Mössbauer Spectroscopy of the Spin-Coupled Fe³⁺−Fe²⁺ Center of Reduced Uteroferrin

Abstract

The mixed valence active center of the diiron-oxo protein uteroferrin (Ufr), a porcine purple acid phosphatase, has been analyzed by Mössbauer spectroscopy. Low and high magnetic field spectra are consistent with an antiferromagnetically spin-coupled center. Theoretical spectra that simulate experimental data have been calculated by diagonalizing the spin Hamiltonian H )Here the subscripts 1 and 2 refer to Fe 3+ (S 1 ) 5/2) and Fe 2+ (S 2 ) 2), respectively. For T e 4.2 K, both irons exhibit resolved magnetic hyperfine splittings which depend on the direction and magnitude of the external field, and the static limit of H applies. For T g 100 K, the spin fluctuations are fast and pure quadrupole doublets are observed unless strong fields are applied. In Ufr, the dominant isotropic exchange is strongly perturbed by the zero field splitting (ZFS). We have also calculated the EPR g eff tensor by diagonalizing the electronic terms of H. Our work confirms the claim (Sage, J. T.; et al. J. Am. Chem. Soc. 1989, 111, 7239) that the anisotropy of g eff arises from the admixture of higher spin manifolds to the S eff ) 1 / 2 ground state by the ZFS. In order to find a solution to the Hamiltonian, we searched in its large parameter space with a genetic algorithm. This highly effective searching procedure allowed us to find an optimal parameter set that simultaneously reproduces Mössbauer spectra and EPR g values. The strategies employed for the search in the parameter space of H are discussed. We have determined the exchange constant J ) 34.7 cm -1 , ZFS parameters D 1 ) -0.10 cm -1 , E 1 ≈ 0, D 2 ) +10.81 cm -1 , and E 2 ) +3.17 cm -1 , and hyperfine tensors ã 1 /g n n ) -(21.4,21.2,17.8) T and ã 2 /g n n ) -(15.2,12.2,14.1) T. We have also interpreted the 4.2 K spectra with an effective S eff ) 1 / 2 Hamiltonian for the ground state and determined effective hyperfine tensors à 1 eff /g n n ) -(45.6,64.1,31.3) T and à 2 eff /g n n ) +(11.4,24.9,16.7) T. For Fe 2+ , the ZFS and the electric and magnetic hyperfine interactions were consistent with the presence of axial and rhombic distortions of the dominant octahedral electrostatic potential. The principal axes of the orthorhombic field, which are rotated by 45°about the z axis of the octahedral field, defined an orbital ground state for Fe 2+ of |x 2 -y 2 〉 symmetry with some |z 2 〉 admixture, consistent with the sign of the electric field gradient. Knowledge of the orbital ground state has allowed us to estimate the intrinsic Fermi contact, orbital, and dipolar hyperfine tensors for Fe 2+ .

First-principle computation of zero-field splittings: Application to a high valent Fe(IV)-oxo model of nonheme iron proteins

“…Thus, often, experimental ZFS parameters have been obtained but their formal relationship to the three-dimensional structure has remained unclear. 8,12,13 It is, therefore, highly desirable to implement accurate first-principle methods which allow a direct mapping of the ZFS parameters to their corresponding crystallographic structures. This in turn will provide a much needed link between spectroscopy and molecular structure.…”

The Molecular and Electronic Structure of Octahedral Tris(phenolato)iron(III) Complexes and Their Phenoxyl Radical Analogues: A Mössbauer and Resonance Raman Spectroscopic Study

“…[24,30] Large electric quadrupole splittings of more than 1 mm s À1 are known to occur for Fe III 2 (m-oxo) dimers. [16,17,33,34] It is the asymmetry in 3d occupation due to the short covalent Fe ± oxo bond which gives rise to the large efg.…”

Exchange Interactions and Covalency in Dinuclear Complexes of Iron(III) and Gallium(III) Containing the Redox‐Noninnocent Ligand 1,2‐Bis(3,5‐di‐tert‐butyl‐2‐hydroxyphenyl)oxamide

“…This does not necessarily indicate the formation of Fe IV ; the large g shifts owe their origin to level mixing of the ground state doublet with excited quadruplet due to competing exchange and ZFS, similar to what was found for 2. This situation resembles moderately strong coupled high-spin Fe III /Fe II dimers (S 1 ϭ 5 / 2 , S 2 ϭ 2) as found in the OH-bridged diiron cores of purpleacid phosphatases, methane monooxygenase, and related model compounds [10] where the g values span the large range 1.5Ϫ2. We achieved a quantitative interpretation of the g values of oxidized 2 from a corresponding spin-Hamiltonian approach based on the simplified coupling scheme shown in Scheme 3 by using Equation (1) into the oxidized system with S 2 ϭ S* ϭ S Fe ϩ S rad using spin projection techniques [11] : D 2 ϭ 2.13 cm…”