Double Resonance Investigation Research Articles

Overhauser shift and dynamic nuclear polarization on carbon fibers

We report on the first experimental magnetic resonance determination of the coupling between electrons and nuclear spins (1H, 13C) in carbon fibers. Our results strongly support the assumption that the electronic spins are delocalized on graphene like structures in the fiber. The coupling between these electrons and the nuclei of the lattice results in dynamic nuclear polarization of the nuclei (DNP), enabling very sensitive NMR experiments on these nuclear spins. For possible applications of graphene in spintronics devices the coupling between nuclei and electrons is essential. We were able to determine the interactions down to 30×10−9(30ppb). We were even able to detect the coupling of the electrons to 13C (in natural abundance). These experiments open the way for a range of new double resonance investigations with possible applications in the field of material science.

Pulsed ENDOR determination of relative orientation of g-frame and molecular frame of imidazole-coordinated heme center of iNOS.

Mammalian nitric oxide synthase (NOS) is a flavo-hemoprotein that catalyzes the oxidation of L-arginine to nitric oxide. Information about the relative alignment of the heme and FMN domains of NOS is important for understanding the electron transfer between the heme and FMN centers, but no crystal structure data for NOS holoenzyme are available. In our previous work [Astashkin, A. V.; Elmore, B. O.; Fan, W.; Guillemette, J. G.; Feng, C. J. Am. Chem. Soc. 2010, 132, 12059-12067], the distance between the imidazole-coordinated low-spin Fe(III) heme and FMN semiquinone in a human inducible NOS (iNOS) oxygenase/FMN construct has been determined by pulsed electron paramagnetic resonance (EPR). The orientation of the Fe-FMN radius vector, R(Fe-FMN), with respect to the heme g-frame was also determined. In the present study, pulsed electron-nuclear double resonance (ENDOR) investigation of the deuterons at carbons C2 and C5 in the deuterated coordinated imidazole was used to determine the relative orientation of the heme g-frame and molecular frame, from which R(Fe-FMN) can be referenced to the heme molecular frame. Numerical simulations of the ENDOR spectra showed that the g-factor axis corresponding to the low-field EPR turning point is perpendicular to the heme plane, whereas the axis corresponding to the high-field turning point is in the heme plane and makes an angle of about 80° with the coordinated imidazole plane. The FMN-heme domain docking model obtained in the previous work was found to be in qualitative agreement with the combined experimental results of the two pulsed EPR works.

Formation of a Nickel−Methyl Species in Methyl-Coenzyme M Reductase, an Enzyme Catalyzing Methane Formation

An organometallic methyl−nickel species was detected in the enzyme methyl-coenzyme M reductase (MCR). This is the key enzyme in microbial methane production and is probably also involved in anaerobic methane oxidation. Incubation of MCR with 13C-bromomethane results in the formation of an electron paramagnetic resonance (EPR) active nickel−methyl species in the active site of this enzyme. High-resolution pulse electron nuclear double resonance and hyperfine sublevel correlation investigations showed the presence of 13C hyperfine couplings of 18−44 MHz that are associated with the nickel-based EPR signal. The large 13C hyperfine interaction shows unambiguously that the methyl group from bromomethane is directly coordinated to the nickel ion. The EPR structure and parameters are fully supported by density functional theory calculations.

The oxygen vacancy in Ga2O3: a double resonance investigation

When produced under reducing conditions, beta-Ga2O3 is transformed into an n-type semiconductor with delocalized conduction electrons that exhibit a very strong electron spin resonance (ESR) and a strong hyperfine coupling to the gallium nuclei of the host lattice. We apply the Overhauser-shift technique to investigate single crystals of this compound. With extension to the high magnetic field of a W-band spectrometer, we were able to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that is actually averaged out in the ESR signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift becomes evident and the shift clearly approaches a constant value. This value in equilibrium was determined directly from a fit to a sequence of measurements, whereas standard X-band experiments only provided indirect conclusions. The probability densities of the electrons at the nuclei in the two nonequivalent crystallographic positions-the lattice sites with octahedral and tetrahedral coordination-could also be determined directly. The enhanced resolution revealed an otherwise hidden substructure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the oxygen vacancy more precisely and to determine the extension of the electronic wave function of the donor electrons.

Ligand electron-nuclear double resonance of T 1 Trigonal Gd3+ centers in CaF2 with a mixed oxygen-fluorine environment

This paper reports on the results of ligand electron-nuclear double resonance (ENDOR) investigations of T1 trigonal 157Gd3+ centers in the CaF2 compound. It is experimentally found that the nearest environment of an impurity center contains only one 19F ion. Anions in the other coordination shells are identical to those in the pure CaF2 crystal. However, 19F ions in these shells are displaced from their ideal positions in the lattice. The parameters of the ligand hyperfine interaction (LHFI) for 19F nuclei and their coordinates and displacements with respect to the positions in the lattice of the pure CaF2 crystal are determined. It is demonstrated that the unusual isotropic LHFI constant As>0 for Gd3+ ions in the lattice with a mixed oxygen-fluorine nearest environment can be associated with the strong polarization of impurity centers in accordance with the empirical model proposed in [1], provided the structural model of the nearest environment of impurities in the T1 centers [2] is correct. This structural model is confirmed by the analysis of the isotropic hyperfine constant A(s) for 157Gd3+ centers.

Electron paramagnetic resonance and electron nuclear double resonance investigation of NaCl: Ir—a model system for similarly doped silver halides

Iridium-doped Bridgman-grown NaCl single crystals were studied with electron paramagnetic resonance and electron nuclear double resonance after irradiation at 77 K. An orthorhombic-I (IrCl6)4– complex, charge compensated by two next-nearest neighbour vacancies was detected. After annealing at 180 K this complex decays by cither vacancy diffusion or an electronic process. The Ir 2+complex is compared with the Rh2+ complex formed in the same conditions in NaCl crystals grown from solution.

A magnetic double-resonance investigation of the quadrupole interaction in ZnO single crystals

Shallow donors in ZnO are used to investigate the quadrupole interaction in single crystals. From the resolved quadrupole splitting, we determine the electric field gradients at the 67Zn and the 17O site. A magnetic double-resonance technique both increases the sensitivity and the accuracy of the field gradient determination. The data for ZnO are compared to previous results on GaN.

Shallow donors in GaN: A magnetic double resonance investigation

The residual shallow donor in GaN is investigated by electron spin resonance and the Overhauser shift double resonance technique. From the resolved quadrupolar splitting, we determine the electric field gradients at the Ga and the N site. The hyperfine interaction, as determined simultaneously from the Overhauser shift and the Knight shift, supports a delocalized donor wavefunction with an effective Bohr radius of 20–30 Å.

1H and 2D electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) investigations on γ-irradiated betaine arsenate

Electron spin resonance, electron nuclear double resonance and electron spin echo envelope modulation measurements were performed on γ-irradiated betaine arsenate single crystals. The investigations revealed that two radicals, Ṅ(CH 3) 3 and (CH 3) 3N +ĊHCOO −, are formed by the γ-irradiation at room temperature. The concentration of both radicals decays in time with a time constant of a few days. In deuterated betaine arsenate the protons of the CH 2 group are partially replaced by deuterons. The 14N hyperfine coupling tensor of the Ṅ(CH 3) 3 radical as well as the 1H, 2D hyperfine coupling tensor and the 2D quadrupole tensor of the (CH 3) 3N +ĊHCOO − ((CH 3) 3N +ĊDCOO −) radicals were determined.

Electron-nuclear double-resonance investigations on Cr3+ ions in natural MgAl2O4 spinel

The ENDOR spectra of Cr3+ ions in natural MgAl2O4 spinel have been measured at 4.2 K and the hyperfine and quadrupole interactions with the nearest-neighbour 27Al nuclei and with the 53Cr isotope have been investigated. The results show that Cr3+ has the point symmetry 3m, being subjected to the original trigonal expansion at the B sites, but inducing some distortion effects on its surroundings. Further, the isotropic hyperfine constant of the 27Al nuclei has been analysed in an independent bonding model, showing that there is appreciable transfer of unpaired spin to the aluminium ion due to covalency.

Electron spin resonance, electron spin echo, and electron nuclear double resonance investigations of the photoreduction yield of a series of alkylmethylviologens in dihexadecyl phosphate vesicles: effect of added cholesterol

The initial event in photosynthesis, photoinduced charge separation, takes place in a membrane bound proton/chlorophyll complex - the photosynthetic reaction center. An electron spin resonance (ESR), electron spin echo (ESE), and electron nuclear double resonance (ENDOR) investigation was performed on a series of alkylmethylviolgens which were solubilized in dihexadecyl phosphate (DHP) vesicles and rapidly frozen at 77 K. Photoreduction yields were measured by ESR, while ESE and matrix ENDOR were used to determine the location of the viologen radical in the lipid bilayer with respect to the water interface and the hydrocarbon interior, respectively. The effect of various concentrations on cholesterol on both the photoreduction yield and the location of the viologen probe within the DHP vesicle was measured. The ESE and ENDOR results indicate that the increased lipophilicity of the longer chained viologens effects deeper solubilization into the bilayer. This increased lipophilicity correlates with an enhancement of the photoreduction yield due to a reduced back electron transfer rate. Addition of cholesterol results in a disruption of the integrity of the vesicle interface and some increase in the photoreduction yield. This increase is mostly probably due to a decrease in effective interface charge density of the vesicle as a resultmore » of the intercalation of cholesterol molecules into the bilayer.« less

Pairs of F centres and CN−molecules in KCl: An ENDOR analysis

Abstract Pair defects consisting of F centres and CN− molecules as substitutional impurity anions (FH(CN−) centres) exhibit a strong coupling between the F centres and the CN− molecules. This leads to an optical pumping of the CN− vibrations via the F centre absorption band. We performed an Electron Nuclear Double Resonance (ENDOR) investigation in order to analyse the microscopic structure of these aggregate centres. The CN− molecule occupies a [110] nearest anion position with respect to the F centre in two dynamic dipole orientations even at low temperature.

Optically and thermally driven bistability of FH(OH−) centres in KBr: Structure of the bistable configurations

Abstract Pairs of F centres and OH− molecules (FH(OH−) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH− molecules reside on a next nearest anion site (4th shell) relative to the F centre. The difference lies in the orientation of the OH− dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.

Electron spin resonance and electron-nuclear double-resonance investigation of a new Cr3+ defect on an Nb site in LiNbO3:Mg:Cr

For LiNbO3 crystals heavily doped with Mg and codoped with Cr a single isotropic ESR line with g=1.971+or-0.002 and a minimal linewidth of 1.8+or-0.2 mT has been observed. With ENDOR it could be shown unambiguously that the new defect is Cr3+ substituting for Nb. The 53Cr hyperfine and quadrupole constants are A/sub ///=50.8+or-0.2 MHz, Aperpendicular to =51.0+or-0.2 MHz and P=0.10+or-0.05 MHz. The crystal field at the chromium site is smaller by more than an order of magnitude than for previously reported Cr3+ centres in LiNbO3, indicating that those centres are not on the Nb site.

Mn2+ defects in LiNbO3: an electron nuclear double resonance (ENDOR) investigation of the Mn2+ site and the local disorder

With ENDOR, Mn2+ centres in both stoichiometric and congruent LiNbO3 have been investigated. An analysis of four identified Li shells with 24 Li neighbours gives sufficient proof for the Mn2+ occupying a Li site. In stoichiometric LiNbO3 both neighbouring on-axis Nb nuclei were also identified and found to experience changed electric field gradients compared with remote Nb nuclei and NMR data in the literature. Signs of the parameters of the Mn2+ spin Hamiltonian (A/sub //, perpendicular to /<0, D>O, P>0) have been determined. Most centres probably contain a defect such as charge-compensating Li vacancy in the first Li shell. As inferred from line-broadening effects, additional defects, especially in congruent crystals, have a measurable influence on the relaxation of nuclei near to the Mn2+.

Rotational energy transfer in CH3F: The ΔJ=n, ΔK=0 processes

We report the measurement of the rates o ΔJ=n, ΔK=0(‖n‖≤10) processes for CH3F–CH3F collisions at 300 K. The data are derived from a time-resolved millimeter/submillimeter-infrared double resonance investigation of both the 12CH3F and the 13CH3F isotopic species. The rates were obtained via a nonlinear least-squares analysis of the data using a numerical simulation of rotational energy transfer in methyl fluoride. These rates are shown to be quantifiable in terms of the scaling law of infinite order sudden collision theory and the statistical power gap law. As a result, the numerous ΔJ=n, ΔK=0(‖n‖&amp;gt;1) rates can be understood in terms of only two parameters, independent of isotopic species. Using these results and the results of our earlier studies of K-changing processes, we discuss how rotational energy transfer in the CH3F system in general can be described in terms of a small number of collisional processes and parameters.

Infrared–infrared double resonance investigation of the N=2 and N=3 stretching manifolds of silane and the local mode model

We report on our IR–IR double resonance experiment study of the excited stretching state of silane involving two or three vibrational quanta, and on the interpretation of our results by using the local mode model. The DR spectra were recorded by means of two separately tunable IR beams, one of which was used to pump selected lines inside the υ3 or 2υ3 IR bands, and the other (the probe) to observe transitions from the pumped vibration–rotation states. We characterized 7 out of the 14 vibrational states belonging to these manifolds for the first time, and the A1+F2 diad localized previously at 6362 cm−1 has been studied with much higher accuracy than before. Only the two highest states of the N=3 manifold are still unknown. The vibrational energies are very well reproduced by the local mode model (≂1 cm−1 rms deviation), and the harmonic interbond couplings have been determined more precisely than before. The rotational structures of the excited states are also well accounted for by Champion’s theory of vibration–rotation coupling in Td, adapted to the description of stretching vibrations in terms of local modes. More high resolution (0.01 cm−1 or better) data are needed however to fully assess this point.

An electron nuclear double resonance investigation of redox-induced electronic structural change at CuA2+ in cytochrome c oxidase.

We measured an electronic change at cysteine ligand(s) of the CuA2+ center brought on by reduction of other metal centers within cytochrome c oxidase, notably cytochrome a. This change specifically manifested itself as a modification in magnetic hyperfine coupling to the beta-protons of the beta-carbons adjacent to the cysteine sulfur in the CuA2+ coordination sphere. The electron nuclear double resonance ENDOR signals of these beta-protons had previously been assigned through study of selectively deuterated yeast oxidase. In the present study the ENDOR signals of the CuA2+ center were compared from the following forms of oxidase: resting (a3+.CuA2+.a3+3.CuB2+); mixed valence, 2-electron-reduced CO-ligated oxidase (a3+.CuA2+.a2+3CO.CuB+), and a more completely reduced mixed-valence CO-ligated oxidase. In agreement with previous studies on 3-electron-reduced oxidase, the latter more completely reduced oxidase showed cytochrome a preferentially reduced with respect to CuA, implying that the majority of paramagnetic CuA2+ centers had reduced cytochrome a partners. The ENDOR-resolved splitting of the beta-proton hyperfine features substantially decreased in going from the first two more oxidized forms to the more fully reduced latter form. Thus, the electronic structure of the CuA2+ center specifically monitored by hyperfine couplings to cysteine protons changed in response to a reductive event elsewhere in the protein. This structural change may correlate with the anticooperative redox interaction recently reported between cytochrome a and CuA.

EPR and electron nuclear double resonance investigation of oxidized hydrogenase II (uptake) from Clostridium pasteurianum W5. Effects of carbon monoxide binding.

Two different hydrogenases have been isolated from Clostridium pasteurianum W5. Hydrogenase II (uptake) is active in H2 oxidation while hydrogenase I (bidirectional) is active both in H2 oxidation and evolution. Previous EPR and electron nuclear double resonance (ENDOR) studies of oxidized hydrogenase I have now been complemented by analogous studies on oxidized 57Fe-enriched hydrogenase II and its CO derivative (using 12CO and 13CO). Binding of CO greatly changes the EPR spectrum of oxidized hydrogenase II, and use of 13CO leads to resolved hyperfine splitting from interaction with a single 13CO molecule (AC approximately 34 MHz). This coupling is over 50% larger than that seen for hydrogenase I. 57Fe ENDOR disclosed two types of iron site in both oxidized hydrogenase II and its CO derivative. Combination of EPR, ENDOR, and Mössbauer results shows that site 1 has AFe1 = 18 MHz shifting to approximately 30 MHz upon CO binding and consisting of two Fe atoms and site 2 has A2 approximately 7 MHz shifting to approximately 10 MHz and containing a single Fe. These results are very similar to those seen for hydrogenase I, which indicates that a structurally similar 3Fe cluster, believed to be the catalytically active site, is present in both. Proton ENDOR shows a solvent exchangeable resonance only in the CO derivative of hydrogenase II. This indicates a structural difference between hydrogenases I and II that is brought out by CO binding. No evidence of 14N coordination to the cluster is seen for either enzyme.

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.