Geneviève Chottard

Modification of the structural and redox properties of cytochrome c by heteropolytungstate binding

Complex formation between horse heart ferricytochrome c and large three-dimensional polyanions has been investigated, in order to study the influence of surface electrostatic interactions on the structural and redox properties of cytochrome c. Cytochrome c binds the large heteropolytungstates (NaSb9W21O86)18- and (KAs4W40O140)27- with a 1/1 polyanion/cytochrome c ratio, and the smaller ion (SiW11O39)8- with a 2/1 ratio. Upon complexation, cytochrome c undergoes structural changes that are dependent on the size and charge of the polyanion, and on the pH and ionic strength of the medium. Three different forms of complexed cytochrome c have been characterized by optical and EPR spectroscopies, in the pH range 6.5-8: an N form, close to the native structure, an A form, analogous to cytochrome c in acidic medium, and a novel B form in which the heme pocket is open but the iron remains low-spin. The redox potential of cytochrome c is lowered to 250-220 mV (vs. NHE) in the N form, and to 80 mV in the B form.

Chemical Reactions Between Copper Pigments and Oleoresinous Media

Abstract study demonstrates that resin and fatty acids are able to extract copper(II) ions from verdigris (copper acetate) and verditer (basic copper carbonate). Ligand exchange reactions of basic copper carbonate with fatty acids and resin acids are much slower than is the case with copper acetate. The browning of paint layers is closely correlated with the relative ease of copper extraction; the copper diffuses in the form of fatty acid or resin carboxylic acid complexes. These complexes are formed in the painting layer during grinding of the pigment with binding media containing oleoresin acids, as well as being formed at the interface of this layer and organic upper layers, such as varnishes.

A Pentacoordinated Di-N-carboxamido-dithiolato-O-sulfinato-iron(III) Complex Related to the Metal Site of Nitrile Hydratase.

Postcoordination oxidation by dioxygen of one of the thiolate groups in a pentadentate N(2)S(3) ligand results in an iron(III) complex with two N-carboxamido, two thiolato, and one O-sulfinato ligands (see the CAMERON representation). This novel mixed coordination is similar to that determined for the inactive form of the nitrile hydratase from Rhodococcus sp. N-771, but differs by the O versus S binding of the sulfinato ligand.

The Role of the Dodecamer Subunit in the Dissociation and Reassembly of the Hexagonal Bilayer Structure of Lumbricus terrestris Hemoglobin

The dissociation of the 3500-kDa hexagonal bilayer (HBL) hemoglobin (Hb) of Lumbricus terrestris upon exposure to Gdm salts, urea and the heteropolytungstates [SiWO] (SiW), [NaSbWO] (SbW) and [BaAsWO] (AsW) at neutral pH was followed by gel filtration, SDS-polyacrylamide gel electrophoresis, and scanning transmission electron microscopy. Elution curves were fitted to sums of exponentially modified gaussians to represent the peaks due to undissociated oxyHb, D (200 kDa), T+L (50 kDa), and M (25 kDa) (T = disulfide-bonded trimer of chains a-c, M = chain d, and L = linker chains). OxyHb dissociation decreased in the order Gdm•SCN > Gdm•Cl > urea > Gdm•OAc and AsW > SbW > SiW. Scanning transmission electron microscopy mass mapping of D showed 10-nm particles with masses of 200 kDa, suggesting them to be dodecamers (a+b+c)d. OxyHb dissociations in urea and Gdm•Cl and at alkaline pH could be fitted only as sums of 3 exponentials. The time course of D was bell-shaped, indicating it was an intermediate. Dissociations in SiW and upon conversion to metHb showed only two phases. The kinetic heterogeneity may be due to oxyHb structural heterogeneity. Formation of D was spontaneous during HBL reassembly, which was minimal (≤ 10%) without Group IIA cations. During reassembly, maximal (60%) at 10 mM cation, D occurs at constant levels (15%), implying the dodecamer to be an intermediate.

Molecular Shape, Dissociation, and Oxygen Binding of the Dodecamer Subunit of Lumbricus terrestris Hemoglobin

Small angle x-ray scattering of the 213-kDa dodecamer of Lumbricus terrestris Hb yielded radius of gyration = 3.74 ± 0.01 nm, maximum diameter = 10.59 ± 0.01 nm, and volume = 255 ± 10 nm3, with no difference between the oxy and deoxy states. Sedimentation velocity studies indicate the dodecamer to have a spherical shape and concentration- and Ca2+-dependent equilibria with its constituent subunits, the disulfide-bonded trimer of chains a-c and chain d. Equilibrium sedimentation data were fitted best with a trimer-dodecamer model, ln K4 = 7 (association K in liters3/g3) at 1°C and 4 at 25°C, providing ΔH = −20 kcal/mol and ΔS = 4.4 eu/mol. Oxydodecamer dissociation at pH 8.0, in urea, GdmCl, heteropolytungstate K8[SiW11O39] and of metdodecamer at pH 7, was followed by gel filtration. Elution profiles were fitted with exponentially modified gaussians to represent the three peaks. Two exponentials were necessary to fit all the dissociations except in [SiW11O39]−8. Equilibrium oxygen binding measurements at pH 6.5-8.5, provided P50 = 8.5, 11.5-11.9 and 11.9-13.5 torr, and n50 = 5.2-9.5, 3.2-4.9, and 1.8-2.7 for blood, Hb, and dodecamer, respectively, at pH 7.5, 25°C. P50 was decreased 3- and 2-fold in ~100 mM Ca2+ and Mg2+, respectively, with concomitant but smaller increases in cooperativity.

Characterisation of as-synthesised MOx-MCM-41 (M = Mo, W) mesophases using Raman spectrometry and 29Si MAS NMR

Abstract MCM-41 mesostructured silicas containing molybdenum and tungsten have been prepared in an acidic medium at room temperature. Two procedures were considered: the so-called oxo–peroxo route with low-nuclearity peroxo metal precursors (route A) and the oxo–polyoxo pathway, involving or generating polyoxometalates (route B). The native materials have been characterised by chemical analysis, powder X-ray diffraction, Raman spectrometry and 29 Si MAS NMR. The results show that, in route A, the use of peroxo compounds leads to a distribution of the low-nuclearity metal species throughout the silica-surfactant assembly whereas Keggin type polyoxometalates are generated in route B. The different dispersions of the MO x species (M=Mo, W) on and into the silica matrix and, hence, the formation of MO 3 (route B), previously observed for calcined materials, can be explained on the basis of different specific interactions in these organic–inorganic mesophases.

Resonance Raman studies of hemoglobin complexes with nitric oxide, nitrosobenzene and nitrosomethane: Observation of the metal-ligand vibrations

Summary Characteristic low frequency bands have been observed at 549, 631 and 450 cm−1 in Hb-NO, Hb-CH3NO and Hb-C6H5NO respectively. The assignment of the 549 cm−1 band in Hb-NO to the Fe-NO stretching vibration has been ascertained by isotopic substitution 14N → 15N. Its excitation profile suggests that the Raman intensity of this axial vibration originates in delocalization of the II electrons of the porphyrin ring toward the Fe-NO moiety.

New insights into the catalytic activity of polyoxometalates for oxidation with hydrogen peroxide

Abstract Heteropolyacids (HPA) with the Keggin structure, H 3 [PM 12 O 40 ] (M = Mo, W), used as precursors for the oxidation of organic substrates in homogeneous or biphase systems are degraded in the presence of excess H 2 O 2 . This process has been studied by spectrometric methods (UV spectroscopy, Raman diffusion, …). Aqueous solutions of H 3 [PM 12 O 40 ] treated with an excess of hydrogen peroxide (30 ≤ [H 2 O 2 ]/[M] ≤ 355) lead to the peroxo species [M 2 O 3 (O 2 ) 4 (H 2 O) 2 ] 2- , , and {PO 4 [MO(O 2 ) 2 ] 4 } 3- , , which have been proposed previously and independently as active-oxygen-to-olefin transfer agents in homogeneous systems or under phase transfer reaction conditions. Other precursors with the Keggin moiety, H 5 [BW 12 O 40 ] and H 4 [SiW 12 O 40 ] or with the Dawson structure, H 6 [P 2 W 18 O 62 ], have been considered in order to establish a relationship between the catalytic activity and the products formed in the HPA-H 2 O 2 systems.

Novel tungsten catalysts grafted onto polymeric materials: a comparison with phase transfer catalysis

Summary The reaction of hydrogen peroxide with tungstic acid, “H 2 WO 4 ”, or H 3 [PW 12 O 40 ]aq and orthophosphoric acid has been studied, and the formation of novel oxoperoxotungstophosphate species, [PwxOy] ω- has been demonstrated. Some of them have been isolated and can be grafted onto polymeric supports. They are active oxygen-to-olefin transfer agents, in association with H 2 O 2 ; these novel catalysts are compared with phase transfer systems.

Structure and vibrational spectra of K6[Mo2(μ2-S)(CN)12]·4H2O, a complex with a linear MoSMo bridge

Abstract The structure of potassium μ-thio-bis[hexacyanomolybdate(IV)] tetrahydrate, K6[Mo2(CN)12(μ2-S)]·4H2O, consists of two pentagonal pyramidal Mo(CN)6 moieties sharing an axial S2− ligand with a staggered configuration. The space group is P 1 triclinic with cell dimensions a = 10.201(4), b = 9025(3), c = 9.680(3) A, α = 109.41(3), β = 109.72(3), γ = 104.41(3)°, Dx = 1.92 g cm−3 for Z = 1. The structure was solved by conventional methods to R = 0.032 for 4929 reflections. Each molybdenum atom is coordinated to six cyanide anions and to a S2− ligand so as to produce a structure of approximate D5d symmetry for the entire anion. The most interesting feature chemically of the structure is the entire axial unit NCMoSMoCN which is practically linear with a very short MoS bond (2.1716(2) A). Vibrational and electronic spectra suggest that there is no major stereochemical change for the anion from the solid to solution at room temperature. The distinction between D5h and D5d symmetry groups is not possible from the vibrational analysis. The existence of two identified configurations for the anion in the solid state (C2v or D5d) is in favor of a stereochemical non-rigidity of the anion in solution at ambient temperature.