Mössbauer Study of Reduced Rubredoxin As Purified and in Whole Cells. Structural Correlation Analysis of Spin Hamiltonian Parameters

Abstract

The [Fe(II)(Cys)(4)](2-) site of rubredoxin from Clostridium pasteurianum (Rd(red)) has been studied by Mössbauer spectroscopy in both purified protein and whole cells of Escherichia coli overproducing it. Excellent fits were obtained to an S = 2 spin Hamiltonian for D = 5.7(3) cm(-1), E/D = 0.25(2), delta = 0.70(3) mm/s, DeltaE(Q) = -3.25(2) mm/s, eta = 0.75(5), A(x) = -20.1(7) MHz, A(y) = -11.3(2) MHz, and A(z) = -33.4(14) MHz. These parameters were analyzed with crystal-field theory for the (5)D manifold of iron(II), revealing a d(z(2)) orbital ground state that is admixed by approximately 0.21 d(x(2) - y(2)). The spin-Hamiltonian parameters are consistent within the (5)D theory, apart from the zero-field splitting parameter, D. This problem was solved by extending the crystal-field treatment with spin-orbit coupling to spin-triplet d-d excited states of the iron. Theoretical estimates are given for the spin-triplet (D(T)) and spin-quintet contributions (D(Q)) to D based on excitation energies derived from time-dependent density functional theory, TD-DFT. The computational results were interpreted in terms of crystal-field theory, yielding the Racah parameters B = 682 cm(-1) and C = 2583 cm(-1). The theoretical analysis gives the relative magnitudes D(Q):D(T):D(ss) = 51%: 42%:7% (D(ss) originates from spin-spin interaction). The DFT analysis corroborates the pivotal role of the torsion angles (omega(i)) of the C-S(i) bonds in shaping the electronic structure of the iron(II) site. Rd(red) in overexpressing whole cells accounts for 60% of the Mössbauer absorption. The Rd(red) spectra from whole cells are virtually identical to those of the purified protein. By using the theoretical omega dependence of the spin Hamiltonian parameters, the torsions for Rd(red) in whole cells and purified protein samples are estimated to be the same within 2 degrees. These findings establish Mössbauer spectroscopy as a structural tool for investigating iron sites in whole cells.

An Fe IV O complex of a tetradentate tripodal nonheme ligand

“…Such excited states can be admixed into the ground state by spin-orbit coupling, and this admixture would affect the D value but would not lead to a reduction of A x and A y . It has recently been shown that 40% of the zero-field splitting of the S ϭ 2 state of Fe II rubredoxin is attributable to mixing with excited S ϭ 1 configurations and that such mixing does not affect the A tensor (50).…”

First principles approach to the electronic structure, magnetic anisotropy and spin relaxation in mononuclear 3d-transition metal single molecule magnets

“…It must be noted that spin-Hamiltonian parameters are also sensitive to the S-Co-S angles. In fact it has been shown, that even for the JT active [Fe(SPh) 4 ] 2-complexes simple A c c e p t e d M a n u s c r i p t ligand field arguments that would stabilize the elongated or compressed geometries cannot explain the experimentally observed Mössbauer parameters [169,170]. In fact the SH parameters of such iron centers were found to be rather sensitive to more complicated patterns of distortion such as those described by the variation of the S-Fe-S-R torsion angles.…”

Anisotropic double exchange in a valence-delocalized [Fe2.5+Fe2.5+] cluster. Spin–orbit coupling in the Anderson–Hasegawa double exchange model

“…2a) in the ½Fe 2þ a Fe 3þ b and ½Fe 3þ a Fe 2þ b localizations. The x a , y a , z a (x b , y b , z b ) axes are directed along the edges of the cube in which the Fe a S 4 (Fe b S 4 ) tetrahedra is inscribed [36,57,59]; z a kZ and z b k(ÀZ) are directed perpendicular to the common S-S edge, X-axis (Fig. 4 in Ref.…”

“…[52]). For the Fe 2+ ion, we consider a distortion, which compresses the tetrahedron along the local z a and z b (and cluster Z) axes and results in the non-degenerate 5 A 1 ðd z 2 Þ ground state [36,57,59] in the localized states. The ground state Slater determinant wave functions w 0 ð 5 A 1 ;mÞðFe 2þ Þ and u 0 ( 6 A 1 , m)(Fe 3+ ) are the following [36,37]: [37].…”

“…/6 (g k = 2.00, g ? = 2(1 À 3k(Fe 2+ )/D 1n )) and the positive axial ZFS parameter (D 1 (Fe 2+ ) = 3k(Fe 2+ ) 2 /D 1n ) of the mononuclear [Fe 2+ S 4 ] centers of rubredoxins and its synthetic model systems in the flattened tetrahedral coordination with the 5 A 1 ground state, g kexp = 2.00, g ?exp = 2.08 [55][56][57][58][59].…”