Showing papers by "Jean-Marc Latour published in 2002"

Direct inhibition by nitric oxide of the transcriptional ferric uptake regulation protein via nitrosylation of the iron

Abstract: Ferric uptake regulation protein (Fur) is a bacterial global regulator that uses iron as a cofactor to bind to specific DNA sequences. The function of Fur is not limited to iron homeostasis. A wide variety of genes involved in various mechanisms such as oxidative and acid stresses are under Fur control. Flavohemoglobin (Hmp) is an NO-detoxifying enzyme induced by NO and nitrosothiol compounds. Fur recently was found to regulate hmp in Salmonella typhimurium, and in Escherichia coli, the iron-chelating agent 2,2'-dipyridyl induces hmp expression. We now establish direct inhibition of E. coli Fur activity by NO. By using chromosomal Fur-regulated lacZ reporter fusion in E. coli, Fur activity is switched off by NO at micromolar concentration. In vitro Fur DNA-binding activity, as measured by protection of restriction site in aerobactin promoter, is directly sensitive to NO. NO reacts with Fe(II) in purified FeFur protein to form a S = 12 low-spin FeFur-NO complex with a g = 2.03 EPR signal. Appearance of the same EPR signal in NO-treated cells links nitrosylation of the iron with Fur inhibition. The nitrosylated Fur protein is still a dimer and is stable in anaerobiosis but slowly decays in air. This inhibition probably arises from a conformational switch, leading to an inactive dimeric protein. These data establish a link between control of iron metabolism and the response to NO effects.

Structural and density functional studies of uranium(III) and lanthanum(III) complexes with a neutral tripodal N-donor ligand suggesting the presence of a U-N back-bonding interaction.

Abstract: The crystal structures of two trisiodide octacoordinated uranium(III) complexes of tris[(2-pyrazinyl)methyl]amine (tpza), which differ only by the ligand occupying the eighth coordination site (thf or MeCN), and of their lanthanum(III) analogues have been determined. In the acetonitrile adducts the M-N(pyrazine) distances are very similar for U(III) and La(III), while the U-N(acetonitrile) distance is 0.05 A shorter than the La-N(acetonitrile) distance. In the [M(tpza)I(3)(thf)] complexes in which the monodentate acetonitrile ligand, a weak pi-acceptor ligand, is replaced by a thf molecule, a sigma-donor only, the mean value of the distance U-N(pyrazine) is 0.05 A shorter than the mean value of the La-N(pyrazine) distance. Since we are comparing isostructural compounds of ions with very similar ionic radii, these differences indicate the presence of a stronger M-N interaction in the U(III) complexes and therefore suggest the presence of a covalent contribution to the U-N bonding. The selectivity of the tpza ligand toward U(III) complexation (with respect to that of La(III)) in the presence of sigma-donor-only ligands has been quantified by the value of K(U(tpza))/K(La(tpza)) measured to be 3.3 +/- 0.5. The analysis of the metal-N-donor ligand bonding was carried out by a quasi-relativistic density functional theory study on small model compounds, of formula I(3)M-L (M = La, Nd, U; L = acetonitrile, pyrazine) and I(3)M-(pyrazine)(3) (M = La, U). The structural data obtained from geometry optimizations on these systems reproduce experimental trends, i.e., a decrease in the M-N distance from La to U, combined with an increase of the C-N distance in the acetonitrile derivatives. A detailed orbital analysis carried out on the resulting optimized complexes did not reveal any orbital interaction between the trivalent lanthanide cations (Ln(3+)) and the N-donor ligands. In contrast, a back-donation electron transfer from 5f U(3+) orbitals to the pi* virtual orbital of the ligand was observed for both acetonitrile and pyrazine. Evaluation of the total bonding energy between the MI(3) and L fragments shows that this orbital interaction leads to a stabilization of the uranium(III) system compared to the lanthanide species.

A Dinuclear Manganese(II) Complex with the {Mn2(μ-O2CCH3)3}+ Core: Synthesis, Structure, Characterization, Electroinduced Transformation, and Catalase-like Activity

Abstract: Reactions of MnII(PF6)2 and MnII(O2CCH3)2·4H2O with the tridentate facially capping ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in ethanol solutions afforded the mononuclear [MnII(bpea)](PF6)2 (1) and the new binuclear [Mn2II,II(μ-O2CCH3)3(bpea)2](PF6) (2) manganese(II) compounds, respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Complex 1 crystallizes in the monoclinic system, space group P21/n, with a = 11.9288(7) A, b = 22.5424(13) A, c =13.0773(7) A, α = 90°, β = 100.5780(10°), γ = 90°, and Z = 4. Crystals of complex 2 are orthorhombic, space group C2221, with a = 12.5686(16) A, b = 14.4059(16) A, c = 22.515(3) A, α = 90°, β = 90°, γ = 90°, and Z = 4. The three acetates bridge the two Mn(II) centers in a μ1,3 syn−syn mode, with a Mn−Mn separation of 3.915 A. A detailed study of the electrochemical behavior of 1 and 2 in CH3CN medium has been made. Successive controlled potential oxidations at 0.6 and 0.9 V vs Ag/Ag+ for a 10 mM solution of 2 allowed the selective an...

Iron carbonyl, nitrosyl, and nitro complexes of a tetrapodal pentadentate amine ligand: synthesis, electronic structure, and nitrite reductase-like reactivity.

Abstract: The tetrapodal pentaamine 2,6-C5H3N[CMe(CH2NH2)2]2 (pyN4, 1) forms a series of octahedral iron(II) complexes of general formula [Fe(L)(1)]Xn with a variety of small-molecule ligands L at the sixth coordination site (L = X = Br, n = 1 (2); L = CO, X = Br, n = 2 (3); L = NO, X = Br, n = 2 (4); L = NO+, X = Br, n = 3 (5); L = NO2-, X = Br, n = 1 (6)). The bromo complex, which is remarkably stable towards hydrolysis and oxidation, serves as the precursor for all other complexes, which may be obtained by ligand exchange, employing CO, NO, NOBF4, and NaNO2, respectively. All complexes have been fully characterised, including solid-state structures in most cases. Attempts to obtain single crystals of 6 produced the dinuclear complex [Fe2[mu 2-(eta 1-N: eta 1-O)-NO2](1)2]Br2PF6 (7), whose bridging NO2- unit, which is unsupported by bracketing ligands, is without precedent in the coordination chemistry of iron. Compound 2 has a high-spin electronic configuration with four unpaired electrons (S = 2), while the carbonyl complex 3 is low-spin (S = 0), as are complexes 5, 6 and 7 (S = 0 in all cases); the 19 valence electron nitrosyl complex 4 has S = 1/2. Complex 4 and its oxidation product, 5 ([Fe(NO)]7 and [Fe(NO)]6 in the Feltham-Enemark notation) may be interconverted by a one-electron redox process. Both complexes are also accessible from the mononuclear nitro complex 6: Treatment with acid produces the 18 valence electron NO+ complex 5, whereas hydrolysis in the absence of added protons (in methanolic solution) gives the 19 valence electron NO. complex 4, with formal reduction of the NO2- ligand. This reactivity mimicks the function of certain heme-dependent nitrite reductases. Density functional calculations for complexes 3, 4 and 5 provide a description of the electronic structures and are compatible with the formulation of iron(II) in all cases; this is derived from the careful analysis of the combined IR, ESR and Mossbauer spectroscopic data, as well as structural parameters.

Hydrogenperoxo-[(bztpen)Fe(OOH)]2+ and Its Deprotonation Product Peroxo-[(bztpen)Fe(O2)]+, Studied by EPR and Mössbauer Spectroscopy − Implications for the Electronic Structures of Peroxo Model Complexes

Abstract: The purple hydrogenperoxo species [Fe(bztpen)(OOH)]2+ (1) can be generated in methanol solution by treatment of [Fe(bztpen)Cl]3+ with a large excess of hydrogen peroxide [A. Hazell, C. J. McKenzie, L. P. Nielsen, S. Schindler, M. Weitzer, J. Chem. Soc., Dalton Trans.2002, 310−317]. Addition of 30 equivalents of triethylamine to a solution of [Fe(bztpen)(OOH)]2+ gives the [Fe(bztpen)(O2)]+ (2) species, in which it has been proposed that the peroxo ligand is coordinated in the η2 mode. We have synthesized both 100% 57Fe-enriched non-heme FeIII peroxo species and studied them by EPR and Mossbauer spectroscopy. Species 1 displays a typical S = 1/2 low-spin ferric EPR spectrum, the g values of which have been analysed in terms of the Griffith model [J. S. Griffith, Proc. R. Soc. London, A1956, 234, 23−36]. We have thus obtained insight into the electronic structure of 1. The EPR spectrum of 2 is characteristic of a high-spin FeIII species in a nearly axial ligand field. We have determined all Mossbauer parameters for both complexes by a numerical treatment of applied field Mossbauer data. Species 1 exhibits an isomer shift δ/Fe = 0.16 mm/s and a quadrupole splitting ΔEQ = −2.08 mm/s (T = 4.2 K). In the case of 2, we obtain an isomer shift δ/Fe = 0.63 mm/s and a quadrupole splitting ΔEQ = 1.12 mm/s (T = 4.2 K). These values are close to those published very recently by Bill et al. for [Fe(trispicMeen)(OOH)]2+ (3) and [Fe(trispicMeen)(η2−OO)]+ (4) species, where bztpen and trispicMeen ligands differ by a noncoordinating group [A. J. Simaan, F. Banse, J.-J. Girerd, K. Wieghardt, E. Bill, Inorg. Chem.2001, 40, 6538−6540]. We have also used the extension of the Griffith model developed by Lang et al. for Mossbauer spectroscopy [W. T. Oosterhuis, G. Lang, Phys. Rev. 1969, 178, 439−456] to evaluate the 57Fe hyperfine tensor of 1 and reproduce the experimental data. On comparison with the Mossbauer data available for η2-peroxo FeIII complexes, our Mossbauer study of 2 agrees well with a high-spin FeIII side-on peroxo complex. Very characteristic δ/Fe and ΔEQ ranges for hydrogenperoxo and side-on peroxo FeIII species are now available, which should aid in the detection of comparable species formed in biological systems. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

A Mössbauer Study of [Fe(edta)(O2)]3−Agrees with a High-Spin FeIII Peroxo Complex

Abstract: An isotopically enriched [57Fe(edta)(O2)]3− complex, formed by the addition of H2O2 to a 57FeIII-edta (edta = ethylenediaminetetraacetic acid) complex at elevated pH (pH = 11.3), was studied by Mossbauer spectroscopy. All Mossbauer parameters were determined for this nonheme high-spin ferric peroxo complex with an η2-FeO2 (side-on) arrangement by a numerical treatment of applied field Mossbauer data. The peroxo complex exhibits an isomer shift δ/Fe = 0.65(1) mm s−1 and a quadrupole splitting ΔEQ = +0.72(2) mm s−1 (T = 4.2 K). The isomer-shift value is very similar to the value of δ/Fe = 0.61 mm s−1 published for the side-on peroxo complex [FeIII(N4Py)(η2-OO)]+ (Py = pyridine) [V. Vrajmasu, E. Munck, R. Ho, L. Que, Jr., G. Roefles, B. L. Feringa, J. Inorg. Biochem.2001, 86, 472]. The set of Mossbauer parameters obtained herein is expected to aid the characterization of analogous species that may appear during dioxygen activation by nonheme mononuclear iron enzymes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Magnetization studies of the active and fluoride-inhibited derivatives of the reduced catalase of Lactobacillus plantarum: toward a general picture of the anion-inhibited and active forms of the reduced dimanganese catalases.

Abstract: The magnetic properties of the reduced catalase from Lactobacillus plantarum have been studied for the active enzyme and its fluoride complex through variable field/variable temperature magnetization measurements. The magnetic exchange interaction deduced from these experiments [fluoride complex: - J=1.3(1) cm(-1); active enzyme: - J=5.6(5) cm(-1); H=-2 J S(1) S(2)] are similar to those presently obtained in a re-analysis of the data for the corresponding forms of the Thermus thermophilus enzyme (previously published in 1997, Angew Chem Int Ed Engl 36:1626-1628): phosphate complex: - J=2.1(2) cm(-1); active enzyme - J=5.0(3) cm(-1). These results concur to a unified picture for the two enzymes, consistent with the presence of a hydroxide bridge in the reduced active catalases and its replacement by an aqua bridge in the anion-inhibited enzymes as the main mediators of the magnetic exchange.