Paramagnetic Resonance in Chromous Sulfate Pentahydrates

Ôno, Kazuo; Koide, Shoichiro; Sekiyama, Hisao; Abe, Hidetarô

doi:10.1103/physrev.96.38

Cited by 38 publications

(16 citation statements)

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“…The orbital degeneracy of the ground term 5 E g is removed by the tetragonal and orthorhombic distortions of the ligand field to give the ground state (cos δ‚ 5 B 1 + sin δ‚ 5 A 1 ), where δ is a state mixing parameter. General expressions relating the spin-Hamiltonian parameters to the electronic state configuration were previously derived for the electronically equivalent case of Cr 2+ (75) and where (g X , g Y , and g Z ) are the three components of the intrinsic g tensor; g e is the free electron g factor (2.0023); F is the spin-spin coupling constant (0.8 cm -1 for Mn 3+ ) (76); λ is the spin-orbit coupling constant (+88 cm -1 for free Mn 3+ ) (57); ∆ is the energy gap to the excited ligand-field state 5 T 2g (typically 18-25 000 cm -1 ) (58,59). On the basis of the general solutions for the hyperfine spin-Hamiltonian provided by Abragam and Pryce (see eq 3.7 in ref 77), we derived the expressions in (A4) for the 55 Mn hyperfine tensor of Mn 3+ applicable to the particular case of orthorhombic symmetry.…”

Section: Appendix: Spin Hamiltonian Of Mn 3+ In the Orthorhombic Ligamentioning

confidence: 99%

Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex

et al. 2006

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Biogenesis and repair of the inorganic core (Mn 4 CaO x Cl y ), in the water-oxidizing complex of photosystem II (WOC-PSII), occurs through the light-induced (re)assembly of its free elementary ions and the apo-WOC-PSII protein, a reaction known as photoactivation. Herein, we use electron paramagnetic resonance (EPR) spectroscopy to characterize changes in the ligand coordination environment of the first photoactivation intermediate, the photo-oxidized Mn 3+ bound to apo-WOC-PSII. On the basis of the observed changes in electron Zeeman (g eff ), 55 Mn hyperfine (A Z ) interaction, and the EPR transition probabilities, the photogenerated Mn 3+ is shown to exist in two pH-dependent forms, differing in terms of strength and symmetry of their ligand fields. The transition from an EPR-invisible low-pH form to an EPR-active high-pH form occurs by deprotonation of an ionizable ligand bound to Mn 3+ , implicated to be a water molecule: [Mn 3+ (OH 2 In the absence of Ca 2+ , the EPR-active Mn 3+ exhibits a strong pH dependence (pH ∼6.5-9) of its ligand-field symmetry (rhombicity ∆δ ) 10%, derived from g eff ) and A Z (∆A Z ) 22%), attributable to a protein conformational change. Binding of Ca 2+ to its effector site eliminates this pH dependence and locks both g eff and A Z at values observed in the absence of Ca 2+ at alkaline pH. Thus, Ca 2+ directly controls the coordination environment and binds close to the highaffinity Mn 3+ , probably sharing a bridging ligand. This Ca 2+ effect and the pH-induced changes are consistent with the ionization of the bridging water molecule, predicting that [ A single tight-binding Ca 2+ ion is essential for photosynthethic O 2 evolution activity in ViVo (1-5). Calcium is located within the water-oxidizing complex of photosystem II (PSII-WOC), 1 comprised of an oxo/aquo-bridged inorganic core whose stoichiometry, Mn 4 CaO x , is based on several lines of evidence, including X-ray absorption (6-9), electron paramagnetic resonance (EPR) spectroscopy (10,11), and the recent X-ray diffraction (XRD) data (12, 13). Mn-, Sr-, and Ca-extended X-ray absorption fine structure (EXAFS) spectroscopies accurately place the calcium effector site at 3.3-3.5 Å from Mn atoms. However, the relative position of Ca 2+ within the Mn 4 O x cluster differs according to which type of data is emphasized and the assumptions of the models used to interpret the raw data. Spectroscopic and XRD data have constrained the possible core topologies to only a few types, with the structures given in Chart 1 proposed on the basis of one or more lines of evidence (9)(10)(11)13). These structural proposals have led to a number of mechanistic proposals for the role of calcium in O 2 evolution reviewed elsewhere (11,(14)(15)(16)(17).Dissection of the functional roles of the inorganic cofactors has come from in Vitro studies of the photoactivation process, which is defined as the assembly of the inorganic core during biogenesis and repair of the WOC. In Vitro photoactivation of the WOC is described by the following ...

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Section: Appendix: Spin Hamiltonian Of Mn 3+ In the Orthorhombic Ligamentioning

confidence: 99%

Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex

et al. 2006

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“…A theoretical interpretation of the spin-Hamiltonian was given by Ono et aZ. 20 supposing the 5E ground term of the ion to split under the tetragonal field with the 5EMss set of states lower. On mixing in the 5T2 term via the spin-orbit coupling energy this accounts for the g values and then a small admixture of 5EMs$ under the lower symmetry part of the ligand field can give I E I .…”

Section: Zero-field Splitting In the Hiqh-spin Chromous Ionmentioning

confidence: 99%

Octahedral complexes. Part 2.—The calculation of spin-orbit coupling energies

Griffith¹

1960

Trans. Faraday Soc.

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The general form of the matrix of spin-orbit coupling energy between a pair of terms in an S r r l M l coupling scheme is derived. A table is given exhibiting this form explicitly for all terms occurring in t T e n configurations. The complete matrix of spin-orbit coupling is given for d2 and for the lowest terms of d3 to d7, in each case in the strong-field coupling scheme. As applications the crossing of tge2 5T2 by tg1A1 in d6 is discussed and a calculation is made of the zero-field splitting of the ground term of the high-spin chromous ion due to second-order effects of spin-orbit coupling. For the latter no significant disagreement between theory and experiment is found, thus, contrary to a previous view,ZO making it unnecessary to postulate a spin-spin coupling constant significantly different from zero.

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“…The crystal structure of CrSO 4 ·5H 2 O has now been reported and shows that Cr 2+ has tetraaquo equatorial ligands and disulfato axial ligands and is thus effectively trans -[Cr(H 2 O) 4 (SO 4 ) 2 ] 2- , as occurs for CuSO 4 ·5H 2 O. More importantly, a remarkable EPR study was performed over 40 years ago on single-crystal CrSO 4 ·5H 2 O using microwave frequencies of 27, 46, and 55 GHz. , The magnitude, although not the sign, of the zfs parameter D was determined, along with approximate g values. Quite recently, two HF-EPR studies have appeared on a very similar ion, Mn 3+ , also in a HS 3d 4 , S = 2 environment.…”

Section: Introductionmentioning

confidence: 98%

EPR Spectra from “EPR-Silent” Species: High-Field EPR Spectroscopy of Aqueous Chromium(II)

et al. 1998

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High-field (up to 14.5 T) multifrequency (∼90-440 GHz) electron paramagnetic resonance (EPR) spectroscopy has been used to probe the non-Kramers, S ) 2, Cr 2+ ion in frozen aqueous solution, in the presence and absence of the glassing agent, glycerol. Solutions with both chloride and sulfate counterions were investigated. Detailed analysis based on a combination of analytical and full-matrix solutions to the spin Hamiltonian for an S ) 2 system gave zero-field splitting parameters D ) -2.20(5) cm -1 , E ) 0.0(1) cm -1 , and g ⊥ ≈ g | ) 1.98(2), independent of solvent system and counterion. These results are in agreement with an early single-crystal EPR study of CrSO 4 ‚5H 2 O; however, the present study allows unequivocal determination of the sign of D and shows that in solution [Cr(H 2 O) 6 ] 2+ is a perfectly axial system (tetragonally elongated), as opposed to solid CrSO 4 ‚ 5H 2 O, which showed a measurable E value, indicative of slight orthorhombic distortion as seen in its crystal structure. HF-EPR data is combined with earlier electronic absorption data to provide a complete picture of the electronic structure of Cr 2+ in this chemical environment. The results are also compared to a recent HF-EPR study of a Mn 3+ complex, which showed that electronic excited states affect the ground state of the complex, but these effects are less pronounced for Cr 2+ . The present study also shows the applicability of high-field EPR to aqueous solutions of integer-spin ("EPR-silent") transition metal complexes, as previous studies have employed only solid (single-crystal or polycrystalline) samples.

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Paramagnetic Resonance in Chromous Sulfate Pentahydrates

Cited by 38 publications

References 2 publications

Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex

Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex

Octahedral complexes. Part 2.—The calculation of spin-orbit coupling energies

EPR Spectra from “EPR-Silent” Species: High-Field EPR Spectroscopy of Aqueous Chromium(II)

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Paramagnetic Resonance in Chromous Sulfate Pentahydrates

Cited by 38 publications

References 2 publications

Spectroscopic Evidence for Ca2+Involvement in the Assembly of the Mn4Ca Cluster in the Photosynthetic Water-Oxidizing Complex

Spectroscopic Evidence for Ca2+Involvement in the Assembly of the Mn4Ca Cluster in the Photosynthetic Water-Oxidizing Complex

Octahedral complexes. Part 2.—The calculation of spin-orbit coupling energies

EPR Spectra from “EPR-Silent” Species: High-Field EPR Spectroscopy of Aqueous Chromium(II)

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Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex

Spectroscopic Evidence for Ca²⁺Involvement in the Assembly of the Mn₄Ca Cluster in the Photosynthetic Water-Oxidizing Complex