Electron Nuclear Double Resonance Measurements Research Articles

EPR identification of the negatively charged vacancy in diamond.

Electron-paramagnetic-resonance and electron-nuclear-double-resonance (ENDOR) methods are used to identify the negatively charged state of the isolated vacancy in electron-irradiated synthetic diamond crystals. The ${\mathit{T}}_{\mathit{d}}$ symmetry is confirmed by determining the arrangement of both nearest neighbors and next-nearest neighbors. ENDOR measurements reveal that the effective spin is 3/2.

Iron binding to horse spleen apoferritin: a vanadyl ENDOR spin probe study.

The role of the protein shell in the formation of the hydrous ferric oxide core of ferritin is poorly understood. A VO2+ spin probe study was undertaken to characterize the initial complex of Fe2+ with horse spleen apoferritin (96% L-subunits). A competitive binding study of VO2+ and Fe2+ showed that the two metals compete 1:1 for binding at the same site or region of the protein. Curve fitting of the binding data showed that the affinity of VO2+ for the protein was 15 times that of Fe2+. Electron nuclear double resonance (ENDOR) measurements on the VO(2+)-apoferritin complex showed couplings from two nitrogen nuclei, tentatively ascribed to the N1 and N3 nitrogens of the imidazole ligand of histidine. The possibility that the observed nitrogen couplings are from two different ligands is not precluded by the data, however. A pair of exchangeable proton lines with a coupling of approximately 1 MHz is tentatively assigned to the NH proton of the coordinated nitrogen. A 30-40% reduction in the intensity of the 1H matrix ENDOR line upon D2O-H2O exchange indicates that the metal-binding site is accessible to solvent and, therefore, to molecular oxygen as well. The ENDOR data provide the first evidence for a principle iron(II)-binding site with nitrogen coordination in an L-subunit ferritin. The site may be important in Fe2+ oxidation during the beginning stages of core formation.

Electron nuclear double resonance near ferroelectric and antiferroelectric phase transitions

Abstract A methodology is described for making electron nuclear double resonance (ENDOR) measurements on highly polar materials, such as the KH2PO4 -type of ferroelectrics (KH2PO4, KH2AsO4, RbH2AsO4, RbD2AsO4, CsH2AsO4 and CsD2AsO4) and the antiferroelectric NH4H2AsO4 in the vicinity of their phase transitions. Unlike EPR, the ENDOR lineshapes undergo drastic changes and yield direct information about the motional correlation time of a spin probe in the phase transition region. The ENDOR methodology enhances the spectral resolution and motional sensitivity of the EPR technique by approximately two orders of magnitude, and, thus, opens up another avenue for investigating molecular reorientations and, hence, the mechanism of ferroelectric transitions.

Visible, EPR and electron nuclear double‐resonance spectroscopic studies on the two metal‐binding sites of oxovanadium (IV)‐substituted D‐xylose isomerase

The two metal-binding sites of the D-xylose isomerase from Streptomyces rubiginosus were studied using VO2+ as a sensor for the ligand environment. Titration of the tetrameric enzyme with VO2+, followed by EPR spectroscopy and inhibition studies, show that the first four VO2+ equivalents occupy, in analogy to Co2+, Cd2+ and Pb2+, the binding site B. The visible absorption data and the EPR parameters indicate that a nitrogen ligand is involved in the ligand sphere of the high-affinity B site. The low-affinity A site could be studied selectively by blocking the B site with visible and EPR-silent Cd2+. The visible data and EPR parameters for this site are consistent with a ligand environment composed of oxygen donors without nitrogen ligation. The nitrogen coordination in the high-affinity site could be demonstrated by electron nuclear double-resonance (ENDOR) studies of the 4VO2+ enzyme, and was assigned to a histidine ligand. The 14N resonances are interpreted in terms of a quartet with a coupling value of 13.2 MHz. 1H-ENDOR coupling of 1.7 MHz, exchangeable in D2O, has been assigned to the N-H proton of the histidine. Additional proton ENDOR couplings, which are not exchangeable, are due to protons bound to the carbon atoms of the histidine. For the low-affinity binding site, a nitrogen coordination could be definitely excluded by the ENDOR measurements. Exchangeable 1H-ENDOR couplings observed in this sample were assigned to H2O ligands in the vicinity of VO2+. The results closely relate to what is known from X-ray structure. However, the relative affinities for the two binding sites seem not to be the same for different bivalent cations. In mixed metal samples with four VO2+ and four Co2+ equivalents, the VO2+ is distributed between both binding sites. Small changes in the complex geometry of the A site, indicated by different EPR features, seem to occur if the B site is occupied by Co2+ or by Cd2+.

The nature of structural defects and thermal equilibrium effects on doped a-Si:H as elucidated by electron nuclear double resonance measurements

An increase in the magnetic centre of g = 2.004 in P-doped a-Si:H associated with its rapid quenching from 200 – 250 °C is interpreted in terms of a frozen-in defect model. The nature of frozen-in defects in P- and B- doped a-Si:H is elucidated by electron nuclear double resonance measurements. The magnetic centres of g = 2.004 and g = 2.01 for respective doped a-Si:H are identified as being dangling bond-like centres.

Electron paramagnetic resonance, electron nuclear double resonance, and electron spin echo envelope modulation of Fe(CN)3−6 in a KCl lattice

An electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) study has been carried out on Fe(CN)3−6 in a KCl lattice. The EPR spectrum showed the formation of a large number of centers, which correspond to different configurations of the charge compensating cation vacancies. The two major centers called center Ia and Ib have been well characterized. Both centers have orthorhombic symmetry and show a large g anisotropy. The principle g values are: gx =2.079, gy =3.054, and gz =0.400 for center Ia; gx =2.007, gy =3.177, and gz =0.752 for center Ib. The ligand field splitting and the orbital reduction factor k have been obtained through analyzing the spectra in terms of the generalized spin Hamiltonian. A number of unusual features observed in the EPR spectra have been found to be due to the high level of g anisotropy. The ENDOR and ESEEM measurements performed on center Ia revealed nearly all the coupling tensors between the unpaired electron and the 13C and 14N nuclei. The principle values and tensor orientations were precisely determined by least-squares fitting. The orientations of the coupling tensors give a detailed picture of the complex under the influence of the two cation vacancies. The coupling constants of 13C were found to be an order of magnitude larger than those of 14N. Level anticrossing was observed for the nuclear spin states of 14N.

Electronic structure of Cr3+ in Cs2NaYCl6 and Cs2NaYBr6 lattices. Electron-paramagnetic resonance and electron-nuclear double resonance measurements and multiple scattering Xα calculations

The electronic ground state of isolated Cr3+ introduced into the title compounds has been investigated with electron spin resonance and electron-nuclear double resonance spectroscopy. Simultaneously a multiple scattering (MS) Xα study of the (CrCl6)3− cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupole interaction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xα model and the experimental superhyperfine interaction constants but not with the Cr-hyperfine structure constant. It is suspected that the second neighboring Cs play a non-negligible role in the electronic structure of the cluster.

Dependence of the covalency of the (MnF6)4- complex ion upon the Mn2+-F- distance.

The covalency of the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ cluster has been examined by means of cluster-in-vacuo and cluster-in-the-lattice Hartree-Fock-Roothaan (HFR) calculations. The lattice potentials of ${\mathrm{RbMnF}}_{3}$ and ${\mathrm{KMgF}}_{3}$ have been considered. We have found that the covalency parameters ${f}_{s}$ and ${f}_{p}$=${f}_{\ensuremath{\sigma}}$-${f}_{\ensuremath{\pi}}$ change as the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance R changes from 1.90 to 2.32 A\r{}, as ${R}^{\mathrm{\ensuremath{-}}8.2}$ and ${R}^{\mathrm{\ensuremath{-}}3.2}$, respectively. This result might be helpful in future determinations of these parameters under high pressure. Several contributions to the covalency of the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ bond, including the 3d-orbital deformation, the action of the empty 4s and 4p metallic orbitals, and the effects of the crystal lattice, have been analyzed from the HFR wave function. Calculations show that, these three contributions being small, the (${\mathrm{MnF}}_{6}$${)}^{4\mathrm{\ensuremath{-}}}$ complex is highly ionic and its ${A}_{s}$ and ${A}_{p}$ superhyperfine constants behave as local observables, i.e., changes in their observed values from crystal to crystal are mainly determined by changes in the ${\mathrm{Mn}}^{2+}$-${\mathrm{F}}^{\mathrm{\ensuremath{-}}}$ distance.The traditional analysis of the superhyperfine tensor directed to obtain empirical covalency parameters has also been reexamined. Quantitative evaluation of the often-neglected metal-ligand terms and fluoride relaxation accompanying impurity substitution has shown that these two factors play a key role in determining the anisotropic covalency parameters from magnetic-resonance data. From electron-nuclear double-resonance measurements in cubic fluoroperovskites doped with ${\mathrm{Mn}}^{2+}$, we have found ${f}_{p}$=0.60\ifmmode\pm\else\textpm\fi{}0.20 %. The new empirical values of ${f}_{p}$ derived in this work for ${\mathrm{Mn}}^{2+}$:${\mathrm{KZnF}}_{3}$, ${\mathrm{Mn}}^{2+}$:${\mathrm{RbCdF}}_{3}$, and ${\mathrm{Mn}}^{2+}$:${\mathrm{CsCaF}}_{3}$ do not show a definite trend when R changes. More experimental work is needed to determine the nature of this variation.

Evidence for N coordination to Fe in the [2Fe-2S] center in yeast mitochondrial complex III Comparison with similar findings for analogous bacterial [2Fe-2S] proteins

Yeast mitochondrial complex III contains a subunit with a [2Fe-2S] cluster (the Rieske center) that has unusual physical and chemical properties. For apparently similar centers isolated from bacteria, it has been shown by electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) measurements that these [2Fe-2S] centers are coordinated by at least one and probably two nitrogen ligands. This work describes similar ENDOR and ESEEM studies on the intact mitochondrial complex. We find that this [2Fe-2S] cluster exhibits ESEEM and ENDOR properties that appear to be indistinguishable from those observed with the isolated bacterial systems. Furthermore, changes in EPR lineshape that occur as complex III is progressively reduced are not accompanied by any changes in the nitrogen coupling parameters. This spectroscopic evidence for nitrogen coordination is supported by published sequence data on four Rieske iron-sulfur subunits. It seems likely that this is a general characteristic of such [2Fe-2S] redox active centers.

First principles cluster investigation of electronic structure and hyperfine interaction for nitrogen in diamond

The self-consistent field Unrestricted Hartree-Fock cluster procedure has been used to study the location, electronic structure and hyperfine properties of nitrogen impurity in diamond. From the analysis of the potential energy curve for nitrogen along the axis, it was found that nitrogen is located at a position 0.3A away from the substitutional site towards the plane formed by the three nearest neighbour carbon atoms. The calculated values of the magnetic hyperfine constants and nuclear quadrupole coupling constants for14N agree very well with values obtained from electron paramagnetic resonance and electronnuclear double resonance measurements.

Vacancy in silicon: Hyperfine interactions from electron-nuclear double resonance measurements

The Si-$G7$ EPR spectrum, which is attributed to the negative charge state of the divacancy in silicon, was investigated by electron-nuclear double resonance. Hyperfine interactions between the unpaired defect electron and various $^{29}\mathrm{Si}$ nuclei were determined to obtain detailed information about the electron wave function. A total number of 33 hyperfine tensors was determined, of which 20 belong to a general class shell of atoms and 13 to a mirrorplane class shell. In this way the divacancy electron was probed in a region containing more than 100 lattice sites around the defect. Most hyperfine tensors exhibited approximate axial symmetry, a majority of these having their axial direction along a $〈111〉$ crystal bond direction. An analysis of the interactions is given, using a wave function that is a linear combination of atomic orbitals. From a further theoretical approach of the defect wave function, using extended H\uckel theory, preliminary results are given. In their discussion data from the positive divacancy are also included. In agreement with previous conclusions, the largest general class and mirrorplane class interactions were identified with the nearest-neighbor shells of both types. Further matching between hyperfine tensors and specific shells of lattice sites could not yet be made.

A variable temperature ESE–ENDOR resonator for single-crystal studies

A microwave resonator is described for performing electron spin-echo (ESE) and electron spin-echo electron nuclear double resonance (ESE–ENDOR) measurements on single crystals. The ENDOR coil arrangement allows the crystal to be rotated about two independent axes. The system operates in a temperature range from 4 to 300 K using a helium flow cryostat.

An EPR and electron nuclear double resonance investigation of carbon monoxide binding to hydrogenase I (bidirectional) from Clostridium pasteurianum W5.

Previous Mössbauer and electron nuclear double resonance (ENDOR) studies of oxidized hydrogenase I (bidirectional) from Clostridium pasteurianum W5 demonstrated that this enzyme contains two diamagnetic [4Fe-4S]2+ clusters and an iron-sulfur center of unknown structure and composition that is characterized by its novel Mössbauer and ENDOR properties. In the present study we combine ENDOR and EPR measurements to show that the novel cluster contains 3-4 iron atoms. In addition, we have used EPR and ENDOR spectroscopies to investigate the effect of binding the competitive inhibitor carbon monoxide to oxidized hydrogenase I, using 13C-labeled CO and enzyme isotopically enriched in 57Fe. Treatment of oxidized enzyme with CO causes the g-tensor of the paramagnetic center to change from rhombic to axial symmetry. The observation of a 13C signal by ENDOR spectroscopy and analysis of the EPR broadening show that a single CO covalently binds to the paramagnetic center. The 13C hyperfine coupling constant (Ac approximately equal to 21 MHz) is within the range observed for inorganic iron-carbonyl clusters. The observation of 57Fe ENDOR signals from two types of iron site ([A1c] approximately 30-34 MHz; [A2c] approximately 6 MHz) and resolved 57Fe hyperfine interactions in the EPR spectrum from two nuclei characterized by [A1c] confirm that the iron-sulfur cluster remains intact upon CO coordination, but show that CO binding greatly changes the 57Fe hyperfine coupling constants.

Detection of hydrogen in nominally pure GaP:Zn by optically detected electron nuclear double resonance

Optically detected electron nuclear double resonance (ODENDOR) measurements of the triplet antisite center P Ga 3+ in GaP:Zn have revealed the presence of hydrogen atom impurities in a nominally pure sample. Strong ligand 31P signals have also been observed. Assuming a uniform distribution of hydrogen the intensity ratio of the 31P to 1H lines sets a rough lower limit of between 1–10 ppm on the hydrogen concentration. The possibility that the H 1 atoms form part of the antisite defects is also discussed.

17O electron nuclear double resonance characterization of substrate binding to the [4Fe-4S]1+ cluster of reduced active aconitase.

To characterize the binding of substrate to aconitase, we have made 17O electron nuclear double resonance (ENDOR) measurements on reduced active ([4Fe-4S]1+) beef heart aconitase, both in H216O and H217O, in the presence of substrate and the inhibitors, tricarballylate, trans-aconitate, and 1-hydroxy-2-nitro-1, 3-propanedicarboxylate, referred to here as nitroisocitrate; the hydroxyl of the latter also was isotypically labeled with 17O. The hydroxyl oxygen of citrate and isocitrate is exchanged with solvent water by aconitase, but the hydroxyl of nitroisocitrate is not. Thus, the isotopic composition of nitroisocitrate can be chemically controlled, allowing direct identification of any 17O ENDOR signal associated with it. 17O ENDOR signals were observed from Hx17O (mean = 1 or 2) bound to the [4Fe-4S]1+ cluster in samples prepared with trans-aconitate and unlabeled nitroisocitrate. 17O-Labeled nitroisocitrate in H216O bound to the cluster showed a signal from the 17OH group; in H217O it showed 17O ENDOR resonances due to both Hx17O and 17OH of substrate. This result demonstrates that the cluster participates in substrate binding and can simultaneously coordinate the hydroxyl of a substrate (or analogue) and water (or hydroxyl). The sample with citrate in H217O showed only the Hx17O signal, although aconitase exchanges the hydroxyl of substrate with solvent water. The mechanism of action of aconitase is discussed in light of this observation. Comparison shows the ENDOR study to be in agreement with previous Mössbauer and EPR spectroscopic results.

ENDOR and ESEEM study of phosphorus-doped a-Si:H

Hyperfine interactions due to the electron nuclear dipolar coupling between an electron spin and nearby nuclear spins ( 29Si, 31P) in P-doped a-Si:H films (PH 3/SiH 4 = 1 %) were detected for the first time by electron-nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) measurements. The nature of a spin center with g ∼ 2.0043 is discussed on the basis of the experimental results.

Origin of stable spin species in Langmuir-Blodgett films of merocyanine dyes studied by ESR and ENDOR

ESR and electron nuclear double resonance (ENDOR) measurements were performed on Langmuir-Blodgett films of merocyanine dyes with two different donor nuclei, diluted with arachidic acid. The existence of oriented stable radical species was confirmed from the observed ESR and proton ENDOR spectra which showed clear anisotropies both in-plane and out-of-plane of the film. Heat treatment of the system, which destroys the J-aggregates of the dye molecules, brought an increase of spin concentration at the initial stage of heating, followed by a gradual decrease on further heating. Based on those facts, together with the observation of light-induced enhancement of ESR intensity, an intermolecular charge transfer in dye aggregates is proposed as the origin of the radical species.

Direct Observation of `Soliton-Like' Spin Density Distribution in UndopedCis-Rich (CH)xUsing ENDOR

Electron nuclear double resonance (ENDOR) measurements were performed on stretch oriented undoped cis -rich (CH) x at 4 K. The observed ENDOR line shape had spectral turning points and showed a clear anisotropy. The presence of the spin density which is comparable with the prediction of the soliton theory was directly confirmed from the maximum ENDOR frequency. The analysis of the observed line shape using a spectrum simulation method suggested that the spin density is of the form similar as those predicted by the soliton theory for finite on-site Coulomb energy. Especially the spectral turning points observed when the external magnetic field was parallel to the stretch direction of the sample were directly correlated to the existence of the negative spin density predicted by the theory.

Fermion-propagator calculations of excitations in polyenes with the use of a Heisenberg (XYZ) Hamiltonian. II. Applications to large systems

A previously developed formalism for calculating excitation energies in polyenes using an effective-spin Hamiltonian is applied to large systems. The method involves only diagonalization of N\ifmmode\times\else\texttimes\fi{}N matrices (N being the number of sites). A description of the excitation process in terms of local spin flippings is given and values of the triplet and doublet exciton gap for finite polyenes can be estimated. When applied to deformed chains with odd N, midgap states appear. Introduction of electronic correlation renders local ground-state spin densities which are in qualitative agreement with electron-nuclear double-resonance measurements. Excited-state spin densities are also discussed in some cases.

Direct observation of ‘soliton-like’ spin density distribution in undoped cis-rich (CH) x by ENDOR at 4°K

Electron nuclear double resonance (ENDOR) measurements were performed on stretch oriented undoped cis-rich (CH) x at 4°K. The presence of the spin density which is comparable with the prediction of the soliton theory was directly observed for the first time. The maximum value of the spin density ranged from 0.11 to 0.19 depending on the choice of McConnell's constant. The form of the spin density distribution was discussed through the analysis of observed anisotropic spectral line shape using a spectrum simulation method.