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PubMed1992

Mössbauer study of the native, reduced and substrate-reacted Desulfovibrio gigas aldehyde oxido-reductase.

The Desulfovibrio gigas aldehyde-oxido-reductase contains molybdenum and iron-sulfur clusters. Mössbauer spectroscopy was used to characterize the iron-sulfur clusters. Spectra of the enzyme in its oxidized, partially reduced and benzaldehyde-reacted states were recorded at different temperatures and applied magnetic fields. All the iron atoms in D. gigas aldehyde oxido-reductase are organized as [2Fe-2S] clusters. In the oxidized enzyme, the clusters are diamagnetic and exhibit a single quadrupole doublet with parameters (delta EQ = 0.62 +/- 0.02 mm/s and delta = 0.27 +/- 0.01 mm/s) typical for the [2Fe-2S]2+ state. Mössbauer spectra of the reduced clusters also show the characteristics of a [2Fe-2S]1+ cluster and can be explained by a spin-coupling model proposed for the [2Fe-2S] cluster where a high-spin ferrous ion (S = 2) is antiferromagnetically coupled to a high-spin ferric ion (S = 5/2) to form a S = 1/2 system. Two ferrous sites with different delta EQ values (3.42 mm/s and 2.93 mm/s at 85 K) are observed for the reduced enzyme, indicating the presence of two types of [2Fe-2S] clusters in the D. gigas enzyme. Taking this observation together with the re-evaluated value of iron content (3.5 +/- 0.1 Fe/molecule), it is concluded that, similar to other Mo-hydroxylases, the D. gigas aldehyde oxido-reductase also contains two spectroscopically distinguishable [2Fe-2S] clusters.

Aldehyde Oxidoreductases↗

PubMed1992

Mössbauer characterization of the tetraheme cytochrome c3 from Desulfovibrio baculatus (DSM 1743). Spectral deconvolution of the heme components.

Mössbauer spectroscopy was used to study the tetraheme cytochrome c3 from Desulfovibrio baculatus (DSM 1743). Samples with different degrees of reduction were prepared using a redoxtitration technique. In the reduced cytochrome c3, all four hemes are reduced and exhibit diamagnetic Mössbauer spectra typical for low-spin ferrous hemes (S = 0). In the oxidized protein, the hemes are low-spin ferric (S = 1/2) and exhibit overlapping magnetic Mössbauer spectra. A method of differential spectroscopy was applied to deconvolute the four overlapping heme spectra and a crystal-field model was used for data analysis. Characteristic Mössbauer spectral components for each heme group are obtained. Hyperfine and crystal-field parameters for all four hemes are determined from these deconvoluted spectra.

Cytochrome c Group↗

PubMed1992

Multi-frequency EPR and high-resolution Mössbauer spectroscopy of a putative [6Fe-6S] prismane-cluster-containing protein from Desulfovibrio vulgaris (Hildenborough). Characterization of a supercluster and superspin model protein.

The putative [6Fe-6S] prismane cluster in the 6-Fe/S-containing protein from Desulfovibrio vulgaris, strain Hildenborough, has been enriched to 80% in 57Fe, and has been characterized in detail by S-, X-, P- and Q-band EPR spectroscopy, parallel-mode EPR spectroscopy and high-resolution 57Fe Mössbauer spectroscopy. In EPR-monitored redox-equilibrium titrations, the cluster is found to be capable of three one-electron transitions with midpoint potentials at pH 7.5 of +285, +5 and -165 mV. As the fully reduced protein is assumed to carry the [6Fe-6S]3+ cluster, by spectroscopic analogy to prismane model compounds, four valency states are identified in the titration experiments: [6Fe-6S]3+, [6Fe-6S]4+, [6Fe-6S]5+, [6Fe-6S]6+. The fully oxidized 6+ state appears to be diamagnetic at low temperature. The prismane protein is aerobically isolated predominantly in the one-electron-reduced 5+ state. In this intermediate state, the cluster exists in two magnetic forms: 10% is low-spin S = 1/2; the remainder has an unusually high spin S = 9/2. The S = 1/2 EPR spectrum is significantly broadened by ligand (2.3 mT) and 57Fe (3.0 mT) hyperfine interaction, consistent with a delocalization of the unpaired electron over 6Fe and indicative of at least some nitrogen ligation. At 35 GHz, the g tensor is determined as 1.971, 1.951 and 1.898. EPR signals from the S = 9/2 multiplet have their maximal amplitude at a temperature of 12 K due to the axial zero-field splitting being negative, D approximately -0.86 cm-1. Effective g = 15.3, 5.75, 5.65 and 5.23 are observed, consistent with a rhombicity of [E/D] = 0.061. A second component has g = 9.7, 8.1 and 6.65 and [E/D] = 0.108. When the protein is reduced to the 4+ intermediate state, the cluster is silent in normal-mode EPR. An asymmetric feature with effective g approximately 16 is observed in parallel-mode EPR from an integer spin system with, presumably, S = 4. The fully reduced 3+ state consists of a mixture of two S = 1/2 ground state. The g tensor of the major component is 2.010, 1.825 and 1.32; the minor component has g = 1.941 and 1.79, with the third value undetermined. The sharp line at g = 2.010 exhibits significant convoluted hyperfine broadening from ligands (2.1 mT) and from 57Fe (4.6 mT). Zero-field high-temperature Mössbauer spectra of the protein, isolated in the 5+ state, quantitatively account for the 0.8 fractional enrichment in 57Fe, as determined with inductively coupled plasma mass spectrometry.(ABSTRACT TRUNCATED AT 400 WORDS)

Bacterial Proteins↗

PubMed1992

Evidence for polynuclear aggregates of ferric daunomycin. A Mössbauer, EPR, X-ray absorption spectroscopy and magnetic susceptibility study.

The interaction of the antitumor agent daunomycin (DN) with ferric iron has been analysed by Mössbauer spectroscopy, EPR, extended X-ray absorption fine structure (EXAFS), and magnetic susceptibility measurements. In contrast to literature data, at millimolar iron and anthracycline concentrations no solitary Fe(DN)3 complexes are formed in appreciable amounts. The Mössbauer spectroscopic analysis revealed severe dependencies on temperature, on the preparation procedure, the time allowed for equilibration, and on the metal/ligand ratio. The Mössbauer spectra exhibit two components: a broad magnetic sextet and a quadrupole doublet at an Fe/DN molar ratio of 1:3 and exclusively a doublet at a molar ratio of 1:20, indicating an equilibrium of these two spectral components. The EPR spectra are dominated by a signal at g(eff) = 2. Double integration of the EPR signals enabled the determination of their spin density and a correlation between EPR and Mössbauer spectra. The Mössbauer sextet species is EPR invisible and corresponds to magnetically ordered polynuclear aggregates with high magnetic anisotropy. EXAFS and susceptibility measurements provide additional evidence for the formation of polynuclear aggregates of ferric daunomycin. The quadrupole doublet species in the Mössbauer spectra correlates with the g = 2 signal in EPR. This species is also related to a magnetically ordered system, exhibiting, however, superparamagnetic behavior due to less magnetic anisotropy. Since daunomycin forms dimers in aqueous solution at millimolar concentrations, we conclude that the cooperative phenomena observed in EPR and Mössbauer spectra are a consequence of its stacking effects.

Daunorubicin↗

PubMed1993

Overproduction, purification and characterization of the bacterioferritin of Escherichia coli and a C-terminally extended variant.

The bacterioferritin (BFR) of Escherichia coli is an iron-sequestering haemoprotein composed of 24 identical polypeptide chains forming an approximately spherical protein shell with a central iron-storage cavity. BFR and BFR-lambda, a variant with a 14-residue C-terminal extension, have been amplified (120-fold and 50-fold, respectively), purified by a new procedure and characterized. The overproduced BFR exhibited properties similar to those of natural BFR, but the iron content (25-75 non-haem Fe atoms/molecule) was 13-39-fold lower. Two major assembly states of BFR were detected, a 24-subunit protein (tetracosamer) and a novel haem-containing subunit dimer. BFR-lambda subunits assembled into tetracosamers having the same external-surface properties as BFR, presumably because their C-terminal extensions project into and occupy about 60% of the central cavity. As a result, BFR-lambda failed totake up iron under conditions that allowed incorporation into BFR in vitro. The haem content of BFR-lambda (1-2 haems/tetracosamer) was lower than that of BFR (3.5-10.5 haems/tetracosamer) and this, together with a difference in the visible spectra of the two haemoproteins, suggested that the C-terminal extensions in BFR-lambda perturb the haem-binding pockets. A subunit dimer form of BFR-lambda was not detected. A combination of Mössbauer spectroscopy and electron diffraction showed that the BFR loaded with iron in vitro has a ferrihydrite-like iron core, whereas the in-vivo loaded protein has an amorphous core.

Amino Acid Sequence↗

PubMed1993

Zn-exchange and Mössbauer studies on the [Fe-Fe] derivatives of the purple acid Fe(III)-Zn(II)-phosphatase from kidney beans.

In order to perform Mössbauer studies, Zn(II) in the Fe(III)-Zn(II) purple acid phosphatase of the red kidney bean has been exchanged by incubating the semiapoenzyme with 57Fe(II). The resulting Fe(III)-57Fe(II) enzyme has 125% activity, compared with that of the Zn(II) enzyme. It can be oxidized by H2O2 or peroxydisulfate to the Fe(III)-57Fe(III) species with a 30-times lower activity. Incubation of the metal-free apoenzyme with 57Fe(II) in the presence of O2 leads to the 57Fe(III)-57Fe(II) species which is stable in dilute solutions, but partially oxidized during the concentration procedure to the 57Fe(III)-57Fe(III) enzyme. Limited reduction of the oxidized enzyme with ascorbate delivers a mixture of the Fe(II)-Fe(II)/Fe(III)-Fe(III) species, but not the mixed valent Fe(III)-Fe(II) species, indicating that after the transfer of the first electron the second electron of the ascorbate radical is immediately transferred to the second Fe(III). The Mössbauer spectra of the oxidized species show at 4.2 K two quadrupole doublets with delta of 0.51 mm/s and 0.53 mm/s and delta E of 1.46 and 0.96 mm/s indicating high spin Fe(III) in two different binding sites, obviously with a higher asymmetry in the chromophoric Fe(III) site. The values are too low for a mu-oxo bridge. The mixed-valent Fe(III)-Fe(II) species shows two quadrupole doublets with delta values of 0.55 mm/s and 1.14 mm/s and delta E values of 1.43 mm/s and 3.01 mm/s at 70 K for high spin Fe(II) and Fe(III), but the signal of the Fe(II) component shows magnetic patterns at 4.2 K indicating a half-integer spin system with antiferromagnetic coupling. The Fe(II)-Fe(II) system exhibits two quadrupole doublets with delta values of 1.18 mm/s and 1.22 mm/s and with delta E values of 3.69 mm/s and 2.68 mm/s again indicating a higher asymmetry in the originally chromophoric Fe(III)-binding site. Addition of phosphate shows only minor differences in the oxidized enzyme and in the mixed valent Fe(III)-Fe(II) system. Interaction with O2 is discussed.

Acid Phosphatase↗

PubMed1993

Cytochrome c6 from Monoraphidium braunii. A cytochrome with an unusual heme axial coordination.

A soluble monoheme c-type cytochrome (cytochrome c6) has been isolated from the green alga Monoraphidium braunii. It has a molecular mass of 9.3 kDa, an isoelectric point of 3.6 and a reduction potential of 358 mV at pH 7. The determined amino acid sequence allows its classification as a class-I c-type cytochrome. The ferric and ferrous cytochrome forms and their pH equilibria have been studied using 1H-NMR, ultraviolet/visible, EPR and Mössbauer spectroscopies. The pH equilibria are complex, several pKa values and pH-dependent forms being observed. The amino acid sequence, the reduction-potential value and the visible and NMR spectroscopies data in the pH range 4-9 indicate that the heme iron has a methionine-histidine axial coordination. However, the EPR and Mössbauer data obtained for the ferricytochrome show that in this pH range two distinct forms are present: form I, gz = 3.27, gy = 2.05 and gx = 1.05; form II, gz = 2.95, gy = 2.29 and gx = 1.43. While form I has crystal-field parameters typical of a methionine-histidine coordination, those associated with form II would suggest a histidine-histidine axial ligation. This possibility was extensively analyzed by spectroscopic methods and by chemical modification of a histidine residue. It was concluded that form II actually corresponds to an unusual type of methionine-histidine axial coordination. Straightforward examples of this type of coordination have recently been found in other c-type hemeproteins [Teixeira, M., Campos, A. P., Aguiar, A. P., Costa, H. S., Santos, H., Turner, D. L. & Xavier, A. V. (1993) FEBS Lett. 317, 233-236], corroborating our proposal. Since both forms, with very distinct crystal-field parameters, are shown to have the same reduction potential, it may be concluded that the axial and rhombic distortions of the heme-iron ligand field cannot be directly correlated with the heme-reduction potential. The pH-dependence studies have also shown that the form I and form II are interconvertible, with pKa approximately 5. To establish a possible physiological significance for this process, in particular for the interaction of the cytochrome with the membrane-bound electron-transfer complexes b6f and photosystem I, the effect of surfactants on the spectroscopic characteristics of cytochrome c6 has been studied.

Amino Acid Sequence↗

PubMed1993

Overproduction, purification and characterization of the Escherichia coli ferritin.

Recent studies have indicated that Escherichia coli possesses at least two iron-storage proteins, the haem-containing bacterioferritin and ferritin. The ferritin protein has been amplified 600-fold to 11-14% of total cell protein in a bfr mutant and purified to homogeneity with an overall yield of 13%. The cellular ferritin content remained relatively constant throughout the growth cycle and amplification was accompanied by a 2.5-fold increase in cellular iron content. The isolated ferritin contained 5-20 non-haem iron atoms/holomer and resembled the eukaryotic ferritins rather than the prokaryotic bacterioferritins in containing no haem. The 24 subunits of this ferritin (M(r) 19,400) assemble into a spherical protein shell (12 +/- 1 nm diameter, M(r) 465,000) which sequesters at least 2000 iron atoms in vitro to form an electron-dense iron core of 7.9 +/- 1 nm diameter. Electron-microscopic and Mössbauer spectroscopic studies with iron-loaded ferritin showed that the core can be either crystalline (ferrihydrite) or amorphous, depending on the absence or presence of phosphate, respectively. Mössbauer spectroscopy with intact E. coli revealed a novel-high spin Fe(II) component which is enhanced in bacteria amplified for ferritin but not in the parental strain. Western blotting showed that ferritin and bacterioferritin are immunologically distinct proteins. E. coli is thus an organism containing both a ferritin and a bacterioferritin and the relative roles of the two iron-storage proteins are discussed in this study.

Amino Acid Sequence↗

PubMed1994

Mössbauer and magnetic susceptibility studies on iron(II) metallothionein from rabbit liver. Evidence for the existence of an unusual type of [M3(CysS)9]3- cluster.

The magnetic properties of the Fe(II)-binding sites in Fe(II)7-metallothionein (MT) have been studied using Mössbauer spectroscopy and magnetic-susceptibility measurements. In agreement our previous results, simulation of the Mössbauer spectra showed the presence of paramagnetic and diamagnetic subspectra in the ratio 3:4. By comparison with Mössbauer spectra of the inorganic adamantane-like (Et4N)2[Fe4(SEt)10] model compound, the diamagnetic component in Fe(II)7-MT has been assigned to a four-metal cluster in which there is antiferromagnetic coupling between the high-spin Fe(II) ions. It is suggested that the organization of this cluster is similar to that determined in the three-dimensional structure of the protein, containing diamagnetic Zn(II) and/or Cd(II) ions. From magnetic-susceptibility studies, an average magnetic moment of approximately 8.5 microB was obtained for the three remaining bound Fe(II) ions, responsible for the paramagnetic component observed in the Mössbauer studies. This value is slightly lower than that for three completely uncoupled Fe(II) ions, suggesting the existence of a three-metal cluster within which there is weak exchange coupling between adjacent Fe(II) ions. The spin-Hamiltonian formalism including, besides zero-field and Zeeman interaction, also exchange interaction among the three Fe(II) ions in the three-metal cluster, H = -J12 (S1.S2)-J23 (S2.S3)-J13 (S1.S3), was applied to simulate both magnetic-Mössbauer and magnetic-susceptibility data. Reasonable fits were achieved only with values magnitude of J12 = magnitude of J23 = magnitude of J13 = magnitude of J < 1 cm-1. Such a situation could not be reconciled with the chair-like geometry of the [M3(CysS)9]3- cluster determined with paramagnetic metal ions, where significantly stronger coupling would be anticipated (magnitude of J = 50-70 cm-1). However, modest exchange-coupling properties have been reported for a number of crystallographically characterized trinuclear [Fe3(SR)3X6]3- clusters (X = Cl, Br; R = Phe, p-tolyl, 2,6-Me2C6H3) distinguished by the preferential formation of a planar Fe3(mu 2-SR)3 ring [Whitener, M. A., Bashkin, J. A., Hagen, K. S., Girerd, J.-J., Gamp, E. Edelstein, N. & Holm, R. H. (1986) J. Amer. Chem. Soc. 108, 5607-5620]. It is therefore more likely that a pseudo-planar geometry rather than a chair-like geometry is present in the Fe3 cluster of Fe(II)7-MT. This would represent the first example of structural differences on binding divalent metal ions to this protein.

Adamantane↗

PubMed1994

Mössbauer studies on iron(II)-substituted yeast metallothionein.

Iron(II)-substituted yeast metallothionein has been studied with Mössbauer spectroscopy. The iron in the protein is in the high-spin ferrous state. A maximum metal content of four iron(II)/molecule has been determined, with the four metal ions forming a diamagnetic cluster due to the antiferromagnetic exchange interaction between Fe2+ via bridging thiolates. In the case where the iron titration gives a value of less than four iron(II)/apoprotein, the metal ions are magnetically noninteracting, with each individual iron(II) behaving like iron(II) in reduced rubredoxin.

Apoproteins↗

PubMed1996

Mössbauer, electron-paramagnetic-resonance and X-ray-absorption fine-structure studies of the iron environment in recombinant human tyrosine hydroxylase.

Isoforms (1-4) of human tyrosine hydroxylase (TH) have been expressed in Escherichia coli and purified as apoenzymes (metal-free). Apo-human TH binds 1.0 atom Fe(II)/enzyme subunit, and iron binding is associated with an immediate and dramatic (40-fold) increase in specific activity. For X-ray absorption fine structure (XAFS) and electron paramagnetic resonance (EPR) measurements the apoenzyme was reconstituted with 56Fe and for Mössbauer measurements with 57Fe. XAFS measurements at the Fe-K edge of human TH were performed on the native form [Fe(II)-human TH], as well as after addition of stoichiometric amounts of the substrate tetrahydropterin, the inhibitor dopamine and of H2O2. The addition of dopamine or H2O2 oxidizes the ferrous iron of the native human TH to the ferric state. In both redox states the iron is octahedrally coordinated by low-Z backscatterers, thus sulfur coordination can be excluded. From the multiple scattering analysis of the EXAFS region is was surmised that part of the iron coordination is due to (3 +/- 1) imidazols. Addition of tetrahydropterin does not significantly change the iron coordination of the Fe(II) enzyme. The Mössbauer results confirm the valence states and the octahedral coordination of iron as well as the exclusion of sulfur ligation. Both the EPR spectra and the Mössbauer magnetic hyperfine pattern of dopamine- and H2O2-treated native human TH, were analyzed with the spin-Hamiltonian formalism. This analysis provides significantly different features for the two forms of human TH: the ferric iron (S = 5/2) of the H2O2-treated form exhibits a rhombic environment while that of the dopamine-treated form exhibits near-axial symmetry. The specific spectroscopic signature of dopamine-treated human TH, including that of an earlier resonance-Raman study [Michaud-Soret, I., Andersson, K. K., Que, L. Jr & Haavik, J. (1995) Biochemistry 34, 5504-5510] is most likely due to the bidentate binding of dopamine to iron.

Binding Sites↗

PubMed1997

The primary structure of the split-Soret cytochrome c from Desulfovibrio desulfuricans ATCC 27774 reveals an unusual type of diheme cytochrome c.

The complete amino acid sequence of the unusual diheme split-Soret cytochrome c from the sulphate-reducing Desulfovibrio desulfuricans strain ATCC 27774 has been determined using classical chemical sequencing techniques and mass spectrometry. The 247-residue sequence shows almost no similarity with any other known diheme cytochrome c, but the heme-binding site of the protein is similar to that of the cytochromes c3 from the sulphate reducers. The cytochrome-c-like domain of the protein covers only the C-terminal part of the molecule, and there is evidence for at least one more domain containing four cysteine residues, which might bind another cofactor, possibly a non-heme iron-containing cluster. This domain is similar to a sequence fragment of the genome of Archaeoglobus fulgidus, which confirms the high conservation of the genes involved in sulfate reduction.

Amino Acid Sequence↗

PubMed1997

Mössbauer study of 4-hydroxybutyryl-CoA dehydratase--probing the role of an iron-sulfur cluster in an overall non-redox reaction.

4-Hydroxybutyryl-CoA dehydratase from Clostridium aminobutyricum catalyzes the dehydration of 4-hydroxybutyryl-CoA to crotonyl-CoA. Although dehydration is an overall non-redox reaction, the enzyme contains FAD and Fe-S clusters. Previous work has shown that the Fe-S clusters are difficult to reduce and therefore unlikely to be redox-active in catalysis. Here, Mössbauer spectroscopy has been used to characterise the Fe-S clusters in active as well as in air-inactivated enzyme. In zero magnetic field at 80 K and 4.2 K, the spectra of active dehydratase consisted mainly of one species (95%) with quadrupole splitting, deltaE(Q) = 1.00 mm s(-1) and isomer shift, delta = 0.43 mm s(-1). Magnetically perturbed Mössbauer spectra indicated a spin of zero. In the presence of 6 mM crotonyl-CoA, the spectra remained unchanged. Taken together, the data show that there are [4Fe-4S]2+ in the enzyme, most probably two clusters/homotetramer, that the four iron atoms in each cluster are coordinated in an identical fashion, and that there is no direct interaction with substrates. We therefore infer that the Fe-S clusters serve a structural rather than a catalytic role in 4-hydroxybutyryl-CoA dehydratase. In air-inactivated enzyme (10% residual activity), a new doublet appeared (58%) with deltaE(Q) = 0.72 mm s(-1), delta = 0.32 mm s(-1) and S = 0. The assignment of this subspectrum to [3Fe-4S]+ clusters, based on the typical Mössbauer parameters, is contradicted by the finding of spin zero for the species. One possible explanation could be spin-coupling of two [3Fe-4S]+ clusters in close proximity.

Air↗

PubMed2004

Basaltic rocks analyzed by the Spirit Rover in Gusev Crater.

The Spirit landing site in Gusev Crater on Mars contains dark, fine-grained, vesicular rocks interpreted as lavas. Pancam and Mini-Thermal Emission Spectrometer (Mini-TES) spectra suggest that all of these rocks are similar but have variable coatings and dust mantles. Magnified images of brushed and abraded rock surfaces show alteration rinds and veins. Rock interiors contain </=25% megacrysts. Chemical analyses of rocks by the Alpha Particle X-ray Spectrometer are consistent with picritic basalts, containing normative olivine, pyroxenes, plagioclase, and accessory FeTi oxides. Mössbauer, Pancam, and Mini-TES spectra confirm the presence of olivine, magnetite, and probably pyroxene. These basalts extend the known range of rock compositions composing the martian crust.

Extraterrestrial Environment↗

PubMed2004

Mineralogy at Gusev Crater from the Mössbauer spectrometer on the Spirit Rover.

Mössbauer spectra measured on Mars by the Spirit rover during the primary mission are characterized by two ferrous iron doublets (olivine and probably pyroxene) and a ferric iron doublet (tentatively associated to nanophase ferric iron oxide). Two sextets resulting from nonstoichiometric magnetite are also present, except for a coating on the rock Mazatzal, where a hematite-like sextet is present. Greater proportions of ferric-bearing phases are associated with undisturbed soils and rock surfaces as compared to fresh rock surfaces exposed by grinding. The ubiquitous presence of olivine in soil suggests that physical rather than chemical weathering processes currently dominate at Gusev crater.

Extraterrestrial Environment↗

PubMed2005

A thiolate-ligated nonheme oxoiron(IV) complex relevant to cytochrome P450.

Thiolate-ligated oxoiron(IV) centers are postulated to be the key oxidants in the catalytic cycles of oxygen-activating cytochrome P450 and related enzymes. Despite considerable synthetic efforts, chemists have not succeeded in preparing an appropriate model complex. Here we report the synthesis and spectroscopic characterization of [FeIV(O)(TMCS)]+ where TMCS is a pentadentate ligand that provides a square pyramidal N4(SR)apical, where SR is thiolate, ligand environment about the iron center, which is similar to that of cytochrome P450. The rigidity of the ligand framework stabilizes the thiolate in an oxidizing environment. Reactivity studies suggest that thiolate coordination favors hydrogen-atom abstraction chemistry over oxygen-atom transfer pathways in the presence of reducing substrates.

Catalysis↗

PubMed2006

An octahedral coordination complex of iron(VI).

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO4(2-). We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet d2(xy) ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

Chemical Phenomena↗

PubMed2006

Chemical and spectroscopic evidence for an FeV-oxo complex.

Iron(V)-oxo species have been proposed as key reactive intermediates in the catalysis of oxygen-activating enzymes and synthetic catalysts. Here, we report the synthesis of [Fe(TAML)(O)]- in nearly quantitative yield, where TAML is a macrocyclic tetraamide ligand. Mass spectrometry, Mössbauer, electron paramagnetic resonance, and x-ray absorption spectroscopies, as well as reactivity studies and density functional theory calculations show that this long-lived (hours at -60 degrees C) intermediate is a spin S = 1/2 iron(V)-oxo complex. Iron-TAML systems have proven to be efficient catalysts in the decomposition of numerous pollutants by hydrogen peroxide, and the species we characterized is a likely reactive intermediate in these reactions.

Catalysis↗