CONFORMATIONAL CHANGES IN HEME PROTEINS AND MODEL COMPOUNDS

On a mesuré des spectres Mössbauer de hémoglobine, myoglobine et quelques composés modèles de type hème à différentes températures. Chaque échantillon a donné un doublet quadrupolaire dont la largeur des raies et l'éclatement quadrupolaire dépend de la température. On a comparé les spectres des protéines naturelles avec les spectres d'un des composés modèles étudié en détail avec la spectroscopie Mössbauer et on conclut qu'on peut attribuer la variation en fonction de la température à la relaxation de l'oxygène parmi des conformations différentes.Zero-field Mössbauer spectra of oxygenated hemoglobin, myoglobin and some of their model compounds were recorded at various températures. Each sample gave rise to quadrupole doublet with temperature-dependent linewidth and quadrupole splitting. We have compared the native protein spectra with the spectra from one of the model compounds extensively studied by Mössbauer spectroscopy, and we conclude that oxygen relaxation between conformational states is responsible for the temperature dependence in both cases

MÖSSBAUER AND ESR STUDIES IN LOW SYMMETRY IRON COMPLEXES

Quelques complexes de transport de fer naturels ainsi que quelques modèles de complexes minéraux ont fourni des résultats Mössbauer et RPE qui sont bien expliqués par un hamiltonien de spin électronique de symétrie rhomboïdale et non de symétrie trigonale comme le suggèrent les déterminations structurales par rayons X. Un terme d'ordre 4 est important dans le lissage des résultats expérimentaux de RPE et de spectroscopie Mössbauer. Les paramètres de l'hamiltonien de spin sont déterminés à partir des données expérimentales pour la mycobactine P, l'entérobactine et quelques hydroxamates minéraux.Some naturally occurring iron transport complexes and some inorganic model complexes have given Mössbauer and ESR results which are well explained by a spin Hamiltonian model for the electronic state with rhombic symmetry, and not the trigonal symmetry suggested by X-ray structure determinations. A fourth order term is important in fitting the experimental ESR and ME results. The spin Hamiltonian parameters are determined from the experimental data for mycobactin P, enterobactin and some inorganic hydroxamates

Preparation, Crystal Structure, and Physical Properties of a Pyrogallol-bridged Vanadium(III) Complex

The structure, n.m.r. spectrum, and magnetic properties of the vanadium(III) dimer [(acac)4V2{µ-OC6H3(OH)2}2] formed from [VO(acac)2] and an excess of pyrogallol are described

MÖSSBAUER SPECTROSCOPY OF COMPOUND ES : SUPPORT FOR A Fe IV STATE

Le composé ES, produit par la réaction de cytochrome c peroxydase et éthyl peroxide, a été étudié par spectroscopie Mössbauer à température et champ magnétique variable. A 4,2 K le déplacement isomérique est 0,05 mm/s (Fe) ; l'éclatement quadrupolaire est indépendant de la température à 1,55 mm/s, et les largeurs des raies sont égales, approximativement, à la valeur naturelle. L'application d'un champ magnétique à 4,2 K produit un spectre qui est approximativement celui d'une substance diamagnétique, mais on trouve que le champ magnétique effectif dépend de la température. L'application de la méthode de moindres carrés indique l'existence d'un GCE axial avec Vzz > 0. Le champ magnétique interne est négatif et grand seulement en les directions transversales. Le champ interne est en accord avec une configuration d4 S = 1 avec ζ = 400 cm-1 et avec |xz > et | yz > supérieur à | xy > par 6 ζ, environ. La susceptibilité prédite est en accord avec les valeurs publiées. Le modèle prédit que Vzz > 0 ; mais la grandeur de ΔE observée indique que sa valeur est diminuée par la charge dans les orbitals de liaison. Il n'y a pas de caractéristique dans le spectre Mössbauer qui correspond au radical libre observable par RPE donc on conclut que ce dernier est loin de Fe.Compound ES, formed by reacting cytochrome c peroxidase with ethyl peroxide, has been examined by Mössbauer spectroscopy over a range of temperature and applied magnetic field. At 4.2 K the isomer shift is 0.05 mm/s (Fe) ; quadrupole splitting is temperature independent at 1.55 mm/s, and the linewidth approximates the natural value. Application of a 4 T field at 4.2 K produces a spectrum approximating that of a diamagnet, but the effective magnetic field is found to be temperature dependent. Least squares fittings indicate axial efg and magnetic interactions, with Vzz > 0 and a négative internai field which is large only in transverse directions. The internal field is consistent with a d4, S = 1 configuration with ζ = 400 cm-1 having |xz > and |yz > lying about 6 ζ above |xy >. The predicted susceptibility is in reasonable agreement with published values. The model correctly predicts Vzz > 0 ; the size of the observed ΔE, however, indicates reduction by charge in bonding orbitals. There is no observable Mössbauer spectrum feature corresponding to the EPR-visible free radical, implying that it is remote from the iron

Magnetic Properties of Tunicate Blood Cells. I. Ascidia nigra

The magnetic properties of intact and freeze-dried blood cells of the tunicate Ascidia nigra and of model vanadium(III) and (IV) compounds as polycrystalline solids and in aqueous solution have been measured up to 50 kOe with a SQUID susceptometer. Corrections for the samples\u27 diamagnetism were extracted from the temperature dependence of the data without any further assumptions. For vanadium(IV), measured values of the magnetic moment at different values of the applied magnetic field over the temperature range 2–100 K obey a Brillouin function with spin 1/2. For vanadium(III), the magnetic moment data did not obey a Brillouin function and were analyzed in terms of a spin Hamiltonian with S = 1. Measurements on both whole and freeze-dried blood samples give consistent results with vanadium(III) the predominant species. These results are discussed in terms of the mechanisms of vanadium accumulation and the use of vanadium oxidation states as criteria of ascidian taxonomy

Magnetic Properties of Tunicate Blood Cells. II. \u3ci\u3eAscidia ceratodes\u3c/i\u3e

The magnetic properties of intact blood cells of the tunicate Ascidia ceratodes have been measured up to 50 kOe with a SQUID susceptometer. Analysis of total metal contents by plasma emission spectroscopy and V(IV) content by epr indicates that approximately 5% of the accumulated vanadium is +4 vanadyl ion. Measured values of the magnetic moment MP at different values of the applied magnetic field H over the temperature range T = 2–100 K depend on the magnitude of the field indicating magnetic anisotropy of the ground state. The slope of the MP vs. H/T curve at high temperature is significantly higher than expected from electron spin S = 1 per vanadium(III) ion. The model that fits these data best is a dimer with one V(III) S = 1 ion ferromagnetically coupled to a second V(III) S = 1 ion, with spin-coupling constant J = 3.5 cm−1, and 5% of the total vanadium content in the form of a V(IV) S = 1/2 ion. Since vanadium in A. ceratodes is known to reside in at least three different types of blood cell, the excellent fit indicates that the metal is stored predominantly as a dimer regardless of blood cell type. Ferromagnetic coupling implies that the two vanadium ions in the dimer are connected by an unprotonated μ-oxo bridge

Alternative Spin States in Synthetic Analogues of Biological Clusters: Spin-Quartet Ground States and Structures of [Fe4S4(Sph)4]3- and [Fe4Se4(Sph)4]3- As Theirtetramethylammonium Salts

The compounds (Me4N)3[Fe4X4(SPh)4]·2MeCN [X = S (1), Se (2)] were prepared in good yields by chemical reduction of the corresponding dianion cluster salts and were thoroughly characterized as part of an investigation of alternative spin states in cubane-type clusters having the [Fe(μ3-X)4]+ core. Clusters with this core unit are found in reduced ferredoxins and other Fe-S proteins and enzymes. The two compounds obey the Curie-Weiss law over the temperature ranges 2.1-15 K (1) and 7.0-50 K (2) with Curie constants of 1.891 (1) and 1.883 (2) emu K/G, consistent with a S = 3/2 ground state. Magnetization behaviors between 1.8 and 100 K and at fields up to 50 kOe were successfully analyzed in terms of a spin Hamiltonian with 5 = 3/2. Mössbauer spectra revealed isomer shifts similar to those of other reduced clusters and independent of chalcogenide atom X, very small magnetic splittings in large applied magnetic fields, and negative hyperfine fields at all 57Fe sites. Compound 1 crystallizesin monoclinic space group C2/c with a = 22.868 (6) Å,b= 11.211 (2) Å, c = 20.387 (4) Å, β = 90.08 (2)°, and Z = 4. Compound 2 was obtained in orthorhombic space group Fddl with a = 39.386 (8) Å, b = 23.991 (5) Å, c = 11.471 (2) Å, and Z = 8. Refinement of compounds 1/2 with use of 2839/1957 unique data converged at R = 4.44%/3.32%. Clusters 1 and 2 have very similar structures. Core Fe-X bond lengths may be organized into a set of four long plus eight short with mean values (Å) of 2.344 (6) + 2.287 (19) for 1 and 2.488 (4) + 2.403 (12) for 2, with the long bonds approximately perpendicular to the direction of a C2 axis crystallographically imposed in each case. The cores have tetragonally elongated (D2d) configurations with the elongation along the direction of an idealized 4 axis. The magnetic and spectroscopic results, together with crystallographic definition of the clusters, establish beyond question that the reduced clusters [Fe4X4(SPh)4]3-. (X = S, Se) can stabilize electronic configurations with spin-quartet ground states. Examples of “gnonstandard” biological clusters with S \u3e 1/2 are cited, including the S = 3/2 cluster of the Fe protein of Azotobacter vinelandii nitrogenase. All aspects of the present work support the published spin-state assignment of this cluster. While it may be significant that 1 and 2 have elongated tetragonal cores and S = 3/2 ground states, examination of all results for reduced clusters does not yet afford a clear spin-state-structure correlation