol. 129, no 50, 15462-15464 p.
Keyword [en]
Chemistry, Multidisciplinary
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-12424DOI: 10.1021/ja073012uISI: 000251581900026PubMedID: 18027937OAI: oai:DiVA.org:uu-12424DiVA: diva2:40193
Available from: 2007-12-18 Created: 2007-12-18 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols
Open this publication in new window or tab >>Proton-Coupled Electron Transfer from Hydrogen-Bonded Phenols
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Proton-coupled electron transfer (PCET) is one of the elementary reactions occurring in many chemical and biological systems, such as photosystem II where the oxidation of tyrosine (TyrZ) is coupled to deprotonation of the phenolic proton. This reaction is here modelled by the oxidation of a phenol covalently linked to a Ru(bpy)32+-moitey, which is photo-oxidized by a laser flash-quench method. This model system is unusual as mechanism of PCET is studied in a unimolecular system in water solution. Here we address the question how the nature of the proton accepting base and its hydrogen bond to phenol influence the PCET reaction.

In the first part we investigate the effect of an internal hydrogen bond PCET from. Two similar phenols are compared. For both these the proton accepting base is a carboxylate group linked to the phenol on the ortho-position directly or via a methylene group. On the basis of kinetic and thermodynamic arguments it is suggested that the PCET from these occurs via a concerted electron proton transfer (CEP). Moreover, numerical modelling of the kinetic data provides an in-depth analysis of this CEP reaction, including promoting  vibrations  along the O–H–O coordinate that are required to explain the data.

The second part describes the study on oxidation of phenol where either water or an external base the proton acceptor. The pH-dependence of the kinetics reveals four mechanistic regions for PCET within the same molecule when water is the base. It is shown that the competition between the mechanisms can be tuned by the strength of the oxidant. Moreover, these studies reveal the conditions that may favour a buffer-assisted PCET over that with deprotonation to water solution.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 69 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 706
Keyword
Proton-coupled electron transfer, phenol oxidation, hydrogen bonds, artificial photosynthesis, promoting vibrations, proton transfer, laser flash-quench, transient absorption.
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-112060 (URN)978-91-554-7699-1 (ISBN)
Public defence
2010-02-19, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
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Available from: 2010-01-28 Created: 2010-01-08 Last updated: 2010-01-28Bibliographically approved

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Irebo, TaniaSjödin, MartinHammarström, Leif

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