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PubMed2003

Synthesis and spectral characterization of some oxo-centered, trinuclear mixed-valence iron thiocarboxylates.

Some mixed-valence Fe(II) Fe(III) complexes of thiocarboxylic acids and straight chain fatty acids with general formula [Fe(II)Fe(III)(2)O(SOCR)(6)(H(2)O)(3)] and [Fe(II)Fe(III)(2)O(SOCR)(3)(OOCR')(3)(CH(3)OH)(3)] (where, R=CH(3) or C(6)H(5) and R'=C(13)H(27), C(15)H(31) or C(17)H(35)) were synthesized and characterized by elemental analyses, spectral (infrared, electronic and Mössbauer) studies, molar conductance and magnetic susceptibility measurements. The infrared spectra suggested bridging nature of carboxylate and thiocarboxylate anions along with upsilon(asym)(Fe(3)O) vibrations in the complexes. Mössbauer studies revealed two resolved quadrupole doublets at 120-315 K confirming the presence of Fe(II) and Fe(III) moieties in the complexes. This was supported by the observed electronic spectral bands in the complexes at room temperature. The spectrum showed a band at around 13,800 cm(-1) which indicated an intervalence-transfer. Magnetic susceptibility measurements showed weak antiferromagnetic coupling related to mixed-valence pairs with S(2)=2, em leader S(1)=S(3)=5/2 spin exchange model. Conductance data indicated, the complexes were non-electrolytes in nitrobenzene. A structure has been established on the basis of these studies.

Carboxylic Acids↗
PubMed2003

Mössbauer and electronic spectral studies of iron(III) complexes of oximes.

The hydroxo-bridge complexes of the type [Fe(2)(ligand-H)(4)(OH)(2)] with bidentate nitrogen-oxygen donor ligands, viz. 2-hydroxynaphthaldehydeoxime [hnoH(2)], 2-hydroxyacetphenoneoxime [haoH(2)], salicylaldooxime [SalH(2)], 2-hydroxypropiophenoneoxime [hnoH(2)] have been prepared. All the complexes have been characterized by elemental analysis, magnetic moments, electronic and Mössbauer spectral studies. Mössbauer parameters of the complexes clearly suggest high spin configuration of Fe(III) showing lower magnetic moment to that of the spin only value, i.e. 5.92 BM. It may be due to the antiferromagnetic interaction between Fe(III) centers.

Electrons↗
PubMed2004

Study of the relationship of small variations of the molecular structure and the iron state in iron containing proteins by Mössbauer spectroscopy: biomedical approach.

This review considers the results of experimental Mössbauer studies and theoretical calculations of the effect of small variations of protein molecular structure on the iron electronic structure and stereochemistry in order to understand the proteins structural heterogeneity and functional variety. Structural changes in iron containing proteins during various diseases are also considered. These results show the relationship of the small structural variations and Mössbauer parameters of iron containing proteins and demonstrate the possibilities of Mössbauer spectroscopy to obtain new information at the molecular level in biomedical research.

Animals↗
PubMed2004

Vibrational study of the Fe(phen)2(NCS)2 spin-crossover complex by density-functional calculations.

The geometries and vibrational frequencies of an iron(II) spin-crossover complex Fe(phen)2(NCS)2 in the low- and high-spin states have been calculated using DFT (BP86 functional with 6-311G* + Wachters + f, 6-31G* and LANL2DZ basis sets). A redundancy-free set of internal coordinates has been defined and used to obtain a valence force field. The use of the symmetry coordinates of a perfect octahedron as internal coordinates for the FeN6 framework of the complex has enabled the description of the stretching-bending couplings. A comparison of the force constants of free ligands with the corresponding constants of the two spin states has provided a basis for the discussion of the changes in electronic charge distribution in the complex. By use of the generalized harmonic mode scrambling, it is shown that in the low-lying normal vibrations of the complex, the ligands participate as almost rigid units. Energies of these modes are close to the oscillators describing the interaction of the complex with the surrounding molecules in the solid phase.

Electrons↗
PubMed1997

A 4-term energy level scheme for the high-spin ferrous hemoproteins: evidence for the 5E eta, and 5B2 terms as the ground multiplets in hemoproteins with a histidine and a cysteine protein-derived heme ligand, respectively.

We have carried out analysis of the electronic level scheme of the high-spin ferrous hemoproteins by simultaneous fit of the adjustable parameters of a 4-term theoretical model to low-temperature magnetic circular dichroism (MCD), room temperature absorption spectra and available magnetic susceptibility and or Mössbauer data of myoglobin, horseradish peroxidase and cytochrome P450. The high reliability of the ligand field parameter values obtained for deoxymyoglobin is confirmed by good agreement between the predicted and observed magnetic field dependences of MCD and magnetization not used in the fit procedure. In addition, an energy gap between the ground and first excited singlets, estimated to be 4.2 cm-1, agrees well with the value of approximately 4 cm-1 derived from the far-infrared magnetic resonance. Our computer and explicit theoretical analyses give strong evidence that large distinctions in the shape, intensity and temperature behaviour of the MCD of Mb and HRP from those of cytochrome P450 can be described only if the ground manifold in these proteins is 5E eta and 5B2, respectively. The changes in relative energies of the one-electron 3d-orbitals on substitution of an imidazole of histidine for a sulphur anion of cysteine as a protein-derived heme iron ligand are rationalized by the lower ionization potential of the negatively charged sulphur ligand and the higher pi-orbital overlap of its lone pair orbitals with the iron d pi-orbitals compared to the imidazole ligand.

Animals↗
PubMed1995

Recombinant Desulfovibrio vulgaris rubrerythrin. Isolation and characterization of the diiron domain.

The gene encoding Desulfovibrio (D.) vulgaris rubrerythrin (Prickril, B. C., Kurtz, D. M., Jr., LeGall, J., & Voordouw, G. (1991) Biochemistry 30, 1118), a protein of unknown function containing both FeS4 and (mu-oxo)diiron sites, was cloned and overexpressed in Escherichia coli. Upon cell lysis, the overexpressed protein was found in an insoluble form deficient in iron. Iron was incorporated in vitro by dissolving the protein in 3 M guanidinium chloride, adding Fe(II) anaerobically and diluting the denaturant. This recombinant rubrerythrin was found to have properties very similar to those of rubrerythrin isolated from D. vulgaris, except that the recombinant rubrerythrin contained six rather than four (or five) iron atoms per 44 kDa homodimer. Analyses of UV-vis, Mössbauer, and EPR spectra showed that the six iron atoms in recombinant rubrerythrin are organized as two FeS4 and two (mu-oxo/hydroxo)diiron sites. In order to allow examination of the diiron sites in the absence of the FeS4 sites, a truncated gene encoding the N-terminal 152 residues of D. vulgaris rubrerythrin was also cloned and overexpressed as an insoluble protein in E. coli, and iron was incorporated by a procedure analogous to that for recombinant rubrerythrin. This so-called "chopped" rubrerythrin (CRr) was found to consist of an approximately 35 kDa homodimer containing four iron atoms. Spectroscopic characterization indicated that the four iron atoms in CRr are organized as two diiron sites, the majority of which closely resemble the (mu-oxo)diiron(III) sites in E. coli ribonucleotide reductase R2 protein, and a minor fraction of which resemble the mixed-valent diiron(II,III) site in methane monooxygenase hydroxylase.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial Proteins↗
PubMed1995

Effects of photoinhibition on the QA-Fe2+ complex of photosystem II studied by EPR and Mössbauer spectroscopy.

Effects of photoinhibition on the iron-quinone electron acceptor complex of oxygen-evolving photosystem II have been studied using low-temperature EPR and Mössbauer spectroscopy. Photoinhibition of spinach photosystem II membrane particles at 4 degrees C decreases the EPR signal arising from the interaction of QA- with Fe2+ to 30% in 90 min under our conditions. The free radical EPR signal from QA- induced by cyanide treatment of the iron [Sanakis, Y., et al. (1994) Biochemistry 33, 9922-9928] declines with the same kinetics as the QA-Fe2+ EPR signal. In contrast, Fe2+ is present in about 70% of the centers after 90 min of photoinhibition, as shown by its EPR-detected interaction with NO and by its Mössbauer absorption. Complete oxidation of this Fe2+ population to Fe3+ by ferricyanide is possible only in the presence of glycolate, which lowers the redox potential of the Fe3+/Fe2+ couple. In a fraction of PSII centers, which reach 30% after 90 min of photoinhibition, the iron cannot be detected. It is concluded that photoinhibition of oxygen-evolving photosystem II affects both QA and Fe2+. However, the photoinhibitory impairment of the QA redox functioning precedes the modification of the non-heme iron. In a considerable portion of the photoinhibited centers, which do not have functional QA, the non-heme iron is still present and redox active, but its redox potential is increased relative to that in the normal centers. This is probably due to a minor modification of the bicarbonate ligation site.(ABSTRACT TRUNCATED AT 250 WORDS)

Electron Spin Resonance Spectroscopy↗
PubMed1995

Reaction of NO with the reduced R2 protein of ribonucleotide reductase from Escherichia coli.

The active R2 protein of ribonucleotide reductase from Escherichia coli contains a catalytically essential tyrosine radical at position 122 (Tyr122.) that is formed during the reaction of dioxygen with the nearby diiron(II) center. To gain insight into the mode of dioxygen binding, the reaction of the O2 analog NO with the diiron(II) centers of R2red has been investigated by spectroscopic methods. R2red reacts with NO to form an adduct with visible absorption features at 450 and 620 nm and Mössbauer parameters (delta = 0.75 mm/s, delta EQ = -2.13 and -1.73 mm/s) typical of those observed for S = 3/2 [FeNO]7 complexes of other non-heme iron proteins. However, unlike other non-heme [FeNO]7 complexes, this adduct is EPR silent. Our Mössbauer studies show that each iron site of R2red binds one NO to form local S = 3/2 [FeNO]7 centers which then couple antiferromagnetically (J approximately 5 cm-1, H = JS1.S2) to afford an [FeNO]2 center (77% of total iron). This [FeNO]2 center decomposes with a first-order rate constant of 0.013 min-1 to form R2met, accompanied by the release of N2O. These observations suggest that both iron(II) ions of the two diiron(II) centers of R2red have available sites for NO binding, in agreement with the crystallographic results on R2red, and that the bound NO molecules are sufficiently close to each other to permit N-N bond formation to produce N2O. These observations support the proposal that dioxygen binding may also involve both metal ions of the diiron(II) center to form a (mu-1,1-, or mu-1,2-peroxo)-diiron(III) center. This observed reactivity of R2red with NO may contribute to the in vivo inhibition of ribonucleotide reductase by NO.

Binding Sites↗
PubMed1995

Reexamining the Mössbauer effect as a means to cleave DNA.

The ability of a DNA-bound Mössbauer isotope to absorb resonant gamma-radiation and subsequently, upon decay, induce DNA double-strand breaks by emission of low-energy Auger electrons was examined with a simple plasmid DNA cleavage assay. This mechanism was postulated by Mills et al. [Mills et al. (1988) Nature 336, 787] in the observed ablation of tumor cell growth with 57Fe(III)-bleomycin and Mössbauer radiation. The observed linearization of supercoiled pAA15 plasmid DNA upon treatment with five or more 57Fe(III)-bleomycin per plasmid precluded its use for testing Mössbauer effect induced cleavage. An alternative 57Fe-DNA-binding complex, [57Fe(phen)2(DPPZ)](PF6)2.H2O (57(1)), was synthesized and found to tightly bind DNA (K = 9.8 x 10(5) M-1) yet not induce DNA nicks or cuts at loadings of less than 500/pAA15 plasmid. Mössbauer irradiation of 57(1)/pAA15 samples under frozen and solution conditions does not result in observable linearization of the plasmid DNA over control samples. Some linearization is observed in all irradiated samples but is attributed only to the photoelectric effect.

Animals↗
PubMed1993

Spectroscopic characterization of 57Fe-reconstituted rubrerythrin, a non-heme iron protein with structural analogies to ribonucleotide reductase.

Rubrerythrin, a contraction of rubredoxin and hemerythrin, is the trivial name given to a non-heme iron protein isolated from Desulfovibrio vulgaris (Hildenborough). This protein, whose physiological function is unknown, was first characterized by J. LeGall et al. [(1988) Biochemistry 28, 1636] as being a homodimer of subunit M(r) = 21,900 with four Fe per homodimer distributed as two rubredoxin-type FeS4 centers and one hemerythrin-type diiron cluster. Subsequent analysis of the amino acid sequence of the rubrerythrin gene [Kurtz, D. M., Jr., & Prickril, B.C. (1991) Biochem. Biophys. Res. Commun. 181, 137] revealed an internal homology which suggested that each subunit can accommodate one diiron cluster. Here, we report a procedure for reconstitution of the as-isolated D. vulgaris rubrerythrin with 57Fe. The reconstituted protein was characterized by optical, electron paramagnetic resonance, and Mössbauer spectroscopies. The results indicate successful incorporation of 57Fe into the two types of sites and strongly suggest that each subunit of rubrerythrin can indeed accommodate one diiron cluster as well as one rubredoxin-type center. Combined with amino acid sequence analysis, the spectroscopic characterization further suggests that the rubrerythrin subunit contains a diiron site whose structure is more closely related to that in ribonucleotide reductase than to that in hemerythrin.

Bacterial Proteins↗
PubMed1992

Crystal structures of aconitase with isocitrate and nitroisocitrate bound.

The crystal structures of mitochondrial aconitase with isocitrate and nitroisocitrate bound have been solved and refined to R factors of 0.179 and 0.161, respectively, for all observed data in the range 8.0-2.1 A. Porcine heart enzyme was used for determining the structure with isocitrate bound. The presence of isocitrate in the crystals was corroborated by Mössbauer spectroscopy. Bovine heart enzyme was used for determining the structure with the reaction intermediate analogue nitroisocitrate bound. The inhibitor binds to the enzyme in a manner virtually identical to that of isocitrate. Both compounds bind to the unique Fe atom of the [4Fe-4S] cluster via a hydroxyl oxygen and one carboxyl oxygen. A H2O molecule is also bound, making Fe six-coordinate. The unique Fe is pulled away approximately 0.2 A from the corner of the cubane compared to the position it would occupy in a symmetrically ligated [4Fe-4S] cluster. At least 23 residues from all four domains of aconitase contribute to the active site. These residues participate in substrate recognition (Arg447, Arg452, Arg580, Arg644, Gln72, Ser166, Ser643), cluster ligation and interaction (Cys358, Cys421, Cys424, Asn258, Asn446), and hydrogen bonds supporting active site side chains (Ala74, Asp568, Ser571, Thr567). Residues implicated in catalysis are Ser642 and three histidine-carboxylate pairs (Asp100-His101, Asp165-His147, Glu262-His167). The base necessary for proton abstraction from C beta of isocitrate appears to be Ser642; the O gamma atom is proximal to the calculated hydrogen position, while the environment of O gamma suggests stabilization of an alkoxide (an oxyanion hole formed by the amide and side chain of Arg644). The histidine-carboxylate pairs appear to be required for proton transfer reactions involving two oxygens bound to Fe, one derived from solvent (bound H2O) and one derived from substrate hydroxyl. Each oxygen is in contact with a histidine, and both are in contact with the side chain of Asp165, which bridges the two sites on the six-coordinate Fe.

Aconitate Hydratase↗
PubMed1994

Further characterization of the spin coupling observed in oxidized hydrogenase from Chromatium vinosum. A Mössbauer and multifrequency EPR study.

Hydrogenase from Chromatium vinosum contains 1 Ni, 11-12 Fe, and ca. 9 sulfides. EPR and Mössbauer studies of the enzyme prepared in four different oxidation states show that the enzyme contains two Fe4S4 and one Fe3S4 cluster. In the oxidized (2+) state, the Mössbauer parameters of the two Fe4S4 clusters are typical for this cluster type. Upon reduction, however, these clusters do not exhibit the familiar g = 1.94 signal. The unusual nature of the reduced clusters is also borne out by the Mössbauer spectra which exhibit fairly small magnetic hyperfine interactions similar to those of centers I and II of the Desulfovibrio gigas enzyme. The Mössbauer spectra of the Fe3S4 cluster in the oxidized (1+) and reduced states are typical for this cluster type. The C. vinosum hydrogenase undergoes a reversible redox reaction at Em = +150 mV (vs NHE). Above +150 mV the EPR spectra exhibit signals (previously called signals 2 and 4) that reflect a weak interaction between Ni(III) and an Fe-containing moiety. By clamping the Ni in the diamagnetic Ni(II).CO form, we have discovered that signal 2 (X-band resonances at g = 2.01, 1.974, and 1.963) involves the Fe3S4 cluster and an as yet unidentified paramagnetic moiety. The "coupled" system exhibits magnetic hyperfine interactions quite different from those of the uncoupled [Fe3S4]1+ cluster. We have not yet been able to assign a spin to the coupled state but some of the features of the state are reminiscent of an S = 1 system. The Mössbauer data suggest, but do not prove, that an extra Fe site may be present that shuttles between low-spin Fe(III) and low-spin Fe(II) with Em = +150 mV. The Fe(III) may be located between the Ni(III) and the Fe3S4 cluster enabling it to mediate the interaction between the cluster and the Ni site. In this picture, the Fe(III) site is part of the coupled state that gives rise to signal 2. Other possibilities for signal 2 involve a ligand-based oxidation of the [Fe3S4]1+ cluster or generation of a nearby radical.

Amino Acid Sequence↗
PubMed1994

Cyanide binding at the non-heme Fe2+ of the iron-quinone complex of photosystem II: at high concentrations, cyanide converts the Fe2+ from high (S = 2) to low (S = 0) spin.

The primary electron acceptor complex of photosystem II, QAFe2+, can bind a number of small molecules at the iron site, including cyanide [Koulougliotis, D., Kostopoulos, T., Petrouleas, V., & Diner, B. A. (1993) Biochim. Biophys. Acta 1141, 275-282)]. In the presence of NaCN (30-300 mM) at pH 6.5, the reduced state, QA-Fe2+, produced either by illumination at < or = 200 K or by reduction in the dark with sodium dithionite, is characterized by a g = 1.98 EPR signal. The light- or dithionite-induced g = 1.98 signal decays with increasing pH above 6.5 and is almost totally absent at pH 8.1 and NaCN concentrations above 300 mM. However, at high pH (8.1), the g = 1.98 signal still forms transiently before it decays with a t1/2 of approximately 30 min in spinach BBY preparations treated with 100 mM NaCN. Complementary to the disappearance of the g = 1.98 signal with increasing pH or incubation time, a new EPR signal develops at g = 2.0045. This signal has the characteristics of the semiquinone, QA-, uncoupled from its magnetic interaction with the iron. Prolonged incubation of a high pH, high cyanide treated sample in a cyanide-free medium at pH 6 restores the ability of the sample to develop the cyanide-induced g = 1.98 signal at pH 6.5. This indicates that the iron is not physically dissociated during the high pH cyanide treatment. The high pH, high cyanide effects are accompanied by the conversion of the characteristic Fe2+ (S = 2) Mössbauer doublet [isomer shift (Fe) = 1.19 mm/s, quadrupole splitting = 2.95 mm/s] to a new one with parameters (isomer shift = 0.26 mm/s, quadrupole splitting = 0.36 mm/s) characteristic of an Fe2+(S = 0) state.(ABSTRACT TRUNCATED AT 250 WORDS)

Anions↗
PubMed1994

Resonance Raman evidence for an Fe-O-Fe center in stearoyl-ACP desaturase. Primary sequence identity with other diiron-oxo proteins.

The stearoyl-ACP delta 9 desaturase from plants is a new example of a growing number of proteins that contain oxo- or hydroxo-bridged diiron clusters. On the basis of differences in primary sequence motifs providing the cluster ligands and upon structural differences elucidated by X-ray crystallography, we now propose that the presently known, soluble diiron-oxo proteins can be grouped into two classes, I and II. Class I contains hemerythrin, myohemerythrin, and, possibly, purple acid phosphatase. Class II contains ribonucleotide reductases, bacterial hydrocarbon hydroxylases (methane monooxygenase, toluene-4-monooxygenase, and phenol hydroxylase), rubrerythrin, and stearoyl-ACP desaturases. Through the use of resonance Raman spectroscopy, we have detected symmetric (vs = 519 cm-1) and asymmetric (vas = 747 cm-1) vibrational modes in the castor stearoyl-ACP delta 9 desaturase, which are typical of oxo-bridged diiron clusters. These frequencies shift by -18 and -34 cm-1, respectively, in H218O, proving that the bridging ligand is readily exchangeable with solvent (t1/2 = 7 min). Calculation of an approximately 123 degrees Fe-O-Fe angle from the position of vs and vas and from the 18O-dependent shift in these frequencies suggests that the diiron-oxo cluster in the desaturase is triply bridged in the diferric state. In the diferrous state, the two iron sites of the cluster are structurally inequivalent, as shown by differential temperature dependence of the Mössbauer quadrupole splittings. For the class II diiron-oxo proteins, primary sequence alignments reveal conserved amino acid residues which act as iron cluster ligands, participate in a hydrogen-bonding network, and are potentially involved in O2 binding and activation. Based on this conservation, a structural model for the stearoyl-ACP delta 9 desaturase active site is proposed that has strong similarity to both ribonucleotide reductase and methane monooxygenase. However, after single turnover of the diferous state with 18O2, 18O is not detected in the oxo bridge of the castor desaturase. This is in contrast to the outcome observed for ribonucleotide reductase, suggesting the desaturase and ribonucleotide reductase differ in certain aspects of their respective O2-activation reactions.

Amino Acid Sequence↗
PubMed1991

Mössbauer spectroscopy of iron-ovotransferrin: a crystal field interpretation.

Mössbauer spectra from frozen solutions of ovotransferrin were recorded in a variety of applied external magnetic fields and at various temperatures in a small applied field. The results were fitted to a simple model for the electronic structure at the iron site. This model requires admixtures of the free ion 6S and 4P states, indicating a weak cubic crystal field. Possible implications of this model regarding the binding site are discussed.

Binding Sites↗