High-resolution Electron Spin Resonance Research Articles

Electron Spin Exchange Processes in Strongly Coupled Spin Triads

The complexes of Cu2+ hexafluoroacetylacetonate with two pyrazol-substituted nitronyl nitroxides are the choice systems to study the spin dynamics of strongly exchange-coupled spin triads. The large values of exchange coupling (ca. 100 cm-1) and high-resolution electron paramagnetic resonance (EPR) at Q- and W-bands (35 and 94 GHz) allowed us to observe and interpret specific characteristics of these systems. An electron spin exchange process has been found between different multiplets of the spin triad, which manifests itself as a significant shift of the EPR line position with temperature. We propose that the spin exchange process is caused by the modulation of exchange interaction between copper and nitroxides by lattice vibrations. The estimations of the rate of exchange process and model calculations essentially support the observed phenomena. The studied characteristics of strongly coupled spin triads explain previously obtained results, agree with literature, and should be accounted for in future investigations of similar spin systems.

High-Resolution ESR Study of the H···CH3, H···CHD2, D···CH2D, and D···CD3 Radical Pairs in Solid Argon

High-resolution electron spin resonance (ESR) spectra of radical pairs of a hydrogen atom that coupled with a methyl radical (H...CH3, H...CHD2, D...CH2D, and D...CD3) were observed for X-ray irradiated solid argon containing selectively deuterium-labeled methanes, CH4, CH2D2, and CD4, at 4.2 K. The double-quartet 1H-hyperfine (hf) splittings of ca. 26 and 1.16 mT at the Deltam(s) = +/-1 and Deltam(s) = +/-2 transitions, which are one-half of the isotropic 1H-hf splittings of an isolated H-atom and a CH3 radical, were attributed to the H...CH3 pair. The 1H-hf splittings at the Deltam(s) = +/-1 transition were further split by the fine structure (fs) due to the electron dipole-dipole coupling. Because of the high-resolution spectra, three different sets of the fs splitting, d, are clearly resolved in the spectra of both the H...CH3 and the D...CD3 pairs. The separation distance (inter-spin distance), R, between the H-atom and the CH3 radical being in pairs was evaluated from the d values based on a point-dipole interaction model. For the case of the H...CH3 pair, the observed d values of 4.2, 4.9, and 5.1 mT yield the respective separations, R = 0.87, 0.83, and 0.82 nm, to probe the trapping site of the pair in an Ar crystalline lattice (fcc). For the pair with R = 0.87 nm, for example, we propose that the CH3 radical occupies a substitutional site and the counter H-atom occupies either the interstitial tetrahedral sites directed away from the CH3 radicals by a distance of 0.87 nm or the interstitial octahedral sites by a distance of 0.88 nm. When a mixture of CH4 and CD4 in a solid Ar matrix was irradiated, only two different radical pairs, H...CH3 and D...CD3, were observed. This result clearly demonstrates that the hydrogen atom and methyl radicals, which undergo a pairwise trapping, can originate from the same methane molecule.

Self-Induced Docking Site of a Deeply Embedded Peripheral Membrane Protein

As a first step toward understanding the principles of the targeting of C2 domains to membranes, we have carried out a molecular dynamics simulation of the C2 domain of cytosolic phospholipase A2 (cPLA2-C2) in a 1-palmitoyl-2-oleoyl-phosphatidylcholine bilayer at constant pressure and temperature (NPT, 300 K and 1 atm). Using the high-resolution crystal structure of cPLA2-C2 as a starting point, we embedded two copies of the C2 domain into a preequilibrated membrane at the depth and orientation previously defined by electron paramagnetic resonance (EPR). Noting that in the membrane-bound state the three calcium binding loops are complexed to two calcium ions, we initially restrained the calcium ions at the membrane depth determined by EPR. But the depth and orientation of the domains remained within EPR experimental errors when the restraints were later removed. We find that the thermally disordered, chemically heterogeneous interfacial zones of phosphatidylcholine bilayers allow local lipid remodeling to produce a nearly perfect match to the shape and polarity of the C2 domain, thereby enabling the C2 domain to assemble and optimize its own lipid docking site. The result is a cuplike docking site with a hydrophobic bottom and hydrophilic rim. Contrary to expectations, we did not find direct interactions between the protein-bound calcium ions and lipid headgroups, which were sterically excluded from the calcium binding cleft. Rather, the lipid phosphate groups provided outer-sphere calcium coordination through intervening water molecules. These results show that the combined use of high-resolution protein structures, EPR measurements, and molecular dynamics simulations provides a general approach for analyzing the molecular interactions between membrane-docked proteins and lipid bilayers.

High field ENDOR as a characterization tool for functional sites in microporous materials

The determination of the details of the spatial and electronic structure of functional sites (centers) in any system, be it in materials chemistry or in biology, is the first step towards understanding their function. When such sites happen to be paramagnetic in any point of their activity cycle, the tool box offered by a variety of high resolution electron paramagnetic resonance (EPR) spectroscopic techniques becomes very attractive for their characterization. This tool box has been considerably expanded by the developments in high field (HF) EPR in general, and HF electron nuclear double resonance (ENDOR), in particular. These have led to numerous new applications in the fields of biology, physics, chemistry and materials sciences. This overview focuses specifically on recent applications of pulsed HF ENDOR spectroscopy to microporous materials, such as zeotype materials, presenting the new opportunities it offers. First, a brief description of the theoretical basis required for the analysis of the HF ENDOR spectrum is given, followed by a description of the pulsed techniques used to record spectra and assign the signals, along with a brief presentation of the required instrumentation. Next, specific applications are given, including transition metal ions and complexes exchanged into zeolite cages, transition metal substitution into frameworks of zeolites, aluminophosphate molecular sieves, and silicious mesoporous materials, the interaction of NO with Lewis sites in zeolite cages and trapped S. We end with a discussion of the advantages and the shortcomings of the method and conclude with a future outlook.

Methionine Radical Cation: Structural Studies as a Function of pH Using X- and Q-Band Time-Resolved Electron Paramagnetic Resonance Spectroscopy

A comprehensive high resolution electron paramagnetic resonance (EPR) characterization of the l-methionine radical cation and its N-acetyl derivative in liquid solution at room temperature is presented. The cations were generated photochemically in high yield by excimer laser excitation of a water soluble dye, anthraquinone sulfonate sodium salt, the excited triplet state of which is quenched by electron transfer from the side chain sulfur atom of methionine or N-acetylmethionine. The radicals were detected by continuous wave (CW) time-resolved electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz) and Q-band (35 GHz) microwave frequencies. At pH values well below the pK(a) of the protonated amine nitrogen, the cation forms a dimer with another ground-state methionine molecule through a S-S three-electron bond. In basic solution, the lone pair on the nitrogen of the amino acid is available to make an intramolecular S-N three-electron bond with the side chain sulfur atom, leading to a five-membered ring structure for the cation. When the amino acid nitrogen is unsubstituted (methionine itself), rapid deprotonation to an aminyl radical takes place at high pH values. If the nitrogen is substituted (N-acetylmethionine), the cyclic structure is observed within its electron spin relaxation time at about 1 micros. Spectral simulation provides chemical shifts (g-factors) and hyperfine coupling constants for all structures, and isotopic labeling experiments strongly support the assignments.

Single Electron Traps at the Surface of Polycrystalline MgO: Assignment of the Main Trapping Sites

Paramagnetic centers at the surface of ionic oxides in the form of trapped electrons can be generated by exposure of the material to alkali metal or hydrogen atoms or of molecular hydrogen under UV irradiation. For many years, it has been assumed that the resulting paramagnetic centers consist of oxygen vacancies filled by one electron. High-resolution electron spin resonance spectra and ab initio quantum chemical calculations show that the paramagnetic centers consist of (H(+))(e(-)) electron pairs formed at morphological irregularities of the surface. At least three different kinds of (H(+))(e(-)) centers, [A], [B], and [C], have been identified with abundances of 80%, 10%, and 8%, respectively. In this work, we compare a wide set of measured and computed g-factors and hyperfine coupling constants of the unpaired electron with the surrounding (25)Mg, (17)O, and (1)H nuclei and we propose a general assignment of the centers. (H(+))(e(-)) pairs formed at Mg(4c) ions at steps and edges account for species [A], centers formed at Mg(4c) ions at reverse corners correspond to species [B], and species [C] originates from (H(+))(e(-)) pairs formed at Mg(3c) ions at corners and kinks.

High-resolution electron spin resonance spectroscopy of XeF• in solid argon. The hyperfine structure constants as a probe of relativistic effects in the chemical bonding properties of a heavy noble gas atom

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

High-field ESR study of the dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO 3

The dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO 3 is probed using the tunable-frequency high-resolution electron spin resonance technique in magnetic fields up to 17 T . A field-induced development of the soliton-like incommensurate superstructure is clearly indicated as a pronounced increase of the magnon spin resonance linewidth Δ B, with a Δ B max at B c ∼13.8 T . The anomaly is explained in terms of the magnon–soliton scattering and suggests that the soliton-like phase exists close to the boundary of the dimerized-incommensurate phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped CuGeO 3 are studied. Some features of the high-field high-resolution tunable-frequency ESR facility recently developed in Tallahassee are presented.

Characterization of the High Resolution ESR Spectra of the Methoxyl Radical Adducts of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO)

Spin-trapping investigators are largely limited by the instability of the radical adducts. Spin trap 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO) forms very stable alkoxyl radical adducts. However, the presence of two chiral centers in the DEPMPO alkoxyl radical adduct results in two diastereomers with distinctive ESR spectra, which complicates the interpretation of the ESR spectra. We have analyzed the high resolution ESR spectra of the DEPMPO/•OCH3 radical adduct. DEPMPO/•OCH3 has been synthesized by the nucleophilic addition of alcohols to DEPMPO. The electron spin resonance (ESR) spectrum of DEPMPO/•OCH3 in oxygen-free methanol solution reveals superhyperfine structure with hyperfine coupling constants as small as 0.3 G. In order to simplify the analysis of the electron spin resonance (ESR) spectrum, we synthesized the DEPMPO/•OCD3 radical adduct. Computer simulation of the DEPMPO/•OCD3 ESR spectrum revealed two diastereomers. Hyperfine coupling constants of γ-protons and 17O from the –OCH3 group were also determined. ESR spectra of DEPMPO/•OCH3 in phosphate buffer have also been characterized. The presence of specific hyperfine couplings from the –OCH3 group can be used for the unambiguous identification of the DEPMPO/•OCH3 radical adducts. We suggest that the analysis of high resolution ESR spectra can be used for the unambiguous characterization of DEPMPO radical adducts.

Field-induced structural evolution in the spin-Peierls compoundCuGeO3:High-field ESR study

The dimerized-incommensurate phase transition in the spin-Peierls compound CuGeO$_3$ is probed using multifrequency high-resolution electron spin resonance (ESR) technique, in magnetic fields up to 17 T. A field-induced development of the soliton-like incommensurate superstructure is clearly indicated as a pronounced increase of the ESR linewidth $\Delta B$ (magnon excitations), with a $\Delta B_{max}$ at $B_{c}\sim$ 13.8 T. The anomaly is explained in terms of the magnon-soliton scattering, and suggests that the soliton-like phase exists close to the boundary of the dimerized-incommensurate phase transition. In addition, magnetic excitation spectra in 0.8% Si-doped CuGeO$_3$ are studied. Suppression of the $\Delta B$ anomaly observed in the doped samples suggests a collapse of the long-range-ordered soliton states upon doping, that is consistent with high-field neutron scattering experiments.

Density Functional Theory Calculations of Nitrogen Hyperfine and Quadrupole Coupling Constants in Oxovanadium(IV) Complexes

Relativistic density functional theory (DFT) calculations of nitrogen hyperfine and quadrupole coupling constants were conducted for a series of oxovanadium complexes with axial and equatorial nitrogen ligands. The computational results qualitatively reproduced the observed experimental trends in nitrogen hyperfine coupling constants with ligand type (amine, imine, and isothiocyanate) and coordination (axial vs equatorial). The best quantitative agreement between calculated and experimental nitrogen coupling constants was obtained using the scalar-relativistic, spin-unrestricted, open-shell Kohn−Sham (SR UKS) method. These results have important implications for the interpretation of high-resolution electron paramagnetic resonance (EPR) spectra of oxovanadium complexes with nitrogen ligands.

Tyrosine Iminoxyl Radical Formation from Tyrosyl Radical/Nitric Oxide and Nitrosotyrosine

The quenching of the Y(D)(.) tyrosyl radical in photosystem II by nitric oxide was reported to result from the formation of a weak tyrosyl radical-nitric oxide complex (Petrouleas, V., and Diner, B. A. (1990) Biochim. Biophys. Acta 1015, 131-140). This radical/radical reaction is expected to generate an electron spin resonance (ESR)-silent 3-nitrosocyclohexadienone species that can reversibly regenerate the tyrosyl radical and nitric oxide or undergo rearrangement to form 3-nitrosotyrosine. It has been proposed that 3-nitrosotyrosine can be oxidized by one electron to form the tyrosine iminoxyl radical (>C=N-O*). This proposal was put forth as a result of ESR detection of the iminoxyl radical intermediate when photosystem II was exposed to nitric oxide (Sanakis, Y., Goussias, C., Mason, R. P., and Petrouleas, V. (1997) Biochemistry 36, 1411-1417). A similar iminoxyl radical was detected in prostaglandin H synthase-2 (Gunther, M. R., Hsi, L. C., Curtis, J. F., Gierse, J. K., Marnett, L. J., Eling, T. E., and Mason, R. P. (1997) J. Biol. Chem., 272, 17086-17090). Although the iminoxyl radicals detected in the photosystem II and prostaglandin H synthase-2 systems strongly suggest a mechanism involving 3-nitrosotyrosine, the iminoxyl radical ESR spectrum was not unequivocally identified as originating from tyrosine. We report here the detection of the non-protein L-tyrosine iminoxyl radical generated by two methods: 1) peroxidase oxidation of synthetic 3-nitroso-N-acetyl-L-tyrosine and 2) peroxidase oxidation of free L-tyrosine in the presence of nitric oxide. A newly developed ESR technique that uses immobilized enzyme was used to perform the ESR experiments. Analysis of the high resolution ESR spectrum of the tyrosine iminoxyl radical generated from free tyrosine and nitric oxide reveals a 28.4-G isotropic nitrogen hyperfine coupling and a 2.2-G proton hyperfine coupling assigned to the proton originally ortho to the phenoxyl oxygen.

High-resolution electron spin resonance spectroscopy of ethyl radicals in solid parahydrogen

High-resolution electron spin resonance (ESR) spectrum of ethyl radicals isolated in solid parahydrogen (>99.5 mol %) has been measured in the temperature range between 3.1 and 6.7 K. This study was carried out by UV photolysis of ethyl iodide doped in solid parahydrogen. Although ESR linewidth of the spectrum is broadened due to anisotropic terms of hyperfine interactions, the linewidth measured in parahydrogen remains comparable to that of the isotropic spectrum measured in liquid ethane [R. W. Fessenden, J. Chem. Phys. 37, 747 (1962)]. Small splittings were found in the resolved ESR signals and assigned as ethyl radicals with A(A′,A″) and E(E′,E″) symmetries, respectively. The ratio in the ESR intensities is not proportional to that of the degeneracy of each symmetry but varies as a function of temperature. We measured the signal intensities as a function of temperature and determined the rotational energy level splitting of methyl groups between the lowest with the A(A′,A″) symmetry and the second-lowest with the E(E′,E″) symmetry in solid parahydrogen to be 5.3±0.7 K. The level splitting is close to that measured by infrared-absorption study in gas phase and theoretical calculation, whereas the splitting measured in conventional matrices have been much smaller. Intermolecular interaction between guest radicals and surroundings in solid parahydrogen are found to be much smaller than those in other conventional matrices giving less shifted spectral parameters.

High-field ESR on quantum spin systems

High-field electron spin resonance (ESR) experiments have been performed on quantum spin systems. A distinct field-induced crossover between the gap-less spinon regime and the gapped sine-Gordon regime is observed for the Heisenberg quantum spin chain; CuBenzoate. In a Shastry and Sutherland model compound SrCu 2(BO 3) 2, a set of multiple-triplet excitations has been resolved by utilizing high-resolution ESR. The splitting of the two-triplet bound state into a few discrete energy levels is considered as the evidence of the extreme localization of the triplet excitations in this compound. The present results indicate the unique and powerful features of ESR in the study of magnetic excitations.

Pulse Electron Paramagnetic Resonance (EPR) and Electron−Nuclear Double Resonance (ENDOR) Investigation of N@C60 in Polycrystalline C60

By performing high-resolution electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments on nitrogen atoms encapsulated in C60, the capability of the quartet spin system to sense small local fields at the site of the atom is demonstrated. Symmetry lowering induced by a phase transition in polycrystalline C60 at 258 K can easily be detected by the appearance of a zero-field splitting of axial symmetry. Freezing of cage rotation is observed via the magnetic dipole interaction with 13C nuclei of the carbon shell.

Electron spin resonance and theoretical studies of the 14N⋅⋅⋅⋅14N and 15N⋅⋅⋅⋅15N spin-pair radicals in neon matrices: The effects of mixing among the 1Σg+, 3Σu+, 5Σg+, and 7Σu+ electronic states

The first experimental and theoretical study of the N⋅⋅⋅⋅N spin-pair radical is reported. Its high-resolution ESR (electron spin resonance) spectrum has been observed in neon matrices and interpreted on the basis of weakly interacting atoms using a model recently developed for the H⋅⋅⋅⋅H spin-pair. To fully interpret the N⋅⋅⋅⋅N radical results it was necessary to include electronic state mixing effects among all possible spin states, namely the 1Σg+, 3Σu+, 5Σg+, and 7Σu+ states. Several different trapping sites were observed which indicated the interaction of N atoms at distinctly different separation distances in the neon lattice. Calculated J values at the complete active space self-consistent field (CASSCF) level (TZP basis set) were compared with the experimental results for the various trapping site distances. The 15N⋅⋅⋅⋅15N radical in the dominant trapping site had magnetic parameters of g=2.0016(2), A(15N)=15.9(1) MHz, D=−178(1)MHz and J=468(2) MHz. Using the point dipole approximation this corresponds to a N⋅⋅⋅⋅N separation distance of 6.41 Å. A most unusual type of magnetic dipole transition was observed that involves a transition between electronic states of formally different S values where S is the total spin quantum number for a given electronic state.

High resolution electron paramagnetic resonance imaging of biological samples with a single line paramagnetic label.

The application of electron paramagnetic resonance imaging (EPRI) to obtain information from biological samples has been limited by the lack of ideal single line radical labels. The commonly used nitroxides exhibit multiple lines causing either hyperfine-based limitations in the maximum obtainable image resolution or hyperfine-based artifacts in the reconstructed image. The use of a novel single-line triarylmethyl paramagnetic label that enables marked enhancement in image quality and resolution is reported. This label exhibits a single line EPR spectrum that is sharp (linewidth approximately 60 mG in the absence of oxygen) and relatively stable in tissues. The potential of this label in enabling high resolution EPR imaging of biological samples was demonstrated in a series of phantoms and isolated biological organs such as the rat kidney. The images demonstrate that resolutions better than 100 microns could be obtained at L-band on samples of up to 20 mm in size.

Detection of Thiyl Radical Adducts Formed during Hydroxyl Radical- and Peroxynitrite-Mediated Oxidation of Thiols—A High Resolution ESR Spin-Trapping Study at Q-band (35 GHz)

Thiyl radicals (RS•) formed during peroxynitrite- or hydroxyl radical-dependent oxidation of thiols, i.e., glutathione (GSH) andL-cysteine (CySH) were trapped with 5,5′-dimethyl-1-pyrrolineN-oxide (DMPO) and analyzed by X-band and Q-band electron spin resonance (ESR) spectroscopy. At X-band, the ESR parameters of DMPO-glutathionyl radical adduct (DMPO/•SG) and DMPO-hydroxyl radical adduct (DMPO/•OH) are nearly similar in aqueous solutions and as a result, except for the low-field spectral line, the remaining spectral lines of DMPO/•SG virtually over-lap with those of the DMPO/•OH adduct. In contrast, at Q-band, most of the spectral lines due to the DMPO/•SG were separated from the DMPO/•OH. Inclusion of a superoxide dismutase (SOD) mimic completely abolished the formation of the DMPO/•OH adduct and not the DMPO/•SG adduct during ONOO−-mediated oxidation of GSH and DMPO. In the presence of formate, the DMPO/•SG spectrum was replaced by the DMPO/•CO−2spectrum which was monitored by Q-band ESR spectroscopy. Thus, spin-trapping at Q-band provides unambiguous proof for the glutathionyl radical-dependent oxidation of formate by peroxynitrite. High resolution Q-band ESR spectra of DMPO/•Scys were also obtained. Biological applications of the Q-band spin-trapping technique to detect thiyl radicals in cellular systems are discussed.

Weak field NMR and ESR spectrometers and magnetometers

Weak field magnetic resonance is usually limited by the signal-to-noise ratio, which decreases drastically with the field. The authors present high-resolution NMR (nuclear magnetic resonance) and ESR (electron spin resonance) sensors developed for Earth field magnetometry and spectrometry. For some of them the resonance signal is amplified by the Overhauser effect or laser optical pumping. The spectrometers and magnetometers are described. The features of magnetometers and many application fields are summarized. The performance of the low field spectrometers and their potential use in spin dynamic studies are also presented. >

Methods for automated analysis and simulation of electron paramagnetic resonance spectra

Three computer programs for the simulation and iterative least-squares fitting of high-resolution electron paramagnetic resonance (EPR) spectra are described. The program eprft assumes constant linewidths, the programs hffit and hffits allow for linewidth variations. The program eprft offers three minimization techniques: evolutionary Monte Carlo, simplex and Marquardt. Procedures for an automated analysis of EPR spectra are tested, using the combination of a search technique and iterative least-squares fitting.