Übergangsmetallkomplexe von Diazenen, XXIII [1]. Struktur und Mößbauer-Daten bi- und tricyclischer Ferracyclen / Transition Metal Complexes of Diazenes, XXIII [1] Structure and Mößbauer Data of Bi- and Tricyclic Ferracycles

Mößbauer spectra of mono and dinuclear iron carbonyl complexes of three cyclic diazenes and the crystal structures of two products formed from (cis-diazene)Fe2(CO)6 and alkynes are reported. The ring size of the diazene ligand influences the stereochemistry of the cycloadducts obtained in the latter reaction; the bicyclic diazaferracycloalkene skeleton of these compounds leads to an exo- and endo-arrangement of one iron carbonyl group relative to the five- and six- membered diazene, rcspectively.

A Triangular Iron(III) Complex Potentially Relevant to Iron(III)-Binding Sites in Ferreascidin

An asymmetric triangular Fe(III) complex has been synthesized by an unusual Fe(II) -promoted activation of salicylaldoxime. Formation of the ligand 2-(bis(salicylideneamino)methyl)phenol in situ is believed to occur through the reductive deoximation of salicylaldoxime by ferrous ions. The trinuclear ferric complex has been characterized on the basis of elemental analysis, IR, variable-temperature magnetic susceptibility, and EPR and Mössbauer spectroscopies. The molecular structure established by X-ray diffraction consists of a trinuclear structure with a [Fe3 (μ3 -O)(μ2 -OPh)](6+) core. Two iron ions are in a distorted octahedral environment having FeN2 O4 coordination spheres, and the five-coordinated third iron ion, with an FeNO4 coordination sphere, is in a trigonal bipyramidal environment. The magnetic susceptibility measurements revealed an St = 5/2 ground state with the antiparallel exchange interactions J = - 34.3 cm(-1) , J' = - 4.7 cm(-1) , and D = - 0.90 cm(-1) . The EPR results are consistent with a ground state of S = 5/2 together with a negative D5/2 value. The Mössbauer isomer shifts together with the quadrupole splitting also provide evidence for the high-spin state of the three ferric sites. Magnetic Mössbauer spectra lead to the conclusion that the internal magnetic fields possibly lie in the plane of the three ferric ions.

Multi-frequency EPR and Mössbauer spectroscopic studies on freeze-quenched reaction intermediates of nitric oxide synthase

It is believed by analogy to chloroperoxidase (CPO) from Caldariomyces fumago that the electronic structure of the intermediate iron-oxo species in the catalytic cycle of nitric oxide synthase (NOS) corresponds to an iron(IV) porphyrin-pi -cation radical. Such species can also be produced by the reaction of ferric NOS with external oxidants within the shunt pathway. We present multi-frequency EPR (9.6, 94, 285 GHz) and Mössbauer spectroscopic studies on freeze-quenched intermediates of the oxygenase domain of nitric oxide synthase which has reacted with peroxy acetic acid within 8-200 ms. The intermediates of the oxygenase domain of both the cytokine inducible NOS (iNOSox) and the neuronal NOS (nNOSox) show an organic radical signal in the 9.6-GHz spectrum overlapping with the spectrum of an unknown species with g-values of 2.24, 2.23 and 1.96. Using 94- and 285-GHz EPR the organic radical signal is assigned to a tyrosine radical on the basis of g-values (i.e. Tyr*562 in nNOSox and Tyr*341 in iNOSox). Mössbauer spectroscopy of (57)Fe-labeled unreacted nNOSox shows a ferric low-spin heme-iron (delta = 0.38 mms(-1), deltaE(Q) = 2.58 mms(-1)). The reaction of nNOSox with peroxy acetic acid for 8 ms leads to the disappearance of the magnetic background characteristic for native nNOSox and a new species with delta = 0.27 mms(-1) and deltaE(Q) = 2.41 mms(-1) is detected at 4.2 K which does not resemble the parameters typical for a Fe(IV) center. It is proposed that this intermediate species corresponds to a ferric low-spin species which magnetically couples to an amino acid radical (presumably Trp*409).

Dynamic structural disorder of the FeO2 moiety in oxymyoglobin studied by nuclear resonant forward scattering of synchrotron radiation

Oxy- as well as deoxymyoglobin exhibit a pronounced temperature dependence of the quadrupole splitting of the heme iron as detected by conventional Mössbauer spectroscopy. With nuclear resonant forward scattering (NFS) of synchrotron radiation, which can be viewed as Mössbauer spectroscopy in the time domain, it is shown that this spectroscopic behavior, although it is phenomenologically similar in the two cases, is based on completely different physical mechanisms. It is demonstrated that stochastic fluctuations of the iron electric field gradient in MbO(2), which are due to the dynamic structural disorder of the FeO(2) moiety, are the reason for the temperature-dependent alterations of the coherent quantum beat pattern in the NFS spectra of MbO(2), in contrast to deoxyMb where transitions between orbital states of iron take place. This subtle spectroscopic difference cannot be inferred from conventional Mössbauer spectroscopy.

Spectroscopic studies of peroxyacetic acid reaction intermediates of cytochrome P450cam and chloroperoxidase

It is generally assumed that the putative compound I (cpd I) in cytochrome P450 should contain the same electron and spin distribution as is observed for cpd I of peroxidases and catalases and many synthetic cpd I analogues. In these systems one oxidation equivalent resides on the Fe(IV)=O unit (d(4), S=1) and one is located on the porphyrin (S'=1/2), constituting a magnetically coupled ferryl iron-oxo porphyrin pi-cation radical system. However, this laboratory has recently reported detection of a ferryl iron (S=1) and a tyrosyl radical (S'=1/2), via Mössbauer and EPR studies of 8 ms-reaction intermediates of substrate-free P450cam from Pseudomonas putida, prepared by a freeze-quench method using peroxyacetic acid as the oxidizing agent [Schünemann et al., FEBS Lett. 479 (2000) 149]. In the present study we show that under the same reaction conditions, but in the presence of the substrate camphor, only trace amounts of the tyrosine radical are formed and no Fe(IV) is detectable. We conclude that camphor restricts the access of the heme pocket by peroxyacetic acid. This conclusion is supported by the additional finding that binding of camphor and metyrapone inhibit heme bleaching at room temperature and longer reaction times, forming only trace amounts of 5-hydroxy-camphor, the hydroxylation product of camphor, during peroxyacetic acid oxidation. As a control we performed freeze-quench experiments with chloroperoxidase from Caldariomyces fumago using peroxyacetic acid under the identical conditions used for the substrate-free P450cam oxidations. We were able to confirm earlier findings [Rutter et al., Biochemistry 23 (1984) 6809], that an antiferromagnetically coupled Fe(IV)=O porphyrin pi-cation radical system is formed. We conclude that CPO and P450 behave differently when reacting with peracids during an 8-ms reaction time. In P450cam the formation of Fe(IV) is accompanied by the formation of a tyrosine radical, whereas in CPO Fe(IV) formation is accompanied by the formation of a porphyrin radical.

Plant adenosine 5'-phosphosulfate reductase is a novel iron-sulfur protein

Adenosine 5'-phosphosulfate reductase (APR) catalyzes the two-electron reduction of adenosine 5'-phosphosulfate to sulfite and AMP, which represents the key step of sulfate assimilation in higher plants. Recombinant APRs from both Lemna minor and Arabidopsis thaliana were overexpressed in Escherichia coli and isolated as yellow-brown proteins. UV-visible spectra of these recombinant proteins indicated the presence of iron-sulfur centers, whereas flavin was absent. This result was confirmed by quantitative analysis of iron and acid-labile sulfide, suggesting a [4Fe-4S] cluster as the cofactor. EPR spectroscopy of freshly purified enzyme showed, however, only a minor signal at g = 2.01. Therefore, Mössbauer spectra of (57)Fe-enriched APR were obtained at 4.2 K in magnetic fields of up to 7 tesla, which were assigned to a diamagnetic [4Fe-4S](2+) cluster. This cluster was unusual because only three of the iron sites exhibited the same Mössbauer parameters. The fourth iron site gave, because of the bistability of the fit, a significantly smaller isomer shift or larger quadrupole splitting than the other three sites. Thus, plant assimilatory APR represents a novel type of adenosine 5'-phosphosulfate reductase with a [4Fe-4S] center as the sole cofactor, which is clearly different from the dissimilatory adenosine 5'-phosphosulfate reductases found in sulfate reducing bacteria.

Valence-tautomerism in high-valent iron and manganese porphyrins

Iron and manganese hemes are "high-valent" when the valence state of the metal exceeds III. Redox chemistry of the high valent metal complexes involves redistribution of holes and electrons over the metal ion and the porphyrin and axial ligands, defined as valence tautomerism. Thus, catalytic pathways of heme-containing biomolecules such as peroxidases, catalases and cytochromes P450 involve valence tautomerism, as do pathways of biomimetic oxygen transfer catalysis by manganese porphyrins, robust catalysts with potential commercial value. Determinants of the site of electron abstraction are key to understanding valence tautomerism. In model systems, metal-centered oxidation is supported by hard anionic axial ligands that are also strongly pi-donating, such as oxo, aryl, bix-methoxy and bis-fluoro groups. Manganese(IV) is more stable than iron(IV) and metal-centered one-electron oxidations occur with weaker pi-donating axial ligands such as bisazido, -isocyanato, -hypochlorito and bis chloro groups. Virtually all known high-valent iron porphyrin complexes oxidized by two-electrons above the ferric state are coordinated by the strongly pi-donating oxo or nitrido ligands. In all well-characterized oxo complexes, iron is in the ferryl state and the second oxidizing equivalent resides on the porphyrin. Complexes with iron(V) have not been definitively characterized. One-electron oxidation of oxomanganese(IV) porphyrin complexes gives the oxomanganese(IV) porphyrin pi-cation redicals. In aqueous solution, oxidation of Mn(III) complexes of tetra cationic N-methylpyridiniumylporphyrin isomers by monooxygen donors yields a transient oxomanganese(V) species.

Activation of class III ribonucleotide reductase from E. coli. The electron transfer from the iron-sulfur center to S-adenosylmethionine

The anaerobic ribonucleotide reductase (ARR) from E. coli is the prototype for enzymes that use the combination of S-adenosylmethionine (AdoMet) and an iron-sulfur center for generating catalytically essential free radicals. ARR is a homodimeric alpha2 protein which acquires a glycyl radical during anaerobic incubation with a [4Fe-4S]-containing activating enzyme (beta) and AdoMet under reducing conditions. Here we show that the EPR-active S = 1/2 reduced [4Fe-4S]+ cluster is competent for AdoMet reductive cleavage, yielding 1 equiv of methionine and almost 1 equiv of glycyl radical. These data support the proposal that the glycyl radical results from a one-electron oxidation of the reduced cluster by AdoMet. Reduced protein beta alone is also able to reduce AdoMet but only in the presence of DTT. However, in that case, 2 equiv of methionine per reduced cluster was formed. This unusual stoichiometry and combined EPR and Mössbauer spectroscopic analysis are used to tentatively propose that AdoMet reductive cleavage proceeds by an alternative mechanism involving catalytically active [3Fe-4S] intermediate clusters.

Iron, neuromelanin and ferritin content in the substantia nigra of normal subjects at different ages: consequences for iron storage and neurodegenerative processes

Information on the molecular distribution and ageing trend of brain iron in post-mortem material from normal subjects is scarce. Because it is known that neuromelanin and ferritin form stable complexes with iron(III), in this study we measured the concentration of iron, ferritin and neuromelanin in substantia nigra from normal subjects, aged between 1 and 90 years, dissected post mortem. Iron levels in substantia nigra were 20 ng/mg in the first year of life, had increased to 200 ng/mg by the fourth decade and remained stable until 90 years of age. The H-ferritin concentration was also very low (29 ng/mg) during the first year of life but increased rapidly to values of approximately 200 ng/mg at 20 years of age, which then remained constant until the eighth decade of life. L-Ferritin also showed an increasing trend during life although the concentrations were approximately 50% less than that of H-ferritin at each age point. Neuromelanin was not detectable during the first year, increased to approximately 1000 ng/mg in the second decade and then increased continuously to 3500 ng/mg in the 80th year. A Mössbauer study revealed that the high-spin trivalent iron is probably arranged in a ferritin-like iron--oxyhydroxide cluster form in the substantia nigra. Based on this data and on the low H- and L-ferritin content in neurones it is concluded that neuromelanin is the major iron storage in substantia nigra neurones in normal individuals.

Anisotropic nuclear inelastic scattering of an iron(II) molecular crystal

Nuclear inelastic scattering (NIS) spectra were recorded for a monocrystal of the spin-crossover complex [Fe(tptMetame)] (ClO (4))(2) (tptMetame = 1,1,1-tris([N-(2-pyridylmethyl)-N-methylamino]-methyl)ethane) at T = 30 K (low-spin state) and at room temperature (high-spin state) for different crystal orientations. The high energy resolution (0.65 meV) allowed us to resolve individual molecular vibrations which were unambiguously identified by density functional calculations. From the NIS spectra for the first time the angular-resolved iron-partial density of phonon states (PDOS) was extracted. The PDOS corroborates a vibrational entropy difference as driving force of the spin transition.

Porphyrin-Fe(III)-hydroperoxide and porphyrin-Fe(III)-peroxide anion as catalytic intermediates in cytochrome P450-catalyzed hydroxylation reactions: a molecular orbital study

The hydroxylation of fluorobenzene and aniline, catalyzed by the porphyrin-Fe(III)-peroxide anion with either a cysteinate- or a histidyl-type of axial ligand as well as the hydroxylation of fluorobenzene, catalyzed by porphyrin-Fe(III)-hydroperoxide with a cysteinate-type of axial ligand as catalytic intermediates, have been investigated by electronic structure calculations in local spin-density approximation. Non-repulsive potential curves are, in contrast with porphyrin-Fe(III)-hydroperoxide, obtained only in the case of porphyrin-Fe(III)-peroxide anion as catalytic intermediate. The mutual substrate-porphyrin orientation with a dihedral angle between the plane of the substrate and the porphyrin plane of 45 degrees is more favorable compared with the parallel orientation between these two planes. This orientation differs for the case of fluorobenzene hydroxylation from the corresponding one calculated by us with the ferryl-oxo-pi-cation radical complex as a catalytic intermediate. The calculated reaction profiles show also the effectiveness of the histidyl-type coordinated porphyrin-Fe(III)-peroxide involved in P450 type of hydroxylation reactions. The calculations demonstrate the predominant role of the O1-O2 moiety of the porphyrin-Fe(III)-peroxide anion in the hydroxylation process of the substrates. The results indicate that the porphyrin-Fe(III)-peroxide anion is an effective catalytic species in hydroxylation reactions. In all the studied cases irrespective of the substrate and the nature of the axial ligand, the potential curves reach minimum at approximately 130-140 pm, expressing the length of an aromatic C-O bond.

Intermediates in the reaction of substrate-free cytochrome P450cam with peroxy acetic acid

Freeze-quenched intermediates of substrate-free cytochrome 57Fe-P450(cam) in reaction with peroxy acetic acid as oxidizing agent have been characterized by EPR and Mossbauer spectroscopy. After 8 ms of reaction time the reaction mixture consists of approximately 90% of ferric low-spin iron with g-factors and hyperfine parameters of the starting material; the remaining approximately 10% are identified as a free radical (S' = 1/2) by its EPR and as an iron(IV) (S= 1) species by its Mossbauer signature. After 5 min of reaction time the intermediates have disappeared and the Mossbauer and EPR-spectra exhibit 100% of the starting material. We note that the spin-Hamiltonian analysis of the spectra of the 8 ms reactant clearly reveals that the two paramagnetic species, e.g. the ferryl (iron(IV)) species and the radical, are not exchanged coupled. This led to the conclusion that under the conditions used, peroxy acetic acid oxidized a tyrosine residue (probably Tyr-96) into a tyrosine radical (Tyr*-96), and the iron(III) center of substrate-free P450(cam) to iron(IV).

Generation of oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes by m-CPBA oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C. Evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-cation radical complexes FeIV = O(tmp*)X (X = Cl-, Br-), by Mössbauer and X-ray absorption spectroscopy

The generation of six-coordinate oxoiron (IV) tetramesitylporphyrin pi-caption radical complexes by m-CPBA (meta-chloroperbenzoic acid) oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at - 78 degrees C was investigated. UV-Vis and EPR spectroscopies indicate that the axial ligand present in the ferric starting derivatives is retained in the high-valent iron complexes. Indirect evidence for the formation of six-coordinate oxoiron (IV) tetramesitylporphyrin complexes FeIV = O(tmp*)X (X=Cl-, Br-) by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) in butyronitrile at - 78 degrees C was also obtained by Mössbauer spectroscopy. Direct confirmation of the presence of a halide ion as second axial ligand of iron in these high-valent iron species was obtained by X-ray absorption spectroscopy. The EXAFS spectra of the samples obtained by m-CPBA oxidation of FeX(tmp) (X=Cl-, Br-) were refined using two different coordination models including both four porphyrinato-nitrogens and the axial oxo group. The two models include (model I) or exclude (model II) the axial halogen. The statistical tests indicate the presence of a halide ion as second axial ligand of iron in both derivatives. The refinements led to the following bond distances: FeIV=O(tmp*)Cl(3):Fe-O=1.66(1),Fe-Cl=2.39(2) and Fe-Np=1.99(1) A;FeIV=O(tmp*)Br(4):Fe-O=1.65(1),Fe-Br=2.93(2), Fe-Np=2.02(1) A. The lengthening of the Fe-X(X=Cl-, Br-) distances relative to those occurring in the ferric precursor porphyrins is, most probably, related to the strong trans influence of the oxoiron(IV) fragment present in 3 or 4.

Transport and utilization of rhizoferrin bound iron in Mycobacterium smegmatis

Transport and metabolization of iron bound to the fungal siderophore rhizoferrin was analyzed by transport kinetics, Mössbauer and EPR spectroscopy. Saturation kinetics (vmax = 24.4 pmol/(mg min), K(m) = 64.4 microM) and energy dependence excluded diffusion and provided evidence for a rhizoferrin transport system in M. smegmatis. Based on the spectroscopic techniques indications for intracellular presence of the ferric rhizoferrin complex were found. This feature could be of practical importance in the search of novel drugs for the treatment of mycobacterial infections. EPR and Mössbauer spectroscopy revealed different ferritin mineral cores depending on the siderophore iron source. This finding was interpreted in terms of different protein shells, i.e. two types of ferritins.

ESEEM and Mössbauer studies of the ferriheme model compound bis(3-aminopyrazole)tetraphenylporphyrinatoiron(III) chloride, [TPPFe(NH2PzH)2]Cl

A model heme complex, bis(3-aminopyrazole)tetraphenylporphinatoiron(III) chloride, [TPPFe (NH2PzH)2]Cl, for which the EPR g-values lead to a rhombicity V/delta = 1.2 if gzz is the largest g-value, have been investigated by electron spin echo envelope modulation (ESEEM) and Mössbauer spectroscopies. The ESEEM studies focus on the proton sum frequency peaks at near twice the proton Larmor frequency. Analysis of the distant proton peak (mainly due to the pyrrole-H) at exactly twice the proton Larmor frequency shows conclusively that gzz is aligned along the normal to the porphyrin plane, and thus the electron configuration is (dxy)2(dxz,dyz)3, with gzz > gyy > gxx. This system is thus another violation to Taylor's "proper axis system" rule. The near proton (the alpha-H and N-H of the axial ligands) peaks provide distance information for those protons from the metal. Magnetic Mössbauer studies of the same complex confirm the (dxy)2(dxz,dyz)3 ground state and indicate that, as is the case for cytochrome P450cam, Axx is the largest magnitude A-value, and is negative in sign. Other low-spin iron(III) porphyrinates also have Axx of negative sign, but usually the magnitude is only about half that of Azz, which is always positive in sign.

Mössbauer studies of Escherichia coli biotin synthase: evidence for reversible interconversion between [2Fe-2S](2+) and [4Fe-4S](2+) clusters

The nature and properties of the iron-sulphur (Fe-S) cluster in as-prepared and reduced biotin synthase of Escherichia coli have been investigated by Mössbauer spectroscopy. Our data clearly demonstrate that in the as-prepared sample, the cluster is present as [2Fe-2S](2+) with isomer shift, delta = 0.29 mm/s and quadrupole splitting, DeltaE(Q) = 0.53 mm/s, indicating incomplete cysteinyl-S coordination. Anaerobic reduction by dithionite in the presence of 55% (v/v) glycerol converts this form to [4Fe-4S](2+) (delta = 0.45 mm/s and DeltaE(Q) = 1.11 mm/s) and is accompanied by some destruction to Fe(2+). This cluster conversion is reversible and when exposed to air, the [4Fe-4S](2+) cluster is quantitatively reconverted to the [2Fe-2S](2+) cluster without any further cluster degradation.

Iron-sulfur interconversions in the anaerobic ribonucleotide reductase from Escherichia coli

The anaerobic ribonucleotide reductase from Escherichia coli contains an iron-sulfur cluster which, in the reduced [4Fe-4S](+) form, serves to reduce S-adenosylmethionine and to generate a catalytically essential glycyl radical. The reaction of the reduced cluster with oxygen was studied by UV-visible, EPR, NMR, and Mössbauer spectroscopies. The [4Fe-4S](+) form is shown to be extremely sensitive to oxygen and converted to [4Fe-4S](2+), [3Fe-4S](+/0), and to the stable [2Fe-2S](2+) form. It is remarkable that the oxidized protein retains full activity. This is probably due to the fact that during reduction, required for activity, the iron atoms, from 2Fe and 3Fe clusters, readily reassemble to generate an active [4Fe-4S] center. This property is discussed as a possible protective mechanism of the enzyme during transient exposure to air. Furthermore, the [2Fe-2S] form of the protein can be converted into a [3Fe-4S] form during chromatography on dATP-Sepharose, explaining why previous preparations of the enzyme were shown to contain large amounts of such a 3Fe cluster. This is the first report of a 2Fe to 3Fe cluster conversion.

Mössbauer and electron paramagnetic resonance studies of the cytochrome bf complex

The (57)Fe-enriched cytochrome bf complex has been isolated from hydrocultures of spinach. It has been studied at different redox states by optical, EPR, and Mössbauer spectroscopy. The Mössbauer spectrum of the native complex at 190 K with all iron centers in the oxidized state reveals the presence of four different iron sites: low-spin ferric iron in cytochrome b [with an isomer shift (delta) of 0.20 mm/s, a quadrupole splitting (DeltaE(Q)) of 1.77 mm/s, and a relative area of 40%], low-spin ferric iron of cytochrome f (delta = 0.26 mm/s, DeltaE(Q) = 1.90 mm/s, and a relative area of 20%), and two high-spin ferric iron sites of the Rieske iron-sulfur protein (ISP) with a bis-cysteine and a bis-histidine ligated iron (delta(1) = 0.15 mm/s, DeltaE(Q1) = 0.70 mm/s, and a relative area of 20%, and delta(2) = 0.25 mm/s, DeltaE(Q2) = 0.90 mm/s, and a relative area of 20%, respectively). EPR and magnetic Mössbauer measurements at low temperatures corroborate these results. A crystal-field analysis of the EPR data and of the magnetic Mössbauer data yields estimates for the g-tensors (g(z)(), g(y)(), and g(x)()) of cytochrome b (3.60, 1.35, and 1.1) and of cytochrome f (3.51, 1.69, and 0.9). Addition of ascorbate reduces not only the iron of cytochrome f to the ferrous low-spin state (delta = 0.43 mm/s, DeltaE(Q) = 1.12 mm/s at 4.2 K) but also the bis-histidine coordinated iron of the Rieske 2Fe-2S center to the ferrous high-spin state (delta(2) = 0.73 mm/s, DeltaE(Q2) = -2.95 mm/s at 4.2 K). At this redox step, the Mössbauer parameters of cytochrome b have not changed, indicating that the redox changes of cytochrome f and the Rieske protein did not change the first ligand sphere of the low-spin ferric iron in cytochrome b. Reduction with dithionite further reduces the two hemes of cytochrome b to the ferrous low-spin state (delta = 0.49 mm/s, DeltaE(Q) = 1.08 mm/s at 4.2 K). The spin Hamiltonian analysis of the magnetic Mössbauer spectra at 4.2 K yields hyperfine parameters of the reduced Rieske 2Fe-2S center in the cytochrome bf complex which are very similar to those reported for the Rieske center from Thermus thermophilus [Fee, J. A., Findling, K. L., Yoshida, T., et al. (1984) J. Biol. Chem. 259, 124-133].

Iron coordination geometry in full-length, truncated, and dehydrated forms of human tyrosine hydroxylase studied by Mössbauer and X-ray absorption spectroscopy

Full-length human tyrosine hydroxylase 1 (hTH1) and a truncated enzyme lacking the 150 N-terminal amino acids were expressed in Escherichia coli and purified either with or without (6 x histidine) N-terminal tags. After reconstitution with 57Fe(II), the Mössbauer and X-ray absorption spectra of the enzymes were compared before and after dehydration by lyophilization. Before dehydration, > 90% of the iron in hTH1 had Mössbauer parameters typical for high-spin Fe(II) in a six-coordinate environment [isomer shift delta (1.8-77 K) = 1.26-1.24 mm s-1 and quadrupole splitting delta EQ = 2.68 mm s-1]. After dehydration, the Mössbauer spectrum changed and 63% of the area could be attributed to five-coordinate high-spin Fe(II) (delta = 1.07 mm s-1 and delta EQ = 2.89 mm s-1 at 77 K), whereas 28% of the iron remained as six-coordinate high-spin Fe(II) (delta = 1.24 mm s-1 and delta EQ = 2.87 mm s-1 at 77 K). Similar changes upon dehydration were observed for truncated TH either with or without the histidine tag. After rehydration of hTH1 the spectroscopic changes were completely reversed. The X-ray absorption spectra of hTH1 in solution and in lyophilized form, and for the truncated protein in solution, corroborate the findings derived from the Mössbauer spectra. The pre-edge peak intensity of the protein in solution indicates six-coordination of the iron, while that of the dehydrated protein is typical for a five-coordinate iron center. Thus, the active-site iron can exist in different coordination states, which can be interconverted depending on the hydration state of the protein, indicating the presence or absence of a water molecule as a coordinating ligand to the iron. The present study explains the difference in iron coordination determined by X-ray crystallography, which has shown a five-coordinate iron center in rat TH, and by our recent spectroscopic study of human TH in solution, which showed a six-coordinated iron center.

FhuF, an iron-regulated protein of Escherichia coli with a new type of [2Fe-2S] center

We previously used fhuF as a sensitive reporter gene of the iron status of Escherichia coli. In this report, the fhuF gene was identified as open reading frame f262b at 99.2 min on the genome sequence map of E. coli K-12. The FhuF protein was labeled with a His-tag and then purified to electrophoretic homogeneity. Based on sulfur determinations and Mössbauer and EPR spectroscopy, FhuF was identified as a [2Fe-2S] protein. The g values (gx = 1.886, gy = 1.961, gz = 1.994) and some of the Mössbauer parameters of FhuF obtained [oxidized protein as isolated: delta EQ,4.2K = 0.474 mm s-1; Fe3+ (reduced protein): delta EQ = 0.978 mm s-1] are not typical of common [2Fe-2S] proteins and indicate that FhuF has unusual structural properties. The primary sequence of FhuF does not show any sequence similarities to known [2Fe-2S] proteins. By site-directed mutagenesis, each of the six cysteines of FhuF was replaced by serine. EPR of the six reduced mutant proteins revealed that the terminal cysteine residues 244, 245, 256, and 259 form the [2Fe-2S]Cys4 cluster. Mutants having the Cys-to-Ser replacement at positions 244, 245, 256, or 259 did not complement a fhuF mutant. The motif Cys-Cys-Xaa10-Cys-Xaa2-Cys in FhuF differs considerably from the motif Cys-Xaa2-Cys-Xaa9-15-Cys-Xaa2-Cys found in other [2Fe-2S] proteins. The unusual Cys-Cys terminal group of the cluster may explain the atypical EPR and Mössbauer spectroscopic properties of the FhuF protein; possibly the tetrahedral symmetry at the ferric ion site is distorted. The phenotype of fhuF mutants and the structural features of the FhuF protein suggest that FhuF is involved in the reduction of ferric iron in cytoplasmic ferrioxamine B.

Molecular orbital study of porphyrin–substrate interactions in cytochrome P450 catalysed aromatic hydroxylation of substituted anilines

The reaction mechanism for the primary reaction step of the hydroxylation of 3-fluoro-6-methylaniline, attacked at different positions (oxygen attack across a C-C bond and direct attack at positions para and ortho with respect to the NH(2)-group) catalysed by a high-valent ferryl-oxo porphyrin a(2u)-cation complex with H(3)CS(-) as an axial ligand, has been investigated on the basis of electronic structure calculations in local spin-density approximation. Non-repulsive potential curves are obtained only in cases of direct attack at the para- and ortho-positions with respect to NH(2), but not for epoxide formation. Comparing the potential curves for the hydroxylation at the positions para and ortho to the NH(2)-group, an attack at the para-position is more likely. The relative orientation of the substrate towards the porphyrin is essentially determined by the interaction between the substituents of the substrate and the porphyrin. Consequently, different geometrical orientations of the substrate are obtained for hydroxylation at the para- and ortho-positions. In both cases of direct attack the substrate plane is not parallel to the porphyrin plane. The decisive role of sulphur in the hydroxylation is demonstrated by the participation of the S(3p)-orbitals in all molecular orbitals involved in the reaction.

The prismane protein resolved--Mössbauer investigation of a 4Fe cluster with an unusual mixture of bridging ligands and metal coordinations

The prismane protein of Desulfovibrio vulgaris, in its isolated, its one-electron-reduced and its oxidized states, was the subject of a detailed Mössbauer investigation. Measurements were recorded in the range 0.295-77 K and in the field range 0-6.2 T (parallel and perpendicular to the gamma beam). The paramagnetic parts of the magnetically split Mössbauer spectra were analyzed with the spin-Hamiltonian formalism, including the nuclear Hamiltonian; the diamagnetic parts result from the nuclear Hamiltonian only. The field-dependent spectra at 295 mK and 4.2 K indicate that the paramagnetic part of the isolated protein represents a spin-coupled 4Fe unit with the spin of one Fe site (5/2) oriented antiparallel to the spins of the other three Fe sites (5/2, 5/2 and 2), yielding a total cluster spin, Stot of 9/2. The Mössbauer parameters of the individual Fe sites indicated that this unit represents a 4Fe cluster with an unusual mixture of bridging and terminal ligands and metal coordinations (hybrid cluster). The diamagnetic part of the isolated protein represents an additional 4Fe unit, which, according to its Mössbauer parameters, is a [4Fe2.5+-4S] cubane. The parameter changes upon one-electron oxidation or reduction and the magnetic properties of the two clusters in the three oxidation states of the protein investigated here reveal that the redox behavior of the prismane protein is exclusively related with the hybrid cluster. [structures: see text] These findings are contrary to the former hypothesis of one or two [6Fe-6S] cluster(s) as the prosthetic group of this protein [Hagen, W. R., Pierik, A. J. & Veeger, C. (1989) J. Chem. Soc. Faraday Trans. 185, 4083-4090; Moura, I., Tavares, P., Moura, J. J. G., Ravi, N., Huynh, B. H., Liu, M.-Y. & LeGall, J. (1992) J. Biol. Chem. 287, 4487-4496]. However, they are in full agreement with the crystal structure of the isolated protein, which, concurrent with our Mössbauer investigation, has been solved.

Compound I and Compound II Analogues from Porpholactones

The tetraaza macrocycles 2-oxa-3-oxotetramesitylporphine (|H(2) 1|) and 2-oxa-3-oxotetrakis(2,6-dichlorophenyl)porphine (|H(2) 2|) and the corresponding iron complexes (|Fe(III)(X) 1| and |Fe(III)(X) 2|; X= Cl(-), OH(-), or SO(3)CF(3)(-)) have been synthesized. These macrocycles are derived from porphyrins by transformation of one pyrrole ring to an oxazolone ring. The resulting lactone functionality serves to restrict but not completely block pi-conjugation around the periphery. These complexes thus share properties with both porphyrins and chlorins. The ferric and high-valent iron complexes have been characterized by a variety of spectroscopic techniques. The molecular structure of |Fe(III)(Cl) 2| has been obtained by X-ray crystallography and shows that the structural changes at the macrocycle periphery do not perturb the coordination sphere of iron relative to the corresponding porphyrin complexes. This is illustrated by the observation that Fe-O frequencies in the resonance Raman spectra of the porpholactone analogues of compounds I and II are not substantially different from those of porphyrins and by the axial appearance of the EPR signals of the high-spin ferric complexes. This is consistent with reports that the Fe=O unit of oxidized porphyrins and chlorins is relatively insensitive to alteration of macrocycle symmetry. Nevertheless, probes of properties of the porpholactone macrocycle ((1)H NMR, resonance Raman skeletal modes) show effects of the asymmetry induced by the oxazolone ring. On the basis of (1)H NMR, EPR, Mössbauer, and resonance Raman data, the singly occupied molecular orbital of oxoferryl porpholactone pi-cation radicals correlates with the a(1u) molecular orbital of porphyrins under D(4)(h)() symmetry. Moreover, the paramagnetic properties and the intramolecular exchange interaction of ferryl iron and the porpholactone pi-radical have been characterized by EPR and magnetic Mössbauer measurements and spin-Hamiltonian analyses. The values J(0) = 17 cm(-)(1) and J(0) = 11 cm(-)(1) obtained for the exchange coupling constants of the oxoferryl porpholactone pi-cation radical complexes |Fe(IV)=O 1|(+) and |Fe(IV)=O 2|(+), respectively, are among the lowest found for synthetic compound I analogues.

Iron uptake and intracellular metal transfer in mycobacteria mediated by xenosiderophores

Growth promotion was tested using M. smegmatis wild type strain, an exochelin-deficient mutant, and M. fortuitum employing a broad variety of xenosiderophores including hydroxamates, catecholates and alpha-hydroxy carboxylic acids. The experiments revealed that utilization of siderophore-bound iron is substrate specific suggesting high-affinity siderophore receptor and transport systems. Concentration-dependent uptake of a selected xenosiderophore (fericrocin) in M. smegmatis showed saturation kinetics and uptake was inhibited by respiratory poisons. In situ Mössbauer spectroscopy of ferricrocin uptake in M. smegmatis indicated rapid intracellular reductive removal of the metal excluding intracellular ferricrocin accumulation. The ultimate intracellular iron pool is represented by a compound (delta = 0.43 mm s-1, delta EQ = 1.03 mm s-1) which has also been found in many other microorganisms and does not represent a bacterioferritin, cytochrome or iron-sulfur cluster. By contrast, iron uptake via citrate-a compound exhibiting a very low complex stability constant-involves ligand exchange with mycobactin. Mycobactin has merely a transient role. The ultimate storage compound is an E. coli-type bacterioferritin, in which over 90% of cellular iron is located.

Exchange Coupling in an Isostructural Series of Face-Sharing Bioctahedral Complexes [LM(II)(&mgr;-X)(3)M(II)L]BPh(4) (M = Mn, Fe, Co, Ni, Zn; X = Cl, Br; L = 1,4,7-Trimethyl-1,4,7-triazacyclononane)

The reaction of the divalent metal halides ZnCl(2), ZnBr(2), MnCl(2).4CH(3)CN, MnBr(2), FeCl(2).4CH(3)CN, CoCl(2).4CH(3)CN, CoBr(2), NiCl(2).6H(2)O, and NiBr(2), respectively, with the macrocycle 1,4,7-trimethyl-1,4,7-triazacyclononane (L) (1:1) in anhydrous acetonitrile, acetone, chloroform, or ethanol affords upon additon of NaBPh(4) the isomorphous series of complexes [LM(II)(&mgr;-X)(3)M(II)L]BPh(4): 1, M = Zn, X = Cl; 2, Zn, Br; 3, Mn, Cl; 4, Mn, Br; 5, Fe, Cl; 6, Co, Cl; 7, Co, Br; 8, Ni, Cl; 9, Ni, Br. Six of these complexes have been structurally characterized by single-crystal X-ray crystallography; they crystallize in the triclinic space group P&onemacr; (No. 2) with Z = 4. Crystal data are as follows 1, a = 16.654(1), b = 17.042(1), c = 17.684(1) Å, alpha = 97.30(1), beta = 93.58(1), gamma = 117.46(1) degrees; 3, a = 16.632(8), b = 17.012(8), c = 17.855(5) Å, alpha = 97.16(3), beta = 93.37(3), gamma = 117.24(3) degrees; 5, a = 16.658(3), b = 17.064(3), c = 17.741(4) Å, alpha = 97.32(3), beta = 93.47(3), gamma = 117.36(3) degrees; 6, a = 16.640(3), b = 17.040(3), c = 17.686(4) Å, alpha = 97.39(3), beta = 93.58(3), gamma = 117.39(3) degrees; 8, a = 16.608(3), b = 16.995(3), c = 17.555(3) Å, alpha = 97.36(1), beta = 93.52(1), gamma = 117.52(1) degrees; 9, a = 16.680(3), b = 17.016(2), c = 17.715(3)Å, alpha = 96.99(1), beta = 93.70(1), gamma = 117.42(1) degrees. All complexes consist of a dinuclear, face-sharing bioctahedral monocation with three &mgr;(2)-Cl or &mgr;(2)-Br bridging ligands and two LM fragments and well-separated tetraphenylborate anions (1:1). The cations cocrystallize in two different forms: an enantiomeric form with (lambdalambdalambda) (or (deltadeltadelta)) conformation at both LM fragments and a meso form with an (lambdalambdalambda) conformation at one LM fragment and (deltadeltadelta) at the other (ratio 1:1). From temperature-dependent magnetic susceptibility measurements (2-293 K) it was established that the spins of the unpaired electrons in 3 (d(5)d(5) high spin), 4 (d(5)d(5)), 6 (d(7)d(7) high spin), 7 (d(7)d(7)), 8 (d(8)d(8)), and 9 (d(8)d(8)) are intramolecularly, weakly antiferromagnetically coupled in each case. Surprisingly, the spins order ferromagnetically in 5 (d(6)d(6) high spin). This is in contrast to the previously reported complex [(thf)(3)Fe(II)(&mgr;-Cl)(3)Fe(II)(thf)(3)][SnCl(5)(thf)] (thf = tetrahydrofuran)(5) for which a new analysis of the temperature-dependence of the magnetic susceptibility and of field-dependent Mössbauer spectra establish a weak intramolecular antiferromagnetic coupling. The origin of this difference is analyzed.

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.

CMP-N-acetylneuraminic acid hydroxylase: the first cytosolic Rieske iron-sulphur protein to be described in Eukarya

Electron paramagnetic resonance (EPR) spectroscopy and analysis of the primary structure of the CMP-N-acetylneuraminic acid hydroxylase revealed that this enzyme is the first iron-sulphur protein of the Rieske type to be found in the cytosol of Eukarya. The dithionite-reduced hydroxylase exhibited an EPR signal known to be characteristic for a Rieske iron-sulphur centre (2Fe-2S), the g-values being 1.78, 1.91 and 2.01, respectively. An analysis of the primary structure of the hydroxylase led to the identification of an amino acid sequence, known to be characteristic for Rieske proteins. Furthermore, possible binding sites for cytochrome b5, the substrate CMP-Neu5Ac and a mononuclear iron centre were also identified.

The interaction of Fe(III), adriamycin and daunomycin with nucleotides and DNA and their effects on cell growth of fibroblasts (NIH-3T3)

The interactions of the iron complexes of the anthracycline antitumour drugs daunomycin (DN) and adriamycin (ADM) with the mononucleotide AMP, herring sperm DNA, plasmic pBR322 and immortalized 3T3 fibroblasts were studied. By means of Mössbauer spectroscopy it was demonstrated that DNA is a powerful ferric iron chelator as compared with AMP, which is not able to compete with DN or acetohydroxamic acid for ferric iron. The difference between AMP and DNA is postulated to be based on the chelate effect. The Mössbauer spectra of the ternary Fe-anthracycline-DNA systems differ from Fe-anthracycline binary complexes, indicating rearrangement reactions. Dialysis experiments clearly disclose the formation of a ternary Fe-ADM-pBR322 complex, the topology of which differs substantially from intercalating ADM. The effect of Fe-ADM complexes (3:1) on the growth of immortalized mouse embryonal fibroblasts (NIH-3T3) was studied in comparison with ADM alone. No significant difference on the inhibition of cell growth was noticed, suggesting comparable cytotoxicity for the compounds. In contrast to literature data, no evidence was found for DNA cleavage by ferric ADM at molar ratios as high as 1/100 (ADM/base pair), even if the ternary systems were prepared in the light and in the presence of reducing or oxidizing agents. Based on our observations it seems that the cytotoxicity of both ADM and Fe-ADM oligomer is not based primarily on intercalation or direct interaction with DNA.