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          Institute: MPI für bioanorganische Chemie     Collection: MPI für bioanorganische Chemie     Display Documents

ID: 681952.0, MPI für bioanorganische Chemie / MPI für bioanorganische Chemie
The Protonation States of Oxo-Bridged MnIV Dimers Resolved by Experimental and Computational Mn K Pre-Edge X-ray Absorption Spectroscopy
Authors:Krewald, Vera; Lassalle-Kaiser, Benedikt; Boron, Thaddeus T.; Pollock, Christopher J.; Kern, Jan; Beckwith, Martha A.; Yachandra, Vittal K.; Pecoraro, Vincent L.; Yano, Junko; Neese, Frank; DeBeer, Serena
Date of Publication (YYYY-MM-DD):2013
Title of Journal:Inorganic Chemistry
Journal Abbrev.:Inorg. Chem.
Start Page:12904
End Page:12914
Review Status:Peer-review
Audience:Experts Only
Abstract / Description:In nature, the protonation of oxo bridges is a commonly encountered mechanism for fine-tuning chemical properties and reaction pathways. Often, however, the protonation states are difficult to establish experimentally. This is of particular importance in the oxygen evolving complex of photosystem II, where identification of the bridging oxo protonation states is one of the essential requirements toward unraveling the mechanism. In order to establish a combined experimental and theoretical protocol for the determination of protonation states, we have systematically investigated a series of Mn model complexes by Mn K pre-edge X-ray absorption spectroscopy. An ideal test case for selective bis-mu-oxo-bridge protonation in a Mn dimer is represented by the system [Mn-2(IV)(salpn)(2)(mu-OHn)(2)](n+). Although the three species [Mn-2(IV)(salPn)(2)(mu-O)(2)], [Mn-2(IV)(salpn)(2)(mu-O)(mu-OH)](+) and [Mn-2(IV)(salpn)(2)(mu-OH)(2)](2+) differ only in the protonation of the oxo bridges, they exhibit distinct differences in the pre-edge region while maintaining the same edge energy. The experimental spectra are correlated in detail to theoretically calculated spectra. A time-dependent density functional theory approach for calculating the pre-edge spectra of molecules with multiple metal centers is presented, using both high spin (HS) and broken symmetry (BS) electronic structure solutions. The most intense pre-edge transitions correspond to an excitation of the Mn Is core electrons into the unoccupied orbitals of local e(g) character (d(z)(2) and d(xy) based in the chosen coordinate system). The lowest energy experimental feature is dominated by excitations of 1s-alpha electrons, and the second observed feature is primarily attributed to 1s-beta electron excitations. The observed energetic separation is due to spin polarization effects in spin-unrestricted density functional theory and models final state multiplet effects. The effects of spin polarization on the calculated Mn K pre-edge spectra, in both the HS and BS solutions, are discussed in terms of the strength of the antiferromagnetic coupling and associated changes in the covalency of Mn-O bonds. The information presented in this paper is complemented with the X-ray emission spectra of the same compounds published in an accompanying paper. Taken together, the two studies provide the foundation for a better understanding of the X-ray spectroscopic data of the oxygen evolving complex (OEC) in photosystem II.
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für bioanorganische Chemie
External Affiliations:Lassalle-Kaiser, B.; Kern, J.; Yachandra, V.K.; Yano, J.; Univ Calif Berkeley, Lawrence Berkeley Natl Lab, Phys Biosci Div, Berkeley, CA 94720 USA.
Boron, T.T., III; Pecoraro, Vincent L.; Univ Michigan, Dept Chem, Ann Arbor, MI 48109 USA.
Kern, J.; SLAC Natl Accelerator Lab, Menlo Pk, CA 94025 USA.
Beckwith, M.A.; DeBeer, S.; Cornell Univ, Dept Chem & Chem Biol, Ithaca, NY 14853 USA.
Identifiers:ISI:000327225900011 [ID No:1]
ISSN:0020-1669 [ID No:2]
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