| 1. |
- Cox, Nicholas, et al.
(författare)
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Electronic Structure of a Weakly Antiferromagnetically Coupled Mn(II)Mn(III) Model Relevant to Manganese Proteins : A Combined EPR, (55)Mn-ENDOR, and DFT Study
- 2011
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Ingår i: Inorganic Chemistry. - : American Chemical Society. - 0020-1669 .- 1520-510X. ; 50:17, s. 8238-8251
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of the electronic structure of the [Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)](ClO(4))(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
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| 2. |
- Cox, Nicholas, et al.
(författare)
-
Electronic Structure of a Weakly Antiferromagnetically Coupled MnIIMnIII Model Relevant to Manganese Proteins : A Combined EPR, 55Mn-ENDOR, and DFT Study
- 2011
-
Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 50:17, s. 8238-8251
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Tidskriftsartikel (refereegranskat)abstract
- An analysis of the electronic structure of the [(MnMnIII)-Mn-II(mu-OH)-(mu-piv)(2)(Me(3)tacn)(2)] (ClO4)(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-mu-hydroxo bridge, bis-mu-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn-III. All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the Mn-55-ENDOR constrains the Mn-55 hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn-II and Mn-III ions. The large effective hyperfine tensor anisotropy of the Mn-II, a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn-III ion strongly perturbs the zero-field energy levels of the (MnMnIII)-Mn-II complex. An estimate of the fine structure parameter (d) for the Mn-III of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn-II ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn-III is consistent with the known coordination environment of the Mn-III ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
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| 3. |
- Kulik, Leonid V., et al.
(författare)
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Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR Spectroscopy
- 2007
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Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC. - 0002-7863 .- 1520-5126. ; 129:44, s. 13421-13435
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Tidskriftsartikel (refereegranskat)abstract
- The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S-0 to S-4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained Mn-55 hyperfine coupling constants of the S-0 and S-2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S-0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S-0 and S-2 are, at 4 K, Mn-4(III, III, III, IV) and Mn-4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and Mn-55-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 angstrom Mn-Mn distance in S-0 to 2.75 angstrom in S-1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between Mn-A and Mn-B, i.e., between the outer Mn and its neighboring Mn of the mu(3)-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S-0 -> S-1 and S-1 -> S-2 transitions.
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