High-resolution Electron Spin Resonance Research Articles

Terahertz EPR spectroscopy using a 36-tesla high-homogeneity series-connected hybrid magnet

Electron Paramagnetic Resonance (EPR) is a powerful technique to study materials and biological samples on an atomic scale. High-field EPR in particular enables extracting very small g-anisotropies in organic radicals and half-filled 3d and 4f metal ions such as MnII (3d5) or GdIII (4f7), and resolving EPR signals from unpaired spins with very close g-values, both of which provide high-resolution details of the local atomic environment. Before the recent commissioning of the high-homogeneity Series Connected Hybrid magnet (SCH, superconducting + resistive) at the National High Magnetic Field Laboratory (NHMFL), the highest-field, high-resolution EPR spectrometer available was limited to 25 T using a purely resistive “Keck” magnet at the NHMFL. Herein, we report the first EPR experiments performed using the SCH magnet capable of reaching the field of 36 T, corresponding to an EPR frequency of 1 THz for g = 2. The magnet’s intrinsic homogeneity (25 ppm, that is 0.9 mT at 36 T over 1 cm diameter, 1 cm length cylinder) was previously established by NMR. We characterized the magnet’s temporal stability (5 ppm, which is 0.2 mT at 36 T over one-minute, the typical acquisition time) using 2,2-diphenyl-1-picrylhydrazyl (DPPH). This high resolution enables resolving the weak g-anisotropy of 1,3-bis(diphenylene)-2-phenylallyl (BDPA), Δg = 2.5 × 10–4 obtained from measurements at 932 GHz and 33 T. Subsequently, we recorded EPR spectra at multiple frequencies for two GdIII complexes with potential applications as spin labels. We demonstrated a significant reduction in line broadening in Gd[DTPA], attributed to second order zero field splitting, and a resolution enhancement of g-tensor anisotropy for Gd[sTPATCN]-SL.

Thermostimulated crystal structure transformation of the polymineral clay submicron particles

Changes in the structure of phyllosilicate microparticles of submicron fractions of polymineral clay with an average size of 290 nm, as well as changes in the positions of impurity ions of transition metals in the crystal lattice, were studied by X-ray diffraction analysis, Fourier transform infrared spectroscopy, and high-resolution electron paramagnetic resonance spectroscopy. The main composition of microparticles included clinochlore, montmorillonite, calcite, and quartz. Structural transformations were stimulated by heating the samples to 1200 K at a rate of 10 K/min. A decrease in the number of crystal structures of microparticles was recorded due to the amorphization of montmorillonite, the decomposition of calcite, and the formation of sillimanite. Structural rearrangements of crystal cells containing impurity paramagnetic ions Fe3+ and Mn2+ in submicron particles of clinochlore and montmorillonite, accompanied by displacements and changes in the chemical bonds of these ions, were recorded by the ion-electron paramagnetic resonance method. It is shown that when microparticles were heated to 1200 K, the impurity paramagnetic ions of transition metals did not go beyond their crystal cells.

High-resolution EPR distance measurements on RNA and DNA with the non-covalent Ǵ spin label.

Pulsed electron paramagnetic resonance (EPR) experiments, among them most prominently pulsed electron-electron double resonance experiments (PELDOR/DEER), resolve the conformational dynamics of nucleic acids with high resolution. The wide application of these powerful experiments is limited by the synthetic complexity of some of the best-performing spin labels. The recently developed n}{}bfacute{G} (G-spin) label, an isoindoline-nitroxide derivative of guanine, can be incorporated non-covalently into DNA and RNA duplexes via Watson-Crick base pairing in an abasic site. We used PELDOR and molecular dynamics (MD) simulations to characterize n}{}bfacute{G}, obtaining excellent agreement between experiments and time traces calculated from MD simulations of RNA and DNA double helices with explicitly modeled n}{}bfacute{G} bound in two abasic sites. The MD simulations reveal stable hydrogen bonds between the spin labels and the paired cytosines. The abasic sites do not significantly perturb the helical structure. n}{}bfacute{G} remains rigidly bound to helical RNA and DNA. The distance distributions between the two bound n}{}bfacute{G} labels are not substantially broadened by spin-label motions in the abasic site and agree well between experiment and MD. n}{}bfacute{G} and similar non-covalently attached spin labels promise high-quality distance and orientation information, also of complexes of nucleic acids and proteins.

Vanadyl Porphyrin Speciation Based on Submegahertz Ligand Proton Hyperfine Couplings

The speciation of vanadyl (VO2+) porphyrins in crude oil (vanadyl petroporphyrins) is an area of ongoing interest in petroleomics. In this paper, we describe a method for the speciation of vanadyl porphyrins that uses electron nuclear double resonance (ENDOR), a high-resolution electron paramagnetic resonance (EPR) spectroscopic technique. We use 1H ENDOR to measure hyperfine couplings between ligand protons and the paramagnetic vanadyl ion as small as about 0.15 MHz. From the measured hyperfine couplings, we directly determine all vanadium–ligand proton distances up to 8 A. This information differentiates porphyrin ligands by their ring substitution pattern and substituent nature. We demonstrate this using a series of vanadyl porphyrin model compounds. Additionally, we demonstrate that the composition of binary vanadyl porphyrins mixtures can be determined. The ability of ENDOR to differentiate types of ligand protons in vanadyl porphyrin mixtures provides a basis for analyzing more complex mixtures of v...

Identification of Si-vacancy related room-temperature qubits in 4H silicon carbide

Identification of microscopic configuration of point defects acting as quantum bits is a key step in the advance of quantum information processing and sensing. Among the numerous candidates, silicon vacancy related centers in silicon carbide (SiC) have shown remarkable properties owing to their particular spin-3/2 ground and excited states. Although, these centers were observed decades ago, still two competing models, the isolated negatively charged silicon vacancy and the complex of negatively charged silicon vacancy and neutral carbon vacancy [Phys. Rev. Lett.\ \textbf{115}, 247602 (2015)] are argued as an origin. By means of high precision first principles calculations and high resolution electron spin resonance measurements, we here unambiguously identify the Si-vacancy related qubits in hexagonal SiC as isolated negatively charged silicon vacancies. Moreover, we identify the Si-vacancy qubit configurations that provide room temperature optical readout.

Induction-detection electron spin resonance with spin sensitivity of a few tens of spins

Electron spin resonance (ESR) is a spectroscopic method that addresses electrons in paramagnetic materials directly through their spin properties. ESR has many applications, ranging from semiconductor characterization to structural biology and even quantum computing. Although it is very powerful and informative, ESR traditionally suffers from low sensitivity, requiring many millions of spins to get a measureable signal with commercial systems using the Faraday induction-detection principle. In view of this disadvantage, significant efforts were made recently to develop alternative detection schemes based, for example, on force, optical, or electrical detection of spins, all of which can reach single electron spin sensitivity. This sensitivity, however, comes at the price of limited applicability and usefulness with regard to real scientific and technological issues facing modern ESR which are currently dealt with conventional induction-detection ESR on a daily basis. Here, we present the most sensitive experimental induction-detection ESR setup and results ever recorded that can detect the signal from just a few tens of spins. They were achieved thanks to the development of an ultra-miniature micrometer-sized microwave resonator that was operated at ∼34 GHz at cryogenic temperatures in conjunction with a unique cryogenically cooled low noise amplifier. The test sample used was isotopically enriched phosphorus-doped silicon, which is of significant relevance to spin-based quantum computing. The sensitivity was experimentally verified with the aid of a unique high-resolution ESR imaging approach. These results represent a paradigm shift with respect to the capabilities and possible applications of induction-detection-based ESR spectroscopy and imaging.

Backbone Orientation and Distance Measurements in Myosin II: Applications of High-Resolution EPR using a Bifunctional Spin Label

We have used electron paramagnetic resonance (EPR) of a bifunctional spin label (BSL) to obtain high-resolution measurements of individual structural elements within the myosin II catalytic domain (CD). Two complementary EPR techniques were employed to measure protein orientation (conventional EPR) and intra-protein distances (dipolar electron-electron resonance, DEER). The use of BSL greatly enhances the resolution of EPR, by virtue of its strongly immobilized and stereoselective bifunctional attachment to the protein backbone at two engineered Cys residues. Crucially, both techniques utilized here permit the elucidation of myosin structure while in complex with actin, generating relevant constraints for the refinement of actomyosin structural models. In the current work, Dictyostelium myosin II was used as our model system. We measured nucleotide-dependent structural transitions of three key helices within the myosin CD. Three double-Cys sites were engineered, with Cys pairs located on the relay helix, helix HK (upper 50kDa domain) and helix HW (lower 50kDa domain), respectively. BSL on a construct with one of these pairs was used to measure myosin orientation relative to oriented actin. BSL on a construct with two pairs was used to measure interprobe distances. The effect of ADP binding was clearly detected by EPR, and subsequently modeled using the orientation and distance measurements as constraints. We find that the structural change induced by ADP in the actin-bound myosin CD is clearly different from that predicted from actin-free crystal structures. This work was funded by grants from NIH (R01 AR32961, T32 AR07612, P30 AR0507220).

Electronic structure and electro-optical properties of ion radicals formed during the reduction of N,N′-dialkylsubstituted salts of 4,4′-bipyridyl

Symmetric N,N′-disubstituted salts of 4,4′-bipyridyl (viologens) are synthesized and described. It is shown by cyclic voltammetry that disubstituted salts of 4,4′-bipyridyl have a tendency toward reversible two-electron reduction. Normal first and second step oxidation-reduction potentials with respect to saturated silver chloride electrodes at various potential scan rates are found. The spectra of the high-resolution electron paramagnetic resonance of viologen cation radicals formed during the reduction of N,N′-disubstituted salts of 4,4′-bipyridyl are obtained and interpreted. It is established that the nature of substitutes and counterions in a viologen molecule considerably influences the distribution of its spin density, voltammetric curves, and electro-optical properties.

High-resolution electron spin resonance analysis of ion bombardment induced defects in advanced low-κ insulators (κ = 2.0-2.5)

Electron spin resonance analysis of defects generated by ion bombardment in different low-permittivity (low-κ) SiO2-based insulating films reveals common types of encountered defects: The EX center (g = 2.0025) and a broad line centered at g = 2.0028 tentatively associated with dangling bonds of carbon atoms backbonded to C or Si atoms. The efficiency of defect generation varies, depending on the type of bombarding ions and the technology of insulator synthesis. However, the two identified defects are observed in all studied cases, suggesting that these defects correspond to the most stable atomic configurations resulting from the network damage of the low-κ materials.

Electron spin resonance analysis of sputtering-induced defects in advanced low-κ insulators (κ=2.0–2.5)

Graphical abstractDisplay Omitted HighlightsWe have carried out a low-temperature ESR study of damage induced by RF ion sputtering in dielectrics.The studied materials are three types of low-permittivity insulators grown on (100)Si.This reveals as universal damage pattern the generation of two types of point defects.These include the EX center and a broad signal ascribed to C dangling bonds in carbosilicide clusters. High-resolution electron spin resonance analysis of defects induced by RF ion sputtering has been performed on three different low-permittivity insulators, including chemical vapour deposited carbon-doped SiO2, spin-on glass with subsequent porogen removal by thermal/ultraviolet curing, and layers obtained by self-assembly. The spectra reveal, as the universal damage pattern in all three types of insulators, generation of EX-centers (g=2.0025) and a broad (ΔBpp?12-15G) signal at g=2.0028 tentatively ascribed to dangling bonds of carbon atoms in a carbosilicide cluster. The differences in defect densities observed between different discharge gasses and different materials are explained by knock off of atoms of different mass from the insulating matrix.

High-Resolution EPR of a Bifunctional Spin Label Reveals Structural Transitions within Myosin's Catalytic Domain

We present a method for obtaining high-resolution information on protein backbone structure and dynamics using electron paramagnetic resonance (EPR) of a bifunctional spin label (BSL) and molecular modeling. Two complementary EPR techniques were employed to measure protein orientation (conventional EPR) and intra-protein distances (dipolar electron-electron resonance, DEER). BSL attaches at Cys positions i and i+4 on a helix, greatly reducing probe mobility relative to the peptide backbone, compared to monofunctional labels. Accurate modeling of BSL provides the coordinates required to directly relate spectroscopic data to backbone structure (both orientation and distance), and dynamics (rotational motion). In the current work, the motor protein Dictyostelium myosin II was used to demonstrate this approach. We measured nucleotide-dependent structural transitions of two key helices within the myosin catalytic domain (CD). Two double-Cys sites were engineered, with one Cys pair located on the relay helix, and the other on a stable helix in the upper 50kD domain. BSL on a construct with one of these pairs was used to measure myosin orientation relative to oriented actin. BSL on a construct with two pairs was used to measure interprobe distances. The effect of ADP binding on both orientation and distance was clearly detected with BSL, but not with a monofunctional label. The significance of this work is twofold: (1) A structural transition in the relay helix upon ADP binding was clearly defined with high resolution. (2) BSL spectra demonstrate superior resolution, compared to monofunctional spin labels, making it possible to directly translate spectroscopic data to protein structure and dynamics.

First Combined in Vivo X-Ray Tomography and High-Resolution Molecular Electron Paramagnetic Resonance (EPR) Imaging of the Mouse Knee Joint Taking into Account the Disappearance Kinetics of the EPR Probe

Although laboratory data clearly suggest a role for oxidants (dioxygen and free radicals derived from dioxygen) in the pathogenesis of many age-related and degenerative diseases (such as arthrosis and arthritis), methods to image such species in vivo are still very limited. This methodological problem limits physiopathologic studies about the role of those species in vivo, the effects of their regulation using various drugs, and the evaluation of their levels for diagnosis of degenerative diseases. In vivo electron paramagnetic resonance (EPR) imaging and spectroscopy are unique, noninvasive methods used to specifically detect and quantify paramagnetic species. However, two problems limit their application: the anatomic location of the EPR image in the animal body and the relative instability of the EPR probes. Our aim is to use EPR imaging to obtain physiologic and pathologic information on the mouse knee joint. This article reports the first in vivo EPR image of a small tissue, the mouse knee joint, with good resolution (≈ 160 μm) after intra-articular injection of a triarylmethyl radical EPR probe. It was obtained by combining EPR and x-ray micro-computed tomography for the first time and by taking into account the disappearance kinetics of the EPR probe during image acquisition to reconstruct the image. This multidisciplinary approach opens the way to high-resolution EPR imaging and local metabolism studies of radical species in vivo in different physiologic and pathologic situations.

High-Resolution EPR of a Bifunctional Spin Label Reveals an ADP Induced Structural Rearrangement of the Actin-Bound Myosin Catalytic Domain

We have used electron paramagnetic resonance (EPR) of a bifunctional spin label (BSL) to measure structural transitions of the catalytic domain (CD) in Dictyostelium myosin II. The use of BSL is a critical feature in this work. The bifunctional attachment of BSL eliminates most of the nanosecond motions characteristic of monofunctional labels, making it possible to measure protein structural transitions with a precision not previously achievable. Two double-Cys constructs were engineered with Cys residues at helical locations i and i+4 (494.498 and 639.643). Residues 494.498 are located on the relay helix and residues 639.643 are located on a helix adjacent to the relay helix. After BSL labeling, myosin was bound to actin in oriented muscle fibers, making it possible to measure helix orientation relative to the fiber axis. Spectra were acquired in APO and ADP states. Simulation of the 494.498.BSL spectra demonstrates that in APO and ADP states there are two highly ordered populations of the relay helix. APO and ADP spectra contain the same two spectral components, but the distribution of these two components differs between states. Similarly, spectra from the 639.643 construct reveal an ADP-induced structural transition, but the difference between APO and ADP spectra demonstrates a 3o rotation of the 639.643 helix. Our results demonstrate structural transitions of two helixes within the CD of actin-bound myosin associated with nucleotide binding. Measuring these transitions is essential to understanding the molecular mechanism of force generation. This is particularly true for the relay helix, which plays a key role in the coupling of myosin ATPase and motor function. Additionally, these results demonstrate the utility of BSL for measuring transitions in protein orientation, order, and dynamics.

ChemInform Abstract: Kinetics of Radical Processes by High Resolution Electron Paramagnetic Resonance

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ESR micro-imaging of LiNc-BuO crystals in PDMS: Spatial and spectral grain distribution

Microcrystals of lithium octa- n-butoxynaphthalocyanine (LiNc-BuO) in a bio-compatible and oxygen-permeable polymer matrix of poly-dimethyl-siloxane (PDMS) can be used for repetitive non-invasive imaging of oxygen in live specimens by means of mm-scale electron spin resonance (ESR) imaging. This probe denoted as “ oxychip” was characterized by high-resolution μm-scale ESR microcopy to reveal the fine details of its spatial and spectral properties. The ESR micro-images of a typical oxychip device showed that while the spatial distribution of the microcrystals in the polymer is fairly homogenous (as revealed by optical microscopy), the ESR signal originates only from a very few dominant crystals. Furthermore, spectral–spatial analysis in a microcrystal and a sub-microcrystal spatial resolution reveals that each crystal has a slightly different g-factor and also exhibits variations in linewidth, possibly due to the slightly different individual crystallization process.

Recent Advances in High-Frequency Electron Spin Resonance Detection Using a Microcantilever

Our recent developments in highly sensitive high-frequency electron spin resonance (ESR) using a microcantilever are reviewed. ESR signals of a Co Tutton salt microcrystal (<1 μg) have been detected at low temperature at frequencies up to 315 GHz under a static magnetic field using a microcantilever and a modulation technique. The achieved sensitivity is about 109 spins/G at 4.5 K. Moreover, we have shown that similar ESR detection using a microcantilever is possible up to 130 GHz under a pulsed magnetic field without using a modulation technique. The achieved sensitivity is about 1011 spins/G at 1.7 K. These results suggest that the ESR detection using a microcantilever is promising for applications to high-resolution and high-sensitivity terahertz ESR.

Magnetic detection of high-resolution electron spin resonance using a microcantilever in the millimeter-wave region up to 240 GHz

Highly sensitive magnetic detection of electron spin resonance (ESR) using a microcantilever is presented. By combining a modulation technique with the use of a piezoresistive cantilever, we successfully observed ESR signals of a tiny single crystal (mass<1 microg) of Co Tutton salt, Co(NH(4))(2)(SO(4))(2) x 6 H(2)O, in the frequency region of 80-240 GHz. The achieved spin sensitivity was approximately 10(9) spins/G at 4.5 K, providing promising applications to high-resolution and high-sensitivity terahertz ESR.

Large-scale synthesis of a persistent trityl radical for use in biomedical EPR applications and imaging

Tetrathiatriarylmethyl radicals are ideal spin probes for biological electron paramagnetic resonance (EPR) spectroscopy and imaging. The wide application of trityl radicals as biosensors of oxygen or other biological radicals was hampered by the lack of affordable large-scale syntheses. We report the large-scale synthesis of the Finland trityl radical using an improved addition protocol of the aryl lithium monomer to methylchloroformate. A new reaction for the formal one-electron reduction of trityl alcohols to trityl radicals using neat trifluoroacetic acid is reported as well. Initial applications show that the compound is very sensitive to molecular oxygen. It has already provided high-resolution EPR images on large aqueous samples and should be suitable for a broad range of in vivo applications.

High resolution EPR spectroscopy of C60F and C70F in solid argon: Reassignment of C70F regioisomers

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric C=C bonds of C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.

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.