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Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH_2/OH Motifs Relevant to H_2O Activation by the Oxygen Evolving Complex in Photosystem II

Reed, Christopher J. and Agapie, Theodor (2018) Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH_2/OH Motifs Relevant to H_2O Activation by the Oxygen Evolving Complex in Photosystem II. Journal of the American Chemical Society, 140 (34). pp. 10900-10908. ISSN 0002-7863. PMCID PMC6277146. http://resolver.caltech.edu/CaltechAUTHORS:20180801-091754883

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Abstract

We report the synthesis of site-differentiated heterometallic clusters with three Fe centers and a single Mn site that binds water and hydroxide in multiple cluster oxidation states. Deprotonation of Fe^(III/II)_3Mn^(II)–OH_2 clusters leads to internal reorganization resulting in formal oxidation at Mn to generate Fe^(III/II)_3Mn^(III)–OH. ^(57)Fe Mössbauer spectroscopy reveals that oxidation state changes (three for Fe^(III/II)_3Mn–OH_2 and four for Fe^(III/II)_3Mn–OH clusters) occur exclusively at the Fe centers; the Mn center is formally Mn^(II) when water is bound and Mn^(III) when hydroxide is bound. Experimentally determined pK_a (17.4) of the [Fe^(III)_2Fe^(II)Mn^(II)–OH_2] cluster and the reduction potentials of the [Fe_3Mn–OH_2] and [Fe_3Mn–OH] clusters were used to analyze the O–H bond dissociation enthalpies (BDE_(O–H)) for multiple cluster oxidation states. BDE_(O–H) increases from 69 to 78 and 85 kcal/mol for the [Fe^(III)Fe^(II)_2Mn^(II)–OH_2], [Fe^(III)_2Fe^(II)Mn^(II)–OH_2], and [Fe^(III)_3Mn^(II)–OH_2] clusters, respectively. Further insight of the proton and electron transfer thermodynamics of the [Fe_3Mn–OH_x] system was obtained by constructing a potential–pK_a diagram; the shift in reduction potentials of the [Fe_3Mn–OH_x] clusters in the presence of different bases supports the BDE_(O–H) values reported for the [Fe_3Mn–OH_2] clusters. A lower limit of the pK_a for the hydroxide ligand of the [Fe_3Mn–OH] clusters was estimated for two oxidation states. These data suggest BDE_(O–H) values for the [Fe^(III)_2Fe^(II)Mn^(III)–OH] and [Fe^(III)_3Mn^(III)–OH] clusters are greater than 93 and 103 kcal/mol, which hints to the high reactivity expected of the resulting [Fe_3Mn═O] in this and related multinuclear systems.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/jacs.8b06426DOIArticle
https://pubs.acs.org/doi/suppl/10.1021/jacs.8b06426PublisherSupporting Information
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6277146PubMed CentralArticle
ORCID:
AuthorORCID
Agapie, Theodor0000-0002-9692-7614
Alternate Title:Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II
Additional Information:© 2018 American Chemical Society. Received: June 19, 2018; Published: July 31, 2018. This research was supported by the NIH (R01-GM102687B) and the Dreyfus Teacher-Scholar Program (T.A.). C.J.R. thanks the Resnick Sustainability Institute at Caltech for a fellowship. We thank Dr. Mike Takase and Dr. Larry Henling for assistance with crystallography, Prof. Jonas Peters for use of his laboratory’s Mössbauer spectrometer, and the Dow Next Generation Educator Fund for instrumentation. The authors declare no competing financial interest.
Group:Resnick Sustainability Institute
Funders:
Funding AgencyGrant Number
NIHR01-GM102687B
Camille and Henry Dreyfus FoundationUNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
Dow Chemical CompanyUNSPECIFIED
PubMed Central ID:PMC6277146
Record Number:CaltechAUTHORS:20180801-091754883
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180801-091754883
Official Citation:Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II. Christopher J. Reed and Theodor Agapie. Journal of the American Chemical Society 2018 140 (34), 10900-10908. DOI: 10.1021/jacs.8b06426
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:88429
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:01 Aug 2018 16:51
Last Modified:05 Dec 2018 17:23

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