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Mechanism for O-O bond formation in a biomimetic tetranuclear manganese cluster - A density functional theory study
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Stockholm University, Faculty of Science, Department of Organic Chemistry.
Number of Authors: 2
2015 (English)In: Journal of Photochemistry and Photobiology. B: Biology, ISSN 1011-1344, E-ISSN 1873-2682, Vol. 152, 162-172 p.Article in journal (Refereed) Published
Abstract [en]

Density functional theory calculations have been used to study the reaction mechanism of water oxidation catalyzed by a tetranuclear Mn-oxo cluster Mn4O4L6 (L = (C6H4)(2)PO4-). It is proposed that the O-O bond formation mechanism is different in the gas phase and in a water solution. In the gas phase, upon phosphate ligand dissociation triggered by light absorption, the O-O bond formation starting with both the Mn-4(III,III,IV,IV) and Mn-4(III,IV,IV,IV) oxidation states has to take place via direct coupling of two bridging oxo groups. The calculated barriers are 42.3 and 37.1 kcal/mol, respectively, and there is an endergonicity of more than 10 kcal/mol. Additional photons are needed to overcome these large barriers. In water solution, water binding to the two vacant sites of the Mn ions, again after phosphate dissociation triggered by light absorption, is thermodynamically and kinetically very favorable. The catalytic cycle is suggested to start from the Mn-4(III,III,III,IV) oxidation state. The removal of three electrons and three protons leads to the formation of a Mn-4(III,IV,IV,IV)-oxyl radical complex. The O-O bond formation then proceeds via a nucleophilic attack of water on the Mn-IV-oxyl radical assisted by a Mn-bound hydroxide that abstracts a proton during the attack. This step was calculated to be rate-limiting with a total barrier of 29.2 kcal/mol. This is followed by proton-coupled electron transfer, O-2 release, and water binding to start the next catalytic cycle.

Place, publisher, year, edition, pages
2015. Vol. 152, 162-172 p.
National Category
Biochemistry and Molecular Biology Organic Chemistry
Identifiers
URN: urn:nbn:se:su:diva-124801DOI: 10.1016/j.jphotobiol.2014.12.005ISI: 000366075400020PubMedID: 25534173OAI: oai:DiVA.org:su-124801DiVA: diva2:890691
Available from: 2016-01-04 Created: 2016-01-04 Last updated: 2016-01-04Bibliographically approved

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Siegbahn, Per E. M.
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