Pyrolyzed Fe-N-C materials are promising platinum-group-metal free catalysts for protonexchange membrane fuel cell cathodes. However, the detailed structure, oxidation and spin states of their active sites is still undetermined. 57 Fe Mössbauer spectroscopy has identified FeN x moieties as the most active sites, with their fingerprint being a doublet in lowtemperature Mössbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (E QS) for a range of FeN x structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe-N-C catalyst, while further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device and 57 Fe Mössbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeN x moieties in Fe(II) low-spin and Fe(III) high-spin states while a minor fraction of sites could exist in Fe(II) S = 1 state. From the analysis of the 57 Fe Mössbauer spectrum under external magnetic field and the comparison of calculated and measured E QS values, we assign the experimental doublet D1 with mean E QS value around 0.9 mm•s-1 to Fe(III)N 4 C 12 moieties in high spin and the experimental doublet D2 with mean E QS value around 2.3 mm•s-1 to Fe(II)N 4 C 10 moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites while D2 may correspond either to bulk sites that are inaccessible to O 2 or to surface sites that bind O 2 weaker than D1.

Iron-sulfur (Fe-S) cluster-containing proteins are essential components of cells. In eukaryotes, Fe-S clusters are synthesized by the mitochondrial iron-sulfur cluster (ISC) machinery and the cytosolic iron-sulfur assembly (CIA) system. In the mammalian ISC machinery, preassembly of the Fe-S cluster on the scaffold protein (ISCU) involves a cysteine desulfurase complex (NFS1/ISD11) and frataxin (FXN), the protein deficient in Friedreich's ataxia. Here, by comparing the biochemical and spectroscopic properties of quaternary (ISCU/NFS1/ISD11/FXN) and ternary (ISCU/NFS1/ISD11) complexes, we show that FXN stabilizes the quaternary complex and controls iron entry to the complex through activation of cysteine desulfurization. Furthermore, we show for the first time that in the presence of iron and L-cysteine, an [Fe(4)S(4)] cluster is formed within the quaternary complex that can be transferred to mammalian aconitase (mACO2) to generate an active enzyme. In the absence of FXN, although the ternary complex can assemble an Fe-S cluster, the cluster is inefficiently transferred to ACO2. Taken together, these data help to unravel further the Fe-S cluster assembly process and the molecular basis of Friedreich's ataxia.

High-valent oxo-metal complexes are involved in key biochemical processes of selective oxidation and removal of xenobiotics. The catalytic properties of cytochrome P-450 and soluble methane monooxygenase enzymes are associated with oxo species on mononuclear iron haem and diiron non-haem platforms, respectively. Bio-inspired chemical systems that can reproduce the fascinating ability of these enzymes to oxidize the strongest C-H bonds are the focus of intense scrutiny. In this context, the development of highly oxidizing diiron macrocyclic catalysts requires a structural determination of the elusive active species and elucidation of the reaction mechanism. Here we report the preparation of an Fe(IV)(µ-nitrido)Fe(IV) = O tetraphenylporphyrin cation radical species at -90 °C, characterized by ultraviolet-visible, electron paramagnetic resonance and Mössbauer spectroscopies and by electrospray ionization mass spectrometry. This species exhibits a very high activity for oxygen-atom transfer towards alkanes, including methane. These findings provide a foundation on which to develop efficient and clean oxidation processes, in particular transformations of the strongest C-H bonds.

The active site of superoxide reductase SOR consists of an Fe2+ center in an unusual [His4 Cys1] square-pyramidal geometry. It specifically reduces superoxide to produce H2O2. Here, we have reacted the SOR from Desulfoarculus baarsii directly with H2O2. We have found that its active site can transiently stabilize an Fe3+-peroxo species that we have spectroscopically characterized by resonance Raman. The mutation of the strictly conserved Glu47 into alanine results in a stabilization of this Fe3+-peroxo species, when compared to the wild-type form. These data support the hypothesis that the reaction of SOR proceeds through such a Fe3+-peroxo intermediate. This also suggests that Glu47 might serve to help H2O2 release during the reaction with superoxide.

The zinc K-edge X-ray absorption spectra of the Fur (ferric uptake regulation) protein isolated from Escherichia coli have been analyzed in frozen solution to determine details of the zinc coordination. The spectra of apoFur and of the cobalt-substituted protein have been analyzed and compared in order to see the influence of the cobalt incorporation on the geometry of the zinc site. EXAFS analysis gave for both samples (apoFur and CoFur) a tetrahedral environment for the zinc atom with two sulfur donor ligands at a distance of 2.3 A from the zinc and two N/O donor ligands at 2.0 A. The two sulfur donor ligands are probably two of the four cysteines present in each Fur monomer and could be Cys92 and Cys95, which are known from mutagenesis studies to be essential for Fur activity [Coy, M., Doyle, C., Besser, J., and Neilands, J. B. (1994) BioMetals 7, 292-298]. The distances obtained from our fits were always too short to be compatible with penta or hexa coordination. The typical pattern observed for the Fourier transform of the EXAFS oscillations suggests the presence of at least one imidazole ligand. The XANES of these two forms of the protein are similar but significantly different. This suggests a change of the conformation of the zinc site upon cobalt incorporation. The present study provides the first unambiguous evidence for the presence of a structural zinc site in the Fur protein from Escherichia coli.

New asymmetrical ligands (H 2 L) have been synthesized to provide both a bridging and a terminal phenolate to a pair of iron ions in order to mimic the binding of a single terminal tyrosinate at the diiron center of the purple acid phosphatases. H 2 L1 is 2-[(bis(2-pyridylmethyl)amino)methyl]-6-[((2-pyridylmethyl)(2-phenol)amino)methyl]-4-methylphenol and H 2 L′1 and H 2 L2 are obtained by replacing the 2-phenol group by the 5-nitro-2-phenol and the 6-methyl-2-phenol residues, respectively. A series of mixed valence diiron complexes [Fe II Fe III L(X) 2 ](Y) have been obtained where (X) 2 is the dianion of m-phenylenedipropionate or (H 2 PO 4 ) 2 and Y )Diferric complexes have been obtained also either by direct synthesis or by iodine oxidation of the mixed valence precursor (L ) L1, 3a (X) 2 ) mpdp, Y ) BPh 4 , 3d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 ; L ) L2, 4d: (X) 2 ) (H 2 PO 4 ) 2 , Y ) PF 6 . Complex 1a [Fe II Fe III L(mpdp)](BPh 4 ) has been characterized by X-ray diffraction techniques. 1a crystallizes in the monoclinic space group P21/a with the following unit cell parameters: a ) 22.038 (9) Å, b ) 16.195 (8) Å, c ) 16.536 (7) Å, β ) 97.26 (1)°, Z ) 4. The significant differences in the Fe-O bond lengths indicate that the metal centers are ordered. The complexes have been studied by electronic spectral, resonance Raman, magnetic susceptibility, Mo ¨ssbauer, NMR, and electrochemical techniques. Mo ¨ssbauer and NMR spectroscopies concur to probe that the valences of the mixed valence compounds are trapped in solution as well as in the solid state at room temperature. The electronic spectrum of the mixed-valence compounds are dominated by a charge transfer transition in the 400-600 nm domain which moves to the 550-660 nm range upon oxidation to the diferric state. In addition they exhibit a weak and broad intervalence transition close to 1100 nm. Electrochemical studies show that the systems exist in the three redox states Fe

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SummaryBacteria adapt to elevated levels of Reactive Oxygen Species (ROS) by increasing the expression of defence and repair proteins, which is regulated by ROS responsive transcription factors. In Bacillus subtilis the zinc protein PerR, a peroxide sensor that binds DNA in the presence of a regulatory metal Mn 2+ or Fe 2+, mediates the adaptive response to H2O2. This study presents the first crystal structure of apoPerR-Zn which shows that all four cysteine residues of the protein are involved in zinc co-ordination. The Zn(Cys)4 site locks the dimerization domain and stabilizes the dimer. Sequence alignment of PerR-like proteins supports that this structural site may constitute a distinctive feature of this class of peroxide stress regulators.