Structure and magnetism of iron-sulfur clusters in proteins

Iron-sulfur clusters are mixed-valence systems exhibiting both localized and delocalized valence states. We discuss here spin-coupling models for two types of oxidized [3Fe-4S] clusters with localized Fe³⁺ valence states; a Heisenberg Hamiltonian with isotropic antiferromagnetic exchange fits the data well. Reduced [3Fe-4S] clusters, on the other hand, contain a trapped Fe³⁺ site and a delocalized Fe³⁺-Fe²⁺ pair. The pair has spin S₁₂=9/2 (formally ferromagnetic coupling) and is antiferromagnetically coupled to the Fe³⁺ S₃=5/2 spin to yield a system spin S=2. We discuss also recent results for [4Fe-4S] clusters such as [3Fe-4S]→[4Fe-4S] conversions, incorporation of other metals into the iron-sulfur core, and the observation of novel spin states.     

Iron sulfur clusters in benzoyl–CoA reductase

The enzyme benzoyl-CoA reductase (BCR) has been studied by Mössbauer spectroscopy in fields up to 7 T and at temperature down to 4.2 K. It has been shown that the oxidized BCR contains three diamagnetic [4Fe-4S]²⁺ centers. Treatment of BCR with dithionite or deazaflavin reduces only one of the three centers from [4Fe-4S]²⁺ to [4Fe-4S]¹⁺. The latter exhibits Mössbauer spectra at 4.2 K in applied fields characteristic for Fe^2.5+Fe^2.5+ (S=9/2) and Fe²⁺Fe²⁺ (S=4) pairs, both coupled antiferromagnetically to total spin S=1/2.     

Cosegregation of tungsten and nitrogen on Fe-9%W-N(100)

Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) have been used to study the cosegregation of tungsten and nitrogen on ferritic Fe-9%W-N(100) single crystals with nitrogen contents ranging from c_N = 11 to 51 wt-ppm. Cosegregation occurs at temperatures T≲600°C depending on the nitrogen content. The thickness of the cosegregated surface layer is estimated by means of Ar⁺ ion depth profiling as being less than two atomic layers. The LEED pattern of the tungsten and nitrogen covered Fe-9%W-51ppmN(100) substrate shows a sharp (1 × 1) structure at a low background intensity indicating epitaxial stabilization of the cosegregated tungsten nitride layer on the bcc (100) surface. The tungsten and nitrogen covered Fe-9%W-11ppmN(100) substrate exhibits a c(2 × 2) structure. On Fe-9%W-51ppmN(100) a temperature-driven phase transition between the cosegregated (1 × 1) and c(2 × 2) surface phases is observed.     

Dimeric Fe (II, III) complex of quinoneoxime as functional model of PAP enzyme: Mössbauer, magneto-structural and DNA cleavage studies

Purple acid phosphatase, (PAP), is known to contain dinuclear Fe₂ ^?+?2,?+?3 site with characteristic Fe^?+?3 ← Tyr ligand to metal charge transfer in coordination. Phthiocoloxime (3-methyl-2-hydroxy-1,4-naphthoquinone-1-oxime) ligand L, mimics (His/Tyr) ligation with controlled and unique charge transfers resulting in valence tautomeric coordination with mixed valent diiron site in model compound Fe-1: [μ-OH-Fe₂ ^?+?2,?+?3 (o-NQ_CH3ox) (o-NSQ_CH3ox)₂ (CAT) H₂O]. Fe-2: [Fe^?+?3(o-NQ_CH3ox) (p-NQ_CH3ox)₂]₂ a molecularly associated dimer of phthiocoloxime synthesized for comparison of charge transfer. ⁵⁷Fe Mössbauer studies was used to quantitize unusual valences due to ligand in dimeric Fe-1 and Fe-2 complexes which are supported by EPR and SQUID studies. ⁵⁷Fe Mössbauer spectra for Fe-1 at 300 K indicates the presence of two quadrupole split asymmetric doublets due to the differences in local coordination geometries of [Fe^?+?3]_A and [Fe^?+?2]_B sites. The hyperfine interaction parameters are δ _A = 0.152, (ΔE _Q)_A = 0.598 mm/s with overlapping doublet at δ _B = 0.410 and (ΔE _Q)_B = 0.468 mm/s. Due to molecular association tendency of ligand, dimer Fe-2 possesses 100% Fe^?+?3(h.s.) hexacoordinated configuration with isomer shift δ = 0.408 mm/s. Slightly distorted octahedral symmetry created by NQ_CH3ox ligand surrounding Fe^?+?3(h.s.) state generates small field gradient indicated by quadrupole split ΔE _Q = 0.213 mm/s. Decrease of isomer shifts together with variation of quadrupole splits with temperature in Fe-1 dimer compared to Fe-2 is result of charge transfers in [Fe₂ ^?+?2,?+?3 SQ] complexes. EPR spectrum of Fe-1 shows two strong signals at g ₁ = 4.17 and g ₂ = 2.01 indicative of S = 3/2 spin state with an intermediate spin of Fe^?+?3(h.s.) configuration. SQUID data of $\chi _m^{{\rm corr}} \mbox{.T}$ were best fitted by using HDVV spin pair model S = 2, 3/2 resulting in antiferromagnetic exchange (J = ?13.5 cm^???1 with an agreement factor of R = 1.89 × 10^???5). The lower J value of antiferromagnetic exchange leads to Fe⁺³μ-(OH) Fe^?+?2 bridging in Fe-1 dimer instead of μ-oxo bridge. The intermolecular association through H-bonds may lead to weakly coupled antiferromagnetic interaction between two Fe-2 molecules having Fe^?+?3(h.s.) centers. Using S = 5/2, 5/2 spin pair model we obtained best-fitted parameters such as J = ?12.4 cm^???1, g = 2.3 with R = 3.58 × 10^???5. Synthetic strategy results in non-equivalent iron sites in Fe-1 dimer analogues to PAP enzyme hence its reconstitution results in pUC-19 DNA cleavage activity, as physiological functionality of APase. It is compared with nuclease activity of Fe-2 RAPase.     

Analysis of the effect of the energy of interatomic interaction and the structure of Fe-Cr alloys on the processes of radiation-induced segregation after ion etching

We present the results of a comparative analysis of profiles of the radiation-induced segregation of chromium after irradiation of model alloys Fe-(9, 11, 14) at % Cr by He⁺ ions with the energy of 30 keV with fluences 10¹⁹–10²¹ ions/m² at 450°C and data on varying the moduli of normal elasticity and the structure of these alloys depending on the Cr concentration. It is shown that an interstitial migration mechanism is the determining factor in the case of chromium segregation near the surface. A correlation between the character of varying the elasticity moduli, order, and value of surface segregation and swelling of the Fe-Cr alloys depending on the Cr concentration is established.     

Characterization of the photolyase-like iron sulfur protein PhrB from Agrobacterium tumefaciens by Mössbauer spectroscopy

High field Mössbauer spectroscopy has been used to characterize the [4Fe-4S] ^{2 +}cluster of the protein PhrB from Agrobacterium tumefaciens which belongs to the cryptochrome/photolyase family (CPF) and which biological function has previously been shown to be DNA repair. Mössbauer spectra taken of the as prepared protein reveal δ = 0. 42 mms ^{? 1}, and Δ E _Q = 1. 26 mms ^{? 1}as well as an asymmetry parameter of η = 0. 8. These parameters are characteristic for a ferredoxin-type [4Fe-4S] ^{2 +}cluster. In order to investigate whether this cluster is involved in DNA-repair the protein has also been studied in its photoactivated state during DNA binding. The so obtained data sets exhibit essentially the same Mössbauer parameters as those of the non-activated PhrB. This indicates that during DNA repair the [4Fe-4S] ^{2 +}cluster of PhrB has no significant amounts of transition states which have conformational changes compared to the resting state of the protein and which have life times of several seconds or longer.     

The magnetostriction of Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys

The ingot of Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys was prepared from high purity elements using a high vacuum arc melting system. The X-ray diffraction patterns indicated that the alloys were disordered bcc A2 structure. The magnetostriction of the alloys was measured and the effect of partial substitution of Ga with Al on the magnetostriction of the alloys was investigated. Fe-9 at% Ga-9 at% Al alloy, the optimizing magnetostrictive alloy was found in Fe-(18−x) at% Ga-x at% Al (3≤x≤13.5) alloys. The saturated magnetostriction of the directional solidification Fe-9 at% Ga-9 at% Al rod is up to 135 ppm for 0 MPa and 221 ppm for 53 MPa. It was found that the alloy has the high linearity of the magnetostriction curve, the low hysteresis and saturated magnetic field, which suggests the directional solidification Fe-9 at% Ga-9 at% Al alloy is a potential candidate for magnetostrictive actuator and transducer applications.     

Electronic structures and magnetoelectric properties of tetragonal BaFeO3: an ab initio density functional theory study

First-principles calculations have been performed to investigate the ground state electronic properties of BaFeO₃ (BFO). Local spin density approximation (LSDA) plus U (LSDA+U) treatment modified the metallic behaviour to insulated one with a band gap of 4.12eV. The spontaneous polarization was found to be 89.3\muC/cm² with Berry phase scheme in terms of the modern theory of polarization. Fe-3d e_g were split into two singlet states (d_z² and d _x²-y²}), and Fe-3d t_2g were split into one doublet states(d_xz and d_yz) and one singlet states(d_xy) after Fe and O displaced along the c axis. Meanwhile the occupation numbers of d_z², d_yz, d_xz and O_T p_z (on the top of Fe) were increased at the expense of those in xy plane. Our results showed that it was the sensitivity of hybridization to ferroelectric distortions, not just the total change of hybridization, that produced the possibility of ferroelectricity. Moreover, the increasing occupation numbers of O_T p_z and Fe d_z² favoured the 180^o coupling between Fe-3d e_g and Fe-3d t_2g, leading to ferromagnetic ordering, which has been confirmed by the increase of magnetic moment by 0.13μ_B per formula unit in the polarized direction. Hence, the magnetization can be altered by the reversal of external electric field.     

Small-angle neutron scattering investigation of the nanostructure of ferritic-martensitic 12%-chromium steels

The nanostructure (nanoparticle distribution) of ferritic-martensitic 12%-chromium steels EK-181 (Fe-12Cr-2W-V-Ta-B) and ChS-139 (Fe-12Cr-2W-V-Ta-B-Nb-Mo) subjected to different modes of mechanical and heat treatments and neutron irradiation has been investigated using small-angle neutron scattering. The samples have been studied in the initial state and after neutron irradiation (IVV-2M reactor) at a temperature of 80°C with fluences of 10¹⁸, 10¹⁹, and 5 × 10¹⁹ cm^?2 (E ≥ 0.1 MeV). The nanostructure of the steels is characterized by precipitations of nanoparticles with two characteristic sizes of 1.0–1.5 and 7–8 nm. The dependence of the nanostructure parameters on the composition of the steels and on the conditions of heat treatment and irradiation has been discussed.     

Density-functional calculation of the Coulomb repulsion and correlation strength in superconducting LaFeAsO

Constrained density functional theory scheme in Wannier functions formalism has been used to calculate Coulomb repulsion U and Hund’s exchange J parameters for Fe-3d electrons in LaFeAsO. Results strongly depend on the basis set. When O-2p, As-4p, and Fe-3d orbitals are included, computation results in U = 3–4 eV. With the basis set restricted to Fe-3d orbitals only, computation gives parameters corresponding to F ⁰ = 0.8 eV, J = 0.5 eV. Local Density Approximation combined with Dynamical Mean-Field Theory calculation with these parameters results in weakly correlated electronic structure. The article is published in the original.     

Self-diffusion of aluminium in the intermetallic compound Fe-48 at.% Al

The self-diffusion coefficient of Al in the B2-type intermetallic compound Fe-48 at.% Al has been determined using the intrinsic diffusion coefficients of Fe and Al and the self-diffusion coefficient of Fe with the help of the Darken-Manning relation. The self-diffusion coefficient of Al in Fe-48 at.% Al is estimated to be a factor of about 0.6 smaller than that of Fe, and the activation energy for the self-diffusion of Al is obtained to be 280 kJ mol^?1 which is a little larger than the value of 262 kJ mol^?1 for the self-diffusion of Fe, indicating that the diffusion mechanisms for both components are nearly equal.     

Mössbauer spectroscopic studies of Fe-20 wt.% Cr ball milled alloy

Interesting differences were noticed in the alloying process during ball milling of Fe-10 wt.% Cr and Fe-20 wt.% Cr alloys by ⁵⁷Fe Mössbauer spectroscopic studies. In both cases, there is almost no diffusion of Fe in Cr or vice versa up to 20 h of milling time. As the powders are milled for another 20 h substantive changes occur in the Mössbauer spectra showing atomic level mixing. But the two compositions behave differently with respect to alloying. Fe-20 wt.% Cr sample does not differ much in the hyperfine field distribution as it is milled from 40 to 100 h. On the other hand, the hyperfine field distribution keeps on changing with milling time for Fe-10 wt.% Cr sample even up to 100 h of milling. The average crystallite size is found to be 7.5 nm for Fe-10 wt.% Cr and 6.5 nm in Fe-20 wt.% Cr after milling.     

Thermal oxidation behaviors and structures of Fe-9Cr-1Mo alloy

Isothermal oxidation behaviors of Fe-9Cr-1Mo alloy were investigated at 600, 700, 750 and 850 ^°C for 72 h in air atmosphere. The oxidation rates were measured using a thermogravimetric analyzer (TGA). The structure and composition of the oxide scale were characterized by X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and conversion electron Mössbauer spectrometry (CEMS). In this study it was found that the oxide layers form duplex structures consisting of Fe and Cr oxides. CEMS spectra are composed of one doublet due to dispersed Fe ³⁺ in Cr₂O₃ oxidation layers produced at high temperatures and two magnetic components due to Fe-Cr-Mo alloy substrate and hematite (Fe, Cr)₂O₃ with lower hyperfine field than pure hematite (52 T). Fe rich oxides are formed at the surface by oxidation at relatively low temperatures of 600 ^°C and 700 ^°C, while Cr rich (Fe, Cr)-oxides are formed in the top surface layers by oxidation at higher than 750 ^°C.     

<Superscript>57</Superscript>Fe NMR study of amorphous and rapidly quenched crystalline Fe-B alloys

Amorphous and crystalline Fe-B alloys (5–25 at % B) were studied using pulsed ⁵⁷Fe nuclear magneticr esonance at 4.2 K. The alloy samples were prepared from a mixture of the ⁵⁷Fe and ¹⁰B isotopes by rapid quenching from the melt. In the microcrystalline Fe-(5–12 at %) B alloys, the resonance frequencies were measured for local states of ⁵⁷Fe nuclei in the tetragonal and orthorhombic Fe₃B phases and also in α-Fe. The resonance frequencies characteristic of ⁵⁷Fe nuclei in α-Fe crystallites with substitutional impurity boron atoms in the nearest neighborhood were also revealed. In the resonance frequency distribution P(f) in the amorphous Fe-(18–25) at % B alloys, there are frequencies corresponding to local Fe atom states with short-range order of the tetragonal and orthorhombic Fe₃B phases. As the boron content decreases below 18 at %, the P(f) distributions are shifted to higher frequencies corresponding to ⁵⁷Fe NMR for atoms exhibiting a short-range order of the α-Fe type. The local magnetic structure of the amorphous Fe-B alloys is also considered.     

Phase composition at surface of Fe-3%Si alloy

The structure, phase and chemical compositions of surface layers in different depths of Fe-3%Si alloy were investigated. According to the X-ray Photoelectron Spectroscopy (XPS) spectrum (penetration depth of up to ∼ 1nm) of the as-prepared sample, a layer of SiO₂ was present on the top. After the subsequent Ar⁺ sputtering (removing the SiO₂ layer), a segregation of Si atoms and two other phases were observed. The phases were described as the cubic c-FeSi and Fe₃Si. The emission⁵⁷Fe M?ssbauer spectra confirmed a presence of these phases. The α-Fe and solid solution of α-Fe + 1wt.%Si were recognized in the Conversion Electron M?ssbauer spectra (penetration depth ∼ 300nm) while the M?ssbauer spectra taken in scattering geometry with detection of 14.4 keV gamma radiation (scanning depth of ∼ 30 μm) indicate Fe-3wt.%Si solid solution as a main phase. Presented at International Colloquium “M?ssbauer Spectroscopy in Materials Science”, Všemina, Czech Republic, June 1–4, 2004. This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Contract No. IAA1041404).     

坡莫合金纳米粉的恒导磁性及其磁导率的压力、频率特性

 用蒸发冷凝法制备了Fe-50wt%Ni和Fe-84wt%Ni坡莫合金的纳米微粉。经透射电镜X光能谱微区成分分析证实：微粉的成分与原材料的一致；微粉的平均粒径分别为16 nm（Fe-50Ni）和19 nm（Fe-84Ni）。在室温和36个不同的流体静压力（0.000 1~2.205 GPa）下原位测量了它们的磁化和起始磁化曲线。结果表明：（1）Fe-50wt%Ni和Fe-84wt%Ni合金纳米微粉均具有恒导滋特性；（2）这两种纳米粉的起始磁导率随静水压的变化分别为μ_i=8.16+18.2p-24.7p²+18.1p³-6.55p⁴+0.908p⁵（Fe-50Ni合金）和μ_i=5.38-0.169p+0.232p²-0.0786p³（Fe-84Ni合金）；（3）纳米粉的起始磁导率μ_i在频率高于10 MHz才降低，而Fe-50Ni合金片的μ_i在频率高于1 MHz，Fe-84Ni高于100 kHz时就开始下降，故纳米压粉磁芯可应用的频率范围可以比相应组分合金片的大1~2个数量级。     

Radiation damage calculations for charged particle emission nuclear reactions

Taking the quantum tunneling and Coulomb barrier into account, the present work proposes the calculation of Primary Knock-on Atom (PKA) energy for charged particle emission reactions. The method proposed in this paper and that used in NJOY have a large difference of PKA energy for charged particle emissions. For a D+T fusion neutron-induced ⁶Li(n,t)⁴He PKA, the maximum PKA energy calculated with NJOY is 2.9 MeV, whereas 14.2 MeV is obtained with the formula proposed in the present work. The subsequent total neutron-induced Displacement per Atom (DPA) calculated with the two methods has a small difference for most isotopes of which the DPA is predominated by neutron scattering reactions, such as ⁵⁶Fe and ⁵⁸Ni. However, the difference can be important for nuclei with charged particle emission channels open at thermal energy, such as ⁶Li, ¹⁰B, and ⁵⁹Ni. Using the damage cross sections (calculated by NJOY with and without modifications) and SRIM/TRIM-2008 calculations, the relative differences on total DPA rates of the compound material 90%⁵⁶Fe-9%⁵⁸Ni-1%⁵⁹Ni are within 1% for two fast spectra, 1.5% for fusion first wall, about 5% for the heavy reflector, and 25% for a pressurized water reactor vessel.     

Effect of W co-doping on the optical,magnetic and electrical properties of Fe-doped BaSnO3

The effect of W co-doping on the optical, magnetic and electrical properties of Fe-doped BaSnO₃ has been studied. Polycrystalline BaSnO₃, BaSn_0.96Fe_0.04O₃ and BaSn_0.95Fe_0.04W_0.01O₃ samples were prepared using solid state reaction. In the analysis of powder X-ray diffraction patterns, the samples were found to be free of secondary phases. Diffuse reflectance spectra evidenced the substitution of Fe and W for Sn in the host BaSnO₃. Micro-Raman spectra confirmed the existence of oxygen vacancies in the samples. Upon W-1% co-doping, the ferromagnetic character of Fe-4% doped BaSnO₃ is suppressed drastically and its Curie temperature is reduced to 310 K from 462 K. The existence of F-centers and ferromagnetic interactions at room temperature is evidenced by the electron paramagnetic resonance and ferromagnetic resonance signals observed in the electron spin resonance spectra of the undoped and Fe-4% doped, (Fe-4% and W-1%) co-doped BaSnO₃ samples respectively. Suppression of ferromagnetism upon W co-doping is due to the fact that each W⁶⁺ ion donates two electrons to the host lattice and it reduces the number of oxygen vacancies that are essential for ferromagnetism to exist in the Fe-doped BaSnO₃ samples.     

Investigation of primary chemical reactions of electrons by emission Mössbauer and positron spectroscopies

A set of early chemical reactions induced in frozen medium around decayed ⁵⁷Co or ^119mSn nuclei by emitted Auger electrons is suggested. The mechanism predicts a correlation between formation probabilities of final products, namely Fe²⁺-Fe³⁺ or Sn²⁺-Sn⁴⁺ ions, positronium atom and molecular hydrogen formed in similar medium after its irradiation by fast positrons and electrons. This correlation indicates a similarity of chemical processes occurring in nanovolumes around Mössbauer ⁵⁷Fe- and ¹¹⁹Sn-nuclei after radioactive decay as well as in tracks produced by fast positrons and electrons. The important role of quasi-free electrons is revealed.     

Experimental evidences of clusters with different short range order in amorphous alloys

⁵⁶Fe,⁵⁷Fe,¹⁰B and¹¹B isotopes were used for binary alloys. The signals of B (40,5 MHz) and Fe (43 MHz) from α-Fe in binary Fe−B crystalline and amorphous alloys were found besides the signals of these nuclei in t-and o-phase or clusters like these phases. The NMR on (51)V impurity nuclei in Fe−B alloys was used as well. Only amorphous Fe-(^<15 at.%B) alloys had the clusters with o-, t-Fe₃B and α-Fe short range order.