X-band Electron Paramagnetic Resonance Spectra Research Articles

Crystal structure and properties of a new five-coordinate manganese superoxide dismutase mimic

A new five-coordinate manganese(II) complex Mn(NTB)(C 8H 4O 4)·DMF·0.5CH 3OH·0.5H 2O, where NTB is tris(2-benzimidazolyl-methyl) amine and C 8H 4O 4 2− is the dianion of terephthalic acid, was prepared and characterized by X-ray crystallography. It crystallizes in the tetragonal system, space group l4 1/ a with a = 28.063(1), c = 22.8680(1) A ̊ , Z = 16 and V = 18009(9) A ̊ 3 . The structure was solved by direct methods and refined by full-matrix least-squares(on F) to residual R( wR) values of 0.0785 (0.076). The manganese in the complex adopts a distorted trigonal-bipyramidal coordination geometry with an N 4O ligand donor set and bears structural similarities to the active site of Mn-superoxide dismutase. The X-band electron paramagnetic resonance spectrum of the complex exhibits a six-line manganese hyperfine pattern with g = 2.005, A = 94 G. The electrochemical properties of the complex were studied in DMF by cyclic voltammetry. The complex can catalyse the dismutation of superoxide (O 2 −) effectively in the riboflavin-methionine-nitro blue tetrazolium assay.

An EPR and pulsed ENDOR study of the structure of tyrosine Z ⋅ in Tris-treated photosystem II

Electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectra of oxidized tyrosines Z and D (Y Z ⋅ and Y D ⋅, respectively) in Mn-depleted photosystem II have been studied in the pH range from 5.5 to 8.0. Above pH 7.0 the hyperfine couplings of 3,5-ring protons of both tyrosines were similar. The hyperfine constants of the strongly-coupled β-methylene protons were slightly different, 28.4 MHz in Y D ⋅ vs. 33.6 MHz in Y Z ⋅. This difference was explained by different orientations of the β-methylene group with respect to the aromatic ring plane. From the similarity of the hyperfine couplings it was concluded that at high pH Y Z ⋅, same as Y D ⋅, is a neutral radical. The EPR and ENDOR spectra of Y D ⋅ did not change in the studied pH range. However, the light-minus-dark spectra below pH 6.0 were considerably different from those at higher pH. To explain these spectra, two models were discussed, with Y Z supposed to be a cation-radical or a neutral radical with spin densities close to those known in literature. Assuming Y Z to be a cation radical with a spin density distribution close to that known for phenolic cation radicals, we were able to explain the shape of the ENDOR, S-(2GHz) and X-band EPR spectra observed at pH 5.5. The deuteron ENDOR spectra of Y Z ⋅ in D 2O-washed sample were similar both at low and high pH, suggesting the change in hydrogen bondings may be orientational without change in their distance. The change of the Y Z ⋅ structure at low pH was suggested to be caused by the protonation of some amino acid residue (possibly, histidine 190 of the D1 protein) in the vicinity of Y Z.

An electron paramagnetic resonance study on the mechanism-based inactivation of adenosylcobalamin-dependent diol dehydrase by glycerol and other substrates

Adenosylcobalamin-dependent diol dehydrase undergoes mechanism-based inactivation by glycerol or other substrates during catalysis. X-band electron paramagnetic resonance spectra of holoenzyme were measured at −130°C after reaction with such substrates. After short time of incubation, broad signals assigned to low-spin Co(II) of cob(II)alamin and doublet signals assigned to an organic radical intermediate derived from each substrate were observed with 1,2-propanediol, 1,2-ethanediol, glycerol and meso-2,3-butanediol with the magnitude of their exchange interaction ( J-value) decreasing in this order. A substrate with the smaller magnitude of exchange interaction between low-spin Co(II) and an organic radical intermediate seems to be an efficient mechanism-based inactivator. Since the magnitude of exchange interaction decreases with the distance between radical species in a radical pair, these results suggest that a stabilizing effect of holoenzyme on radical intermediates during reactions decreases with the distance between Co(II) and a radical.

Slow motion electron spin resonance line shapes of lyotropic liquid crystals in hexagonal phase

In this paper the slow motion electron spin resonance (ESR) line shape theory is extended to hexagonal mesophases. The stochastic Liouville equation is applied to the dynamic description of both local molecular reorientation and azimuthal surface translational diffusion of the lipid around the cylinder axis. The established ESR line shape models allow a separation of contributions to the electron spin relaxation from the two motions, if macroscopically aligned hexagonal spectra are simulated. Such simulations can yield, not only the dynamic parameters such as local ordering parameter, local motional rate, and azimuthal surface diffusion coefficient, but also a disorder parameter accounting for residual cylinder disorder and the disorder effects due to finite cylinder length and curved surface along the cylinder axis. To test these models, they are applied to the analyses of X-band ESR spectra of the hexagonal phase of the sodium dodecyl sulphate (SDS)/decanol/water ternary system. The obtained surface diffusion coefficient and disorder parameter are in good agreement with previously reported values in a deuterium nuclear magnetic resonance (NMR) study. The local dynamic properties, which are not readily available by NMR method, are compared with those obtained for the micellar phase of the SDS/water binary system. Such a comparison reveals that while the local orderings are similar, the local dynamics is much slower in the hexagonal phase than in the micellar phase.

Diluted and non-diluted ferric ions in borate glasses studied by electron paramagnetic resonance

Electron paramagnetic resonance (EPR) spectra of lithium borate glass (1 - x)(0.63B 2O 3 · 0.37Li 2O) · xFe 2O 3, with x varying from 0.001 to 0.1, were measured at different microwave frequencies and temperatures. For low Fe 3+ concentrations (Fe 2O 3 molar contents from 0.001 to 0.01), X-band EPR spectra, consisting of a g ef = 4.3 peak accompanied by a shoulder continuing down to g ef = 9.7, are computer simulated on the basis of a ‘rhombic’ spin-Hamiltonian with Zeeman and fine-structure terms. A good fit to the experimental spectra for various Fe 2O 3 contents is observed with the same values of the spin-Hamiltonian parameters and assuming a Lorentzian lineshape and a linewidth increasing linealry with the concentration of Fe 3+ ions. It is concluded that the spectrum is due to diluted Fe 3+ ions in a relatively strong crystal field of orthorhombic symmetry, with largely distributed fine-structure parameters. From the concentration dependence of the line width, by extending to glasses a theoretical EPR linewidth expression derived for polycrystalline systems, the minimum distance between diluted Fe 3+ ions is estimated as 4.9 Å. A diluted state of Fe 3+ ions in the glass network in this range is also confirmed by the temperature dependence of the g ef = 4.3 resonance which follows a Curie law. For intermediate concentrations of Fe 3+ ions (Fe 2O 3 molar contents from 0.01 to 0.1), the width of the g ef = 4.3 line is proportional to the square root of concentration, still indicating dipolar interactions. On the other hand, the microwave frequency dependence of a broad g ef ≈ 2 line, which coexists at these concentrations with the g ef = 4.3 line, shows that the former line is due to pairs or small clusters of exchange-coupled Fe 3+ ions. The temperature dependence of the g ef ≈ 2 line intensity in 0.1 mol Fe 2O 3 glass is consistent with a more antiferromagnetic character by comparison with the 0.05 mol Fe 2O 3 glass, which is attributed to an appearance, at higher Fe 2O 3 contents, of iron-containing microclusters not incorporated in the random glass network, with smaller distances between the paramagnetic ions. These microcluster are probably the origin of a new narrow line superposed with the broad g ef ≈ 2 line in the low-temperature EPR spectra.

Far-infrared ESR study of cuprate compounds

Electron spin resonance (ESR) was investigated on the spin-Peierls system CuGeO 3 and antiferromagnetic Ba 2Cu 3O 4Cl 2 at X-band and far-infrared regions (FIR). The g-values of CuGeO 3 were determined to be 2.17, 2.26 and 2.11 for H∥ a, H∥ b and H∥ c-axes, respectively. The temperature dependence of FIR and X-band ESR spectra of CuGeO 3 clarified the difference of character among uniform, dimerize and high-field magnetic phases. In the Ba 2Cu 3O 4Cl 2 system, the resonance field for H∥ c drastically changes around 30K, and antiferromagnetic resonance (AFMR) was observed at 5 K.

Electron Paramagnetic Resonance Studies of CdPS3:Co2+Single Crystals

The X-band electron paramagnetic resonance spectra of Co 2+ ion in the monoclinic lattice (C 3 2h ) CdPS 3 has been studied at low temperatures. All details of the spectra are satisfactorily described by a spin-Hamiltonian appropriate for Co 2+ in a crystal field of trigonal symmetry. The observed angular variation of the hyperfine structure of the allowed transition is found to be in good agreement with the theory

Chlorofluorocarbon interaction with DPPC vesicles: an ESR investigation.

The effect of the chlorofluorocarbon halotane on DPPC unilamellar vesicles was investigated by Electron Spin Resonance (ESR) spectroscopy. The X-band ESR spectra showed prominent thermotropic behaviour of the interfacial region of vesicles at different halotane concentrations. A model of molecular interaction between phospholipids and chlorofluorocarbon is also proposed.

Electron paramagnetic resonance studies of type 1 copper in type 2 depleted fungal laccase A

The electron paramagnetic resonance (EPR) spectra of type 1 copper(II) in 63Cu-enriched Coriolus versicolor laccase A (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) have been studied. The X-band EPR spectrum in type 2 copper-depleted [ 63Cu]laccase A exhibited well-resolved ligand superhyperfine structure in the g ⊥ region. This structure was assigned to an interaction with two nitrogens and two protons, and assignment which is consistent with a model in which the two nitrogens belong to two histidine ligands an the two protons are the methylene protons of a coordinating cysteine. It also requires the delocalization of a substantial amount of the type 1 copper(II) unpaired electron density onto the cysteine sulphur.

Electron paramagnetic resonance and magnetic susceptibility studies of dimanganese concanavalin A. Evidence for antiferromagnetic exchange coupling.

The double Mn2+ complex of concanavalin A with bound saccharide (SMMPL) was examined by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. A room temperature X-band (9 GHz) EPR spectrum of SMMPL revealed a relatively weak, broad resonance in contrast to the spectrum with a six-line hyperfine-split pattern observed for the mononuclear, high-spin Mn2+ complex found in Ca2+-Mn2+-concanavalin A with saccharide present (SCMPL). The EPR spectrum of SMMPL at 77 K, however, consisted of a series of overlapping patterns of 11 hyperfine-split lines near g = 2.0 with members of each pattern separated by 47 G, half the value of the hyperfine splitting of SCMPL. These 11-line patterns are preserved at Q-band (35 GHz), indicating that the manganese ions in SMMPL form a spin-coupled, binuclear center. As expected for an exchange-coupled system, the EPR signal of SMMPL at 77 K saturates at a higher microwave power than those for SCMPL or Mn2+ aquoion. There is also a marked loss of EPR signal intensity for SMMPL between 4.2 and 1.4 K, which supports the view that the pair of manganese ions is exchanged-coupled. The temperature dependence of both the magnetic susceptibility and the low-temperature EPR spectral intensity can be explained by a model in which the two high-spin Mn2+ ions of SMMPL are antierromagnetically exchanged-coupled with an isotropic coupling constant J = 1.8 cm-1 (for the spin Hamiltonian Hex = JS1.S2). Zero-field splitting D' was estimated to be 375 G from the EPR spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Photolytic production of Fe 3+ in CaS

Calcium sulfide powder containing iron as an impurity was irradiated with 580, 366 or 254 nm light at 77 K. Irradiation enhanced a broad (16 G peak-to-trough) electron paramagnetic resonance (EPR) signal at g = 2.017 and caused six sharp (∼1 G) lines to appear in the X-band EPR spectrum at 347, 529, 956, 1963, 3547 and 5376 G. Enrichment of CaS with Fe 2+ produced samples with similar photochemistry. It is proposed that irradiation causes the reaction Fe 2+ + trap → Fe 3+ + trap −, whose products give rise to six sharp EPR lines assigned to Fe 3+ and a broad line associated with trap −. Both hyperfine splitting by 57Fe (13 G) and superhyperfine splitting by 33S (11.4 G) are observed in the six line spectrum. The environment of the photo-generated Fe 3+ has less than octahedral symmetry. V 2+ was observed at octahedral sites in unirradiated CaS for the first time, and is characterized by the EPR parameters g = 1.961 and A (hyperfine coupling) = 74.6 × 10 −4 cm −1. EPR signals due to Mn 2+ and Cr 3+ at octahedral sites and Fe 3+ at a low symmetry site were also observed in unirradiated CaS.

ChemInform Abstract: SUPERCONDUCTIVITY, MAGNETIC SUSCEPTIBILITY, AND MAGNETIC RESONANCE STUDIES OF THE PSEUDOBINARY SYSTEMS EUROPIUM MOLYBDENUM SULFIDE‐EUROPIUM MOLYBDENUM SELENIDE (EU1.2MO6S8‐EU1.2MO6SE8) AND LANTHANUM MOLYBDENUM SELENIDE‐EUROPIUM MOLYBDENUM SELENIDE (LAMO6SE8‐EUMO6SE8)

AbstractÄhnlich wie beim früher untersuchten System La.

Superconductivity, magnetic susceptibility, and magnetic resonance studies of the pseudobinary systems europium molybdenum sulfide-europium molybdenum selenide (Eu1.2Mo6S8-Eu1.2Mo6Se8) and lanthanum molybdenum selenide-europium molybdenum selenide (LaMo6Se8-EuMo6Se8)

Superconducting critical temperatures, measured by an ac mutual-inductance technique at ambient pressure, show for series La/sub 1-x/Eu/sub x/Mo/sub 6/Se/sub 8/ approximately the same behaviour as previously reported for the series La/sub 1-x/Yb/sub x/Mo/sub 6/Se/sub 8/; the decrease in the transition temperature with increasing x and the absence of superconductivity above 1.5 K for compositions with x>0.8 may be explained solely in terms of the theory of Abrikosov and Gor'kov. This is not the case for the corresponding sulfide systems. None of the compositions in the series Eu/sub 1.2/Mo/sub 6/(S/sub 1-x/Se/sub x/)/sub 8/ were found to be superconducting at ambient pressure above 1.5 K. Magnetic susceptibility measurements have been carried out by the Faraday method over the temperature range 2-300 K and confirm that the europium ions are divalent over the entire composition range for both series. X-Band electron spin resonance spectra obtained from the europium ions may be explained by assuming a spin Hamiltonian containing crystal field terms that are much larger than the Zeeman interaction term. In addition, measurements on the compositions in the series Eu/sub 1.2/Mo/sub 6/(S/sub 1-x/Se/sub x/)/sub 8/ reveal the presence of a europium-containing impurity phase, EuS/sub 1-x/Se/sub x/, in substantial quantities.

Crystalline state disorder and hyperfine component line widths in ferric hemoglobin chains

In X-band electron paramagnetic resonance spectra from single crystals of horse ferric hemoglobin, observed line widths at the low- and high-field extrema are 30 and 24 g, and as much as 400 G in the intermediate region. This behavior is similar to that of ferric myoglobin. Due to large anisotropy in the g-tensors, the line width variation can be accounted for on the basis of heme orientation disorder. This disorder is characterized by an angle, determined here by two independent methods. In these computations Gaussian disorder on a sphere is assumed. The disorder angle is found to be constant on the sphere and about 4 degrees for both alpha- and beta- chains. Treatment of crystals with heavy water (buffer) increases the disorder. Since ligand nitrogen hyperfine couplings are available from hemoglobin electron nuclear double resonance, single crystal electron paramagnetic resonance spectra can be simulated by superimposing hyperfine bands, where the line width of the component bands is a variable and the disorder model above is employed. Comparison with observed resonances fixes the hyperfine component line widths. These component line widths from ferric hemoglobin in the crystalline state are found to be smaller than those in frozen solution.

ESR spectra of powdered samples of copper(II)-doped bis(diethyldithiocarbamato)nickel(II)—Concentration dependence of linewidths

Room-temperature X-band electron spin resonance spectra of copper(II)-doped nickel diethyldithiocarbamate powders have been studied as a function of copper(II) doping. At low dopant concentrations (less than 5 mole %) the concentration variation of linewidths predicted by a simple dipolar broadening formulation is in good agreement with experiment. At higher dopant concentrations the observed spectra indicate the presence of substantial number of large copper(II) diethyldithiocarbamate clusters in the samples examined.

Kinetic properties and electron paramagnetic resonance spectra of the nitric oxide derivative of hemoglobin components of trout (Salmo irideus).

The binding of nitric oxide to hemoglobin components of trout (Salmo irideus), i.e., Hb trout I and Hb trout IV, has been studied by optical and electron paramagnetic resonance spectroscopy. Kinetic studies show that the Root effect in Hb trout IV is operative also for NO, since a large increase in the dissociation velocity constant (j(4)) is observed as the pH is decreased below 7. Moreover, the time course of the displacement of NO by CO is heterogeneous, suggesting that alpha and beta chains may have different j(4) values. Low-temperature X-band electron paramagnetic resonance spectra have been recorded with Hb trout I and IV saturated with NO at different pH values. The spectra of Hb trout IV are strongly pH-dependent. The high-pH form (pH 8.1) shows axial symmetry and no resolved hyperfine splitting, while the low-pH form is rhombic with a hyperfine splitting of 6.5 G in the g(z) region. The latter form reflects a more distorted site with a more significant delocalization of the unpaired electron on the proximal histidine; both features indicate a destabilization of the ligand binding at low pH. On the other hand, spectra of Hb trout I are axial at both pH values, with hyperfine splitting of 16.5 G, indicating that the site is not distorted and interacts with the ligand very strongly at either pH.

EPR of europium amide: Eu(NH 2) 2

The x-band electron paramagnetic resonance spectrum of europium amide [Eu (NH 2) 2] has been observed at room temperature and at liquid nitrogen temperature. The Lorentzian shaped line is approximately 1000 G wide and the effective g factor is 2.015 ± 0.008. No spectral changes were noted with temperature. The theory of Anderson and Weiss [ Rev. Mod. Phys. 25, 269 (1953)] is used to account for the linewidth in terms of an exchange interaction. The interpretation suggests that europium (II) amide is ferromagnetic and long-range order will occur in the 10–50°K range. A comparison of the physical properties of this compared with other divalent europium (II) compounds is also presented.

Paramagnetic Resonance of Copper in Amorphous and Polycrystalline GeO2

The X-band electron paramagnetic resonance spectra of Cu+ + ions doped into amorphous, tetragonal, and hexagonal GeO2 have been observed at 295°K. Optical absorption and reflection data for, respectively, the doped glass and polycrystalline samples were also obtained. From the experimental results, the symmetry of the cupric complexes and some characteristics of the metal ion's bonding are deduced. In amorphous and tetragonal GeO2, the Cu+ + ions are basically in sixfold coordination. Because these cupric complexes exhibit a significant axial distortion (elongation), however, they have been treated as having square planar symmetry. Since the host cations in amorphous GeO2 are tetrahedrally coordinated, the presence of square planar Cu+ + complexes suggests that, to a significant extent, the cupric ions determine their immediate environment in the glassy host. The Cu+ + ions in hexagonal GeO2 appear to be located within the system of oxygen channels (tunnels) which are present in this crystal.

Some Effects observed by Electron Spin Resonnce of the Diffusion of Manganese into the Calcium and Strontium Fluoride Lattices

The x-band electron spin resonance spectra of calcium or strontium fluoride powder mechanically mixed with manganese fluoride, manganese ammonium fluoride, or manganese dioxide powder were observed immediately after nofixing, after periods of time, and after heating at various temperatures in an argon stream or in air. The proportion of manganese to calcium or strontium was 0.5%. The spectrum immediately after mixing is that of the manganese compound alone. Subsequent spectra depended on the grain size of the alkaline earth fluoride powder. (L.N.N.)