High-spin Fe3 Research Articles

Synthesis, magnetic properties, and Mössbauer spectra of polynuclear iron carboxylates

Several carboxylates of iron(II) and iron(III) have been prepared and their magnetic properties and Mossbauer spectra studied. [Fe4(OMe)6(O2CMe)6], [Fe5O(O2CMe)12][O2CMe], and [Fe3(dmf)3O(O2CPh)6][O2CPh]·dmf (dmf = dimethylformamide) are antiferromagnetic but [Fe4O(O2CMe)10] is ferromagnetic. [Fe(O2CMe)2] and [Fe(py)2(O2CMe)2] are magnetically dilute. The iron(III) compounds show Mossbauer spectra typical of high-spin Fe3+ ions, but the spectrum of [Fe4O(O2CMe)10] is very asymmetric. The characterizations have been supported by appropriate mass-spectrometric, ebulliometric, and conductimetric studies, and electronic and infrared spectra.

FURTHER EXAMPLES OF A LIGAND FIELD CALCULATION FOR THE SIXFOLD COORDINATED COMPLEX Fe(H2O)2+6

FURTHER EXAMPLES OF A LIGAND FIELD CALCULATION FOR THE SIXFOLD COORDINATED COMPLEX Fe(H₂O)²⁺₆

Proton magnetic relaxation dispersion in human fluoromethaemoglobin solutions.

The solvent proton spin-lattice relaxation time of high spin Fe3+ (S=5/2) human A fluoromethaemoglobin aqueous solutions was measured at 14 Larmor frequencies in the range from 2.2 to 96 MHz. The observed paramagnetic relaxation rates are analysed in terms of the Solomon-Bloembergen theory, with the g-tensor value of 2 based on the consideration of the protein tertiary structure. From the H2O (pH 6) haemoprotein solution relaxation data, tau(c) =(9.3+/-0.3) X 10(-10) sec. If the total relaxation rates are corrected for the "outer-sphere" paramagnetic contribution, tau(c)=(6.5+/-0.4) X 10(-10) sec. The latter correction is obtained from the p.m.r. of the non-exchangeable aliphatic protons of C2H4(OD)2 added to the D2O-solution of fluoromethaemoglobin. Assuming that single proton transfer is taking place through the protein channel along the axis normal to the haem (g=2), the protein "binding" site is at a distance of 3.93 to 3.98 A from the haem Fe3+ ion.

Ferromagnetism in metallic FexTaS2 (x≈0.28)

Single crystals of FexTaS2 (x≈0.28) are metallic, ferromagnetic (Tc=73±5 °K), and have a large coercive force of 55 kOe at 4.2 °K. Mössbauer measurements show the iron to be high-spin Fe2+, with a distribution of hyperfine fields at low temperatures. Electron microscopic observations, together with these results, suggest the presence of iron clusters.

Mössbauer-effect study of Co57 and Fe57 impurities in ferroelectric LiNbO3

The Mössbauer effect observed with LiNbO3:Co57 (source) and LiNbO3:Fe57 (absorber) crystals showed the existence of high-spin Fe2+ and Fe3+ valence states. The Fe2+/Fe3+ ratio could be changed by reducing or oxidizing heat treatment. Fe3+ in sources and absorbers shows Mössbauer spectra which are typical for slow electronic relaxation between the crystal field states of the 6S5/2 state ion. For both Fe3+ and Fe2+, the principal axis of the electric field gradient is found to be parallel to the crystallographic c axis. For ferric iron Vzz is positive, while Vzz is negative and strongly temperature dependent for ferrous iron. The impurity site substitution and charge compensation mechanism are discussed.

Formation of Superoxide in the Autoxidation of the Isolated alpha and beta Chains of Human Hemoglobin and Its Involvement in Hemichrome Precipitation

The purpose of the work reported here is to look at O−2 production during autoxidation of the isolated α and β chains of human hemoglobin. The formation of superoxide by isolated chains. in fact, may be involved in pathological phenomena observed in red blood cell diseases where excess of either α or β chains is produced because of inherited defects of synthesis (thalassemus). The autoxidation of the oxygenated chains of human hemoglobin is followed by the transformation of the oxidized molecule (high-spin Fe3+) into a species absorbing as a low-spin Fe3+ compound, that is a hemichrome, which tends to precipitate. The overall process may be described by the following reactions: Depending on conditions the different steps proceed with different relative rates and therefore may overlap to a greater or lesser extent. The results obtained indicate that superoxide is produced during autoxidation of the isolated α and β chains of human hemoglobin. This conclusion has been reached on the basis of an experimental approach based on cooxidation of epinephrine to adrenochrome during O−2 production and inhibition of this effects by superoxide dismutase. In addition the rate of precipitation is, in all cases, very much reduced upon addition of either superoxide dismutase or catalase. These facts seem to imply very strongly that oxygen radicals are involved also in the precipitation following oxidation and hemichrome formation in the chains. In particular the effect of superoxide dismutase implies the involvement of O−2. Thus production of O−2 could well be one of the important events leading to alteration of the red cell membrane and consequently to reduced life span of thalassemic erythrocytes.

Paramagnetic hyperfine structure in an octahedral high-spin Fe2+ complex

Paramagnetic hyperfine structure in an octahedral high-spin Fe2+ complex

Jahn˗Teller effects in ferro˗magnesian minerals : pyroxenes and olivines

In a crystal the replacement of Mg²⁺ in octahedral coordination by high-spin Fe²⁺ should not — even though the two effective ionic radii differ from their mean by less than 4 per cent — result in random solid solution : orbital degeneracy in the ground state increases the stability of Fe²⁺ in a distorted octahedral environment so that, if two octahedral cationic positions are available, iron (the Jahn-Teller ion) will favor the more distorted site. The occupancies of M(1) and M(2) in refined crystal structures of pyroxenes bear out this prediction : M(2) is much more distorted than M(1) and it contains most of the ferrous ion. In olivines, however, the difference in distortion between M(1) and M(2) is less extreme. M(1) is said to be « more distorted » than M(2), but the difference in the types of distortion precludes such comparison. The distortion of M(1) is « quasi-tetragonal », that of M(2) « quasi-trigonal ». Crystal-field calculations for the two sites at both the magnesium and iron end of the olivine series yield an average stabilization energy at M(1) which is 262 cm⁻¹ larger than that at M(2), yet X-ray diffraction results show very little or no ordering of iron. Dynamic Jahn-Teller effects at M(2) can be expected, since such effects are known in Mg₁₋ₓFeₓO, Ca₁₋ₓFeₓO and KMg₁₋ₓFeₓF₃. The inclusion of a Jahn-Teller stabilization energy at M(2) gives a total stabilization energy which is only 95 cm⁻¹ larger than that at M(1).

Reduced Nicotinamide Adenine Dinucleotide Phosphate-Sulfite Reductase of Enterobacteria: I. THE ESCHERICHIA COLI HEMOFLAVOPROTEIN: MOLECULAR PARAMETERS AND PROSTHETIC GROUPS

Abstract NADPH-sulfite reductase (EC 1.8.1.2), an enzyme which catalyzes the 6-electron reduction of sulfite to sulfide, has been purified to homogeneity, according to electrophoretic and ultracentrifugal criteria, from Escherichia coli B. The enzyme, molecular weight 670,000, contains four FMN, four FAD, 20 to 21 atoms of iron, 14 to 15 labile sulfides, and 3 to 4 molecules of a novel type of heme per enzyme molecule. The absorption spectrum of sulfite reductase shows maxima at 278, 386, 455, 587, and 714 nm, the latter three bands being partially bleached by NADPH or dithionite. The enzyme exhibits electron paramagnetic resonance signals with g values of 6.7, 5.25, and 2.0, characteristic of 3 to 4 moles of a high spin Fe3+-heme per mole of enzyme, and this signal is reduced in intensity by addition of NADPH. The hemetype chromophore, which appears to be characteristic of assimilatory sulfite-reducing enzymes in a wide variety of organisms, is extractable with acetone-0.015 n HCI and is highly polar; its chromatographic behavior is clearly different from that of protohemin. Both the enzyme and the extracted heme-type chromophore form spectrally distinct complexes with typical heme ligands (CO, cyanide, hydroxide, and pyridine) in which the Soret band maxima are at unusually short wave lengths with low Soret to α-band absorbance ratios. Formation and decay of the enzyme-CO complex is slow in comparison to other hemoproteins. These data indicate that E. coli sulfite reductase is a self-contained complex of electron transport carriers including a heme-type component which, by virtue of its spectrophotometric, solubility, and chromatographic properties, is clearly distinguishable from previously described hemes.

Mössbauer effect in rubredoxin. Determination of the hyperfine field of the iron in a simple iron-sulphur protein.

1. Rubredoxin isolated from the green photosynthetic bacterium Chloropseudomonas ethylica was similar in composition to those from anaerobic fermentative bacteria. Amino acid analysis indicated a minimum molecular weight of 6352 with one iron atom per molecule. 2. The circular-dichroism and electron-paramagnetic-resonance spectra of Ch. ethylica rubredoxin showed many similarities to those of Clostridium pasteurianum, but suggested that there may be subtle differences in the protein conformation about the iron atom. 3. Mössbauer-effect measurements on rubredoxin from Cl. pasteurianum and Ch. ethylica showed that in the oxidized state the iron (high-spin Fe(3+)) has a hyperfine field of 370+/-3kG, whereas in the reduced state (high-spin Fe(2+)) the hyperfine field tensor is anisotropic with a component perpendicular to the symmetry axis of the ion of about -200kG. For the reduced protein the sign of the electric-field gradient is negative, i.e. the ground state of the Fe(2+) is a [unk] orbital. There is a large non-cubic ligand-field splitting (Delta/k=900 degrees K), and a small spin-orbit splitting (D~+4.4cm(-1)) of the Fe(2+) levels. 4. The contributions of core polarization to the hyperfine field in the Fe(3+) and Fe(2+) ions are estimated to be -370 and -300kG respectively. 5. The significance of these results in interpretation of the Mössbauer spectra of other iron-sulphur proteins is discussed.

Mössbauer studies of adrenodoxin. The mechanism of electron transfer in a hydroxylase iron-sulphur protein.

1. Mössbauer spectra were measured of adrenodoxin purified from porcine adrenal glands. They show similarities to the spectra of the plant ferredoxins. All of these proteins contain two atoms of iron and two of inorganic sulphide per molecule, and on reduction accept one electron. 2. As with the plant ferredoxins the adrenodoxin for these measurements was enriched with (57)Fe by reconstitution of the apo-protein, and subsequently was carefully purified and checked by a number of methods to ensure that it was in the same conformation as the native protein and contained no extraneous iron. 3. The Mössbauer spectra of oxidized adrenodoxin at temperatures from 4.2 degrees K to 197 degrees K show that the iron atoms are probably high-spin Fe(3+), and in similar environments, and experience little or no magnetic field from the electrons. 4. Mössbauer spectra of reduced adrenodoxin showed magnetic hyperfine structure at all temperatures from 1.7 degrees K to 244 degrees K, in contrast with the reduced plant ferredoxins, which showed it only at lower temperatures. This is a consequence of a longer electron-spin relaxation time in reduced adrenodoxin. 5. At 4.2 degrees K in a small magnetic field the spectrum of reduced adrenodoxin shows a sixline Zeeman pattern due to Fe(3+) superimposed upon a combined magnetic and quadrupole spectrum due to Fe(2+). 6. In a large magnetic field (30kG) each hyperfine pattern is further split into two. Analysis of these spectra at 4.2 degrees K and 1.7 degrees K shows that the effective fields at the Fe(3+) and Fe(2+) nuclei are in opposite directions. This agrees with the proposal, first made for the ferredoxins, that the iron atoms are antiferromagnetically coupled. 7. In accord with the model for the ferredoxins, it is proposed that the oxidized adrenodoxin contains two high-spin Fe(3+) atoms which are antiferromagnetically coupled; on reduction one iron atom becomes high-spin Fe(2+).

Mössbauer effect in Scenedesmus and spinach ferredoxins. The mechanism of electron transfer in plant-type iron-sulphur proteins.

1. The Mössbauer spectra of Scenedesmus ferredoxin enriched in (57)Fe were measured and found to be identical with those of two other plant-type ferredoxins (from spinach and Euglena) that had been previously measured. Better resolved Mössbauer spectra of spinach ferredoxin are also reported from protein enriched in (57)Fe. All these iron-sulphur proteins are known to contain two iron atoms in a molecule that takes up one electron on reduction. 2. The Mössbauer spectra at 195 degrees K have electric hyperfine structure only and show that on reduction the electron goes to one of the iron atoms, the other appearing to remain unchanged. 3. In the oxidized state, both iron atoms are in a similar chemical state, which appears from the chemical shift and quadrupole splitting to be high-spin Fe(3+), but they are in slightly different environments. In the reduced state the iron atoms are different and the molecule appears to contain one high-spin Fe(2+) and one high-spin Fe(3+) atom. 4. At lower temperatures (77 and 4.2 degrees K) the spectra of both iron atoms in the reduced proteins show magnetic hyperfine structure which suggests that the iron in the oxidized state also has unpaired electrons. This provides experimental evidence for earlier suggestions that in the oxidized state there is antiferromagnetic exchange coupling, which would result in a low value for the magnetic susceptibility. 5. In a small magnetic field the spectrum of the reduced ferredoxin shows a Zeeman splitting with hyperfine field (H(n)) of 180kG at the nuclei. On application of a strong magnetic field H the spectrum splits into two spectra with effective fields H(n)+/-H, thus confirming the presence of the two antiferromagnetically coupled iron atoms. 6. These results are in agreement with the model proposed by Gibson, Hall, Thornley & Whatley (1966); in the oxidized state there are two Fe(3+) atoms (high spin) antiferromagnetically coupled and on reduction of the ferredoxin by one electron one of the ferric atoms becomes Fe(2+) (high spin).

The weak field representation of the crystal field matrices of d4,6 ions and the electronic absorption spectrum of RbFeF3

The electronic absorption spectrum of RbFeF3 is reported and analysed by means of crystal field theory. It is shown that the strong field formalism is limited for high spin Fe2+ and the weak field representation should be used to interpret the many triplet states. Published spin-orbit weak field matrices for d4,6 are shown to contain errors. The weak field matrices in the absence of spin-orbit coupling have been derived and they are used to assign the spectrum of RbFeF3.

Mössbauer effect study of the state of iron in spinach ferredoxin.

THE ferredoxins are an important group of enzymes which catalyse photochemical reactions in plants and photosynthetic bacteria. They contain non-haem iron which is believed to be close to the active centre but which is in a chemical form 0which has not been unambiguously determined. Spinach ferredoxin is one of the simpler of these compounds, has a molecular weight of about 12,000 and contains two iron atoms/molecule. A Mossbauer study on this system has been made by Bearden and Moss1, and on other non-haem iron proteins by several groups2–4. In this communication, we describe Mossbauer effect measurements made both in the presence and absence of a magnetic field on spinach ferredoxin, and it is concluded that the iron atoms are both low-spin Fe2+ in the oxidized state, and there is one low-spin Fe2+ and one high-spin Fe2+ in the reduced state.

Estimation of ligand-field parameters from Mössbauer spectra

Computer programmes are described for the calculation of the temperature-dependence of the Mossbauer quadrupole splitting of complexes containing octahedral and tetrahedral high-spin Fe2+ and octahedral low-spin FeIII. The derivation of ligand-field parameters from experimental data is critically considered, and contributions from the crystal lattice and covalency are also discussed. An iterative method for fitting trial models to experimental data is described and tested, and can be generally applied to the minimisation of any matrix solution.

Mössbauer Spectrum of Fe2+ in a Square-Planar Environment

The Mössbauer spectrum of high-spin Fe2+, square planar coordinated by oxygens in BaFeSi4O10, has been studied in the temperature range 80°—650°K. The spectrum, a quadrupole doublet, was analyzed by computer fitting, and the results statistically tested using the concept of internal and external consistency. Quadrupole splitting and center shift data are reported for the whole range 80°—650°K, and the widths and relative intensities of the lines have been measured at 80° and 295°K. The quadrupole splitting has been successfully correlated with the magnetic susceptibility and electronic spectrum of Fe2+ in BaFeSi4O10. The lattice contribution is found to be slightly larger in magnitude than the valence contribution, and of opposite sign. By studying both randomized and oriented samples the sign of the quadrupole splitting has been determined experimentally, and the anisotropy of the recoil-free fraction measured. The temperature variation of the center shift has been analyzed in terms of the second-order Doppler shift, and information on the vibrational motion of the iron atom obtained. The theoretical basis of the method used for analyzing the second-order Doppler-shift data is briefly discussed. A correlation between center shift values and coordination number for minerals containing high-spin ferrous iron is noted; and interesting observations concerning the linewidths, and the effects of heat treatment on the Mössbauer spectrum of gillespite, are reported.

Magnetic resonance studies of met-myoglobin and myoglobin azide.

The variation of electron spin resonance lineshapes with orientation of single-crystal myoglobins are studied for high-spin (S=52) Fe3+ in met-myoglobin and low-spin (S=½) Fe3+ in myoglobin azide. It is found that for both these crystals random variations of about 2° in the orientation of the symmetry axes contribute significantly to the observed linewidths. For met-myoglobin, these misorientations are sufficient to explain the observed angular dependence of the linewidths. In the case of myoglobin azide, however, there is an additional broadening due to a 3.5% random variation in the rhombic molecular field. Theoretical expressions describing the effects of random distributions of rhombic field parameters on linewidth are derived and fitted to the observed linewidths. Further lineshape changes are interpreted in terms of an anisotropic-exchange interaction between equivalent paramagnetic sites. A brief account of attempts to study the electron nuclear-double resonance (ENDOR) spectrum of the myoglobin is given.

Electron spin resonance of met-myoglobin: field dependence of g.

The spin Hamiltonian for high-spin Fe3+ in single crystals of met-myoglobin has the form Hs=D(Sz2−25/4)+g| |βHzSz+g⊥β(HxSx+HySy). In view of the large value of D the only ESR signal that can be observed is the transition from the ms=—½ to ms=+½ states. The resonance condition for the ground doublet is described by geffβH=hν. When the magnetic field is applied along the z axis g∥eff=g∥ and when the magnetic field is applied perpendicular to the z axis g⊥eff=3g⊥[1–2(g⊥βH)2(2D)−2]. Through measurement of g⊥eff and g∥eff at 13 and 35 Gc/sec the spin-Hamiltonian parameters were found to be g∥=2.002±0.001, g⊥=1.985±0.002, and 2D=8.76±1.2, cm−1. These results are interpreted by replacing the pure 3d wavefunctions from which the (3d)5 electronic configuration of Fe3+ is usually formed by covalent molecular orbitals which are only part 3d. The values for g∥, g⊥, and D can be explained in terms of a single low-lying pure antibonding state of 4A2 symmetry 2000 cm−1 above the 6A1 ground state and coupled to it by the spin—orbit interaction. Numerical agreement between theory and experiment requires molecular orbits which have an average 3d character of only 68%. These highly covalent orbitals are in very good agreement with the theoretical results of Zerner and Gouterman.