Electron Paramagnetic Resonance Line Shape Research Articles

Stabilization of hydrogen atoms in aggregates of krypton nanoclusters immersed in superfluid helium

Impurity-helium condensates (IHCs) containing krypton and hydrogen atoms immersed in superfluid $^{4}\text{H}\text{e}$ have been studied via cw electron-spin-resonance (ESR) techniques. The IHCs are gel-like aggregates of nanoclusters composed of krypton and hydrogen atoms. We have found that very high average $(\ensuremath{\sim}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3})$ and local $(\ensuremath{\sim}6\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3})$ concentrations of hydrogen atoms can be stabilized in these samples. The analysis of ESR line positions and shapes shows that most of the H atom population resides in solid ${\text{H}}_{2}$ layers on the krypton nanocluster surfaces. High concentrations of atomic hydrogen achieved in IHCs provide an important step in the search for collective magnetic and quantum statistical phenomena at lower temperatures.

ESR probing of quantum critical phenomena in doped S = 1/2 AF quantum spin chain

The results of high-frequency (60–315 GHz) electron spin resonance (ESR) studies of CuGeO3 single crystals containing 0.9% Mn impurity are reported. The quantitative ESR line shape analysis shows that the low-temperature (T < 40 K) magnetic susceptibility of Cu2+ chains diverges as χ ∼ 1/Tα with the critical exponent α = 0.81±0.03 and therefore indicates the onset of a quantum critical (QC) regime. A scenario in which the disorder caused by the Mn impurity in quantum spin chains of CuGeO3 may lead to the coexistence of the quantum critical regime and the spin-Peierls dimerization is discussed. For the quantitative description of the temperature dependences of the line width and g-factor, a model assuming a crossover from the high-temperature semiclassical Nagata and Tazuke limit to the low-temperature quantum case described by Oshikawa and Affleck’s theory is suggested.

Demonstration That CobG, the Monooxygenase Associated with the Ring Contraction Process of the Aerobic Cobalamin (Vitamin B12) Biosynthetic Pathway, Contains an Fe-S Center and a Mononuclear Non-heme Iron Center

The ring contraction process that occurs during cobalamin (vitamin B(12)) biosynthesis is mediated via the action of two enzymes, CobG and CobJ. The first of these generates a tertiary alcohol at the C-20 position of precorrin-3A by functioning as a monooxygenase, a reaction that also forms a gamma lactone with the acetic acid side chain on ring A. The product, precorrin-3B, is then acted upon by CobJ, which methylates at the C-17 position and promotes ring contraction of the macrocycle by catalyzing a masked pinacol rearrangement. Here, we report the characterization of CobG enzymes from Pseudomonas denitrificans and Brucella melitensis. We show that both contain a [4Fe-4S] center as well as a mononuclear non-heme iron. Although both enzymes are active in vivo, the P. denitrificans enzyme was found to be inactive in vitro. Further analysis of this enzyme revealed that the mononuclear non-heme iron was not reducible, and it was concluded that it is rapidly inactivated once it is released from the bacterial cell. In contrast, the B. melitensis enzyme was found to be fully active in vitro and the mononuclear non-heme iron was reducible by dithionite. The reduced mononuclear non-heme was able to react with the oxygen analogue NO, but only in the presence of the substrate precorrin-3A. The cysteine residues responsible for binding the Fe-S center were identified by site-directed mutagenesis. A mechanism for CobG is presented.

Electron spin resonance inEuB6

Cavity measurements of a high frequency (60 GHz) electron spin resonance (ESR) have been carried our for a single crystal of ${\text{EuB}}_{6}$ at temperatures 4.2--50 K in magnetic field up to 7 T, which has been aligned along [001] crystallographic direction. It is found that in the case of homogeneous magnetic field in the sample the ESR spectrum of ${\text{EuB}}_{6}$ is formed by a single line at all temperatures including the ferromagnetic ordering region, whereas the gradient of magnetic field in the samples induces double peak ESR structure at low temperatures. For the quantitative description of the ESR line shape we suggested an analytical approach applicable for the cavity measurements of a metal with an arbitrary value of magnetic permeability including strongly magnetic case and obtained full set of the ESR parameters, namely oscillating magnetization ${M}_{0}$, $g$ factor, and linewidth. Our analysis has explained the visible resonance line shift and revealed the coincidence of the oscillating magnetization defining the resonance amplitude with the static magnetization. The anomalous growth of the linewidth below Curie temperature ${T}_{C}\ensuremath{\approx}15\text{ }\text{K}$ is observed. We argue that ESR in ${\text{EuB}}_{6}$ is not considerably affected by either interaction with the spin polarons or the magnetic phase separation and reflects merely the oscillation of the ${\text{Eu}}^{2+}$ localized magnetic moments, which can be well understood within mean-field approximation.

Solute Effects on Spin Labels at an Aqueous-Exposed Site in the Flap Region of HIV-1 Protease

The effects of solutes on spin-label mobility and protein conformation have been investigated with X-band continuous-wave and pulsed electron paramagnetic resonance (EPR) spectroscopy for spin labels attached to an aqueous-exposed site in the beta-hairpin flap region of HIV-1 protease. Specifically, we examined the effects of glycerol, sucrose, PEG3000, and Ficoll400 for four commonly used nitroxide spin labels and found that the largest perturbations to the EPR line shapes occur for solutions containing PEG3000 and glycerol. From comparisons of the spectral line shapes and distance distribution profiles of spin-labeled HIV-1 protease with and without inhibitor, it was concluded that solutes such as glycerol and PEG3000 alter the line shapes of the spin label in the beta-hairpin flaps of HIV-1 PR by modulation of spin-label mobility through changes in preferential interactions with the solutes. It is noteworthy that the high osmolality of the 40% glycerol solution did not alter the conformation of the flaps as determined from pulsed EPR distance measurements.

Dynamics of Natural Spin Defects in Boron Nitride Nanotubes

In this study, we investigated a possible delocalization of natural spin defect in boron nitride nanotubes (BNNTs). To this end, an electron paramagnetic resonance (EPR) spectroscopy was employed for the BNNTs prepared by balling milling and annealing method. As a result, the EPR lineshape revealed that the spin species were not only originated from a boron site neighboring nitrogen vacancy, but also fairly mobile along the long axis of the BNNTs. Such delocalized characteristics were also supported by the temperature dependence of the full widths at half maximum, deltaH(fwhm) and EPR susceptibilities.

Membrane Hydrocarbon Thickness Modulates the Dynamics of a Membrane Transport Protein

Nitroxide spin labels were incorporated into selected sites within the β-barrel of the bacterial outer-membrane transport protein BtuB by site-directed mutagenesis, followed by chemical modification with a methanethiosufonate spin label. The electron paramagnetic resonance lineshapes of the spin-labeled side chain (R1) from these sites are highly variable, and have spectral parameters that reflect secondary structure and local steric constraints. In addition, these lineshape parameters correlate with crystallographic structure factors for Cα carbons, suggesting that the motion of the spin label is modulated by both the local modes of motion of the spin label and the local dynamics of the protein backbone. Experiments performed as a function of lipid composition and sample temperature indicate that nitroxide spin labels on the exterior surface of BtuB, which face the membrane hydrocarbon, are not strongly influenced by the phase state of the bulk lipids. However, these spectra are modulated by membrane hydrocarbon thickness. Specifically, the values of the scaled mobility parameter for the R1 lineshapes are inversely proportional to the hydrocarbon thickness. These data suggest that protein dynamics and structure in BtuB are directly coupled to membrane hydrophobic thickness.

The Stability of PEDOT:PSS Films Monitored by Electron Spin Resonance

We used electron spin resonance (ESR) and electrical conductivity measurements to track the stability of poly(3,4-ethylene dioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) and ethylene glycol (EG)-treated PEDOT:PSS (EG-PEDOT:PSS) films in air. We observed that the ESR intensity of the PEDOT:PSS films decreased in air with oxidation, along with the conductivity. The rate of decrease depended on the thickness of the film. The EG-treated PEDOT:PSS films showed better stability for both the DC conductivity and the ESR intensity compared with untreated PEDOT:PSS films. PEDOT:PSS, oxidized-PEDOT:PSS, and EG-PEDOT:PSS, all showed a symmetric ESR line shape with the same g-value (g = 2.0037), indicative of polarons as the origin of paramagnetic spin resonance. Low-temperature (100 - 300 K) ESR measurements showed a temperature-independent spin density for all types of samples, in agreement with the metallic phases. The linear temperature-dependent increase of the resonance line width (ΔH pp ) was mostly ascribable to carrier hopping.

Estimation of mean and median pO 2 values for a composite EPR spectrum

Electron paramagnetic resonance (EPR)-based oximetry is capable of quantifying oxygen content in samples. However, for a heterogeneous environment with multiple pO 2 values, peak-to-peak linewidth of the composite EPR lineshape does not provide a reliable estimate of the overall pO 2 in the sample. The estimate, depending on the heterogeneity, can be severely biased towards narrow components. To address this issue, we suggest a postprocessing method to recover the linewidth histogram which can be used in estimating meaningful parameters, such as the mean and median pO 2 values. This information, although not as comprehensive as obtained by EPR spectral–spatial imaging, goes beyond what can be generally achieved with conventional EPR spectroscopy. Substantially shorter acquisition times, in comparison to EPR imaging, may prompt its use in clinically relevant models. For validation, simulation and EPR experiment data are presented.

High-frequency dynamic nuclear polarization using biradicals: A multifrequency EPR lineshape analysis

To date, the cross effect (CE) and thermal mixing (TM) mechanisms have consistently provided the largest enhancements in dynamic nuclear polarization (DNP) experiments performed at high magnetic fields. Both involve a three-spin electron-electron-nucleus process whose efficiency depends primarily on two electron-electron interactions--the interelectron distance R and the correct electron paramagnetic resonance (EPR) frequency separation that matches the nuclear Larmor frequency, /omega(e2)-omega(e1)/ = omega(n). Biradicals, for example, two 2,2,6,6-tetramethyl-piperidine-1-oxyls (TEMPOs) tethered with a molecular linker, can in principle constrain both the distance and relative g-tensor orientation between two unpaired electrons, allowing these two spectral parameters to be optimized for the CE and TM. To verify this hypothesis, we synthesized a series of biradicals--bis-TEMPO tethered by n ethylene glycol units (a.k.a. BTnE)--that show an increasing DNP enhancement with a decreasing tether length. Specifically at 90 K and 5 T, the enhancement grew from approximately 40 observed with 10 mM monomeric TEMPO, where the average R approximately 56 A corresponding to electron-electron dipolar coupling constant omega(d)2 pi = 0.3 MHz, to approximately 175 with 5 mM BT2E (10 mM electrons) which has R approximately 13 A with omega(d)2 pi = 24 MHz. In addition, we compared these DNP enhancements with those from three biradicals having shorter and more rigid tethers-bis-TEMPO tethered by oxalyl amide, bis-TEMPO tethered by the urea structure, and 1-(TEMPO-4-oxyl)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) TOTAPOL is of particular interest since it is soluble in aqueous media and compatible with DNP experiments on biological systems such as membrane and amyloid proteins. The interelectron distances and relative g-tensor orientations of all of these biradicals were characterized with an analysis of their 9 and 140 GHz continuous-wave EPR lineshapes. The results show that the largest DNP enhancements are observed with BT2E and TOTAPOL that have shorter tethers and the two TEMPO moieties are oriented so as to satisfy the matching condition for the CE.

On the Origin of the Polar Order of T4 Lysozyme on Planar Model Surfaces

Site directed spin labeling is used to investigate the origin of the macroscopic alignment of T4 lysozyme vectorially tethered to planar biomimetic surfaces. T4 lysozyme was adsorbed to a quartz-supported dioleoylphosphatidylcholine (DOPC) bilayer by selective binding of the histidine-tagged protein to functionalized headgroups (1,2-dioleoyl-sn-glycero-3-[[N(5-amino-1-carboxypentyl)iminodiacetic acid]succinyl], DOGS NTA) of the bilayer. This results in a polar oriented ensemble of proteins on the surface, which gives rise to angular-dependent electron paramagnetic resonance (EPR) spectra. In order to reveal the mechanism of the protein alignment, the influence of protein coverage on the order of the molecules was addressed. Along the lines described previously for a full monolayer (Jacobsen, et al. Biophys. J. 2005, 88, 4351), the polar orientation of the molecules was inferred from an analysis of the EPR line shape using the stochastic Liouville equation (SLE) approach developed by Freed and co-workers. The simulations reveal that the orientation of the protein is strongly determined by lateral protein-protein interactions. In comparison to the lipid bilayer, a fusion protein of T4 lysozyme (T4L) with Annexin XII was investigated, where the two-dimensional crystallization of Annexin XII on a dioleoylphosphatidylserine (DOPS) bilayer provides a surface layer of regularly anchored T4L molecules. For this system, it is found that the interaction between T4L and Annexin plays a more important role for understanding the structure in the adsorbed state.

Binding of MRI contrast agents to albumin: a high-field EPR study

High-field electron paramagnetic resonance (EPR) experiments to monitor binding of lipophilic Gd(III) complexes to human serum albumin (HSA) are described. It was observed that magnetic interactions between the nitroxide moiety ofn-doxyl-stearic acids bound to HSA and Gd(III) complexes resulted in broadening of nitroxide continuous-wave EPR spectra. The broadening effect can be well described by a one-parameter model of additional Lorentzian broadening At 95 GHz, continuous-wave EPR spectra from Gd(III) complexes are fully resolved from the nitroxide signal allowing for simultaneous and independent line shape analysis. Analysis of the line width broadening effects for spectra from a series ofn-doxyl-stearic acids bound to HSA indicated a progressive decrease of spin label-Gd(III) magnetic interactions along the fatty acid (FA) binding channel, consistent with binding of Gd-DOTAP complex in the vicinity of the main FA binding site. The substantial difference in spin label-metal interactions along the FA binding channel for lipophilic Gd(III) complexes with different chelates is indicative of binding to different sites. We also report measurements of dissociation constant for noncovalent binding of Gd(III) complexes to HSA on the basis of analyses of 95 GHz Gd(III) EPR line shapes.

Spin–Spin Interaction of TEMPO Molecular Chains Formed in an Organic One-Dimensional Nanochannel as Studied by Electron Spin Resonance (ESR)

Abstract The magnitude and dimensionality of spin–spin interactions among 2,2,6,6-tetramethyl-1-piperidinyloxyl radical (TEMPO) molecules in a 1D nanochannel consisting of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) were examined using electron spin resonance (ESR) spectroscopy. Dilution of TEMPO with 2,2,6,6-tetramethyl-1-piperidine (TEMP), a diamagnetic molecule, reduced the dipolar interaction in the TPP/(TEMPO/TEMP) inclusion compound (IC), leading to axially symmetric hyperfine coupling and g-tensors with a molar fraction of 0–0.45: A⊥ = 1.93 ± 0.03 mT and A|| = 0.7 ± 0.1 mT, and g⊥ = 2.0063 ± 0.0003 and g|| = 2.0068 ± 0.0005. The resonance line in TPP/(TEMPO/TEMP) IC narrowed when the molar fraction was more than 0.7, implying the existence of exchange narrowing. The ESR lineshape of TPP/TEMPO IC showed pure 1D spin diffusion between 139 and 166 K, while interchain exchange interactions contributed to the line shape in T &amp;gt; 166 K. The magnitude of intra-chain exchange interaction, |Jintra|/kB, in TPP/TEMPO IC depended on the temperature from ≈0.05 K in 139–166 K to 0.4 K at 383 K. This aspect implies that molecular motion strongly affects the spin–spin interaction between TEMPO molecules in the TPP crystal. The magnetic susceptibility of TPP/TEMPO IC was also measured, and it was found that the exchange interaction was antiferromagnetic.

Electron paramagnetic resonance line shape investigation of the 29Si hyperfine doublet of the E′γ center in a‐SiO2

AbstractWe report an experimental study by electron paramagnetic resonance (EPR) spectroscopy of the E′γ center and of its hyperfine structure, consisting in a pair of lines split by 42 mT. Our results show that two types of E′γ centers can be distinguished on the basis of the EPR line shapes of their main resonance line and hyperfine structure. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Stochastic Liouville equation treatment of the electron paramagnetic resonance line shape of an S-state ion in solution

The current approaches used for the analysis of electron paramagnetic resonance spectra of Gd3+ complexes suffer from a number of drawbacks. Even the elaborate model of [Rast et al., J. Chem. Phys. 113, 8724 (2000)] where the electron spin relaxation is explained by the modulation of the zero-field splitting (ZFS), by molecular tumbling (the so called static contribution), and deformations (transient contribution), is only readily applicable within the validity range of the Redfield theory [Advances in Magnetic Resonance, edited by J.-S. Waugh (Academic, New York, 1965), Vol. 1, p. 1], that is, when the ZFS is small compared to the Zeeman energy and the rotational and vibrational modulations are fast compared to the relaxation time. Spin labels (nitroxides and transition metal complexes) have been studied for years in systems that violate these conditions. The theoretical framework commonly used in such studies is the stochastic Liouville equation (SLE). The authors shall show how the physical model of Rast et al. can be cast into the SLE formalism, paying special attention to the specific problems introduced by the [Uhlenbeck and Ornstein, Phys. Rev. 36, 823 (1930)] process used to model the transient ZFS. The resulting equations are very general and valid for arbitrary correlation times, magnetic field strength, electron spin S, or symmetry. The authors demonstrate the equivalence of the SLE approach with the Redfield approximation for two well-known Gd3+ complexes.

On the local and itinerant properties of the ESR in YbRh2Si2

Below the Kondo temperature the heavy Fermion compound YbRh2Si2 shows a well defined electron spin resonance (ESR) with local Yb3+ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb3+ in YPd3:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic 174YbRh2Si2 we could exclude unresolved hyperfine contributions to the lineshape.

Power Saturation of ESR Signal in Ammonium Tartrate Exposed to60Co γ-Ray Photons, Electrons and Protons

In this paper we present an investigation of the electron spin resonance (ESR) line shape of ammonium tartrate (AT) dosimeters exposed to radiation with different linear energy transfer (LET). We exposed our dosimeters to gamma-ray photons ((60)Co), 7 MeV and 14 MeV initial energy electrons, and 19.3 MeV initial energy protons. The differences in the power saturation behavior of ESR spectra of AT irradiated with photons, electrons and protons could be correlated to the effective LET of the radiation beams. We analyzed the behavior of peak-to-peak amplitude as a function of microwave power, and we developed a fitting procedure that permits us to obtain the dependence of the homogeneity parameter of the line shape on the LET of the radiation using the Castner saturation theory. This simple procedure allows us to distinguish the LET of the radiation beam.

On the possible manifestation of harmonic-anharmonic dynamical transition in glassy media in electron paramagnetic resonance of nitroxide spin probes

Continuous wave (cw) electron paramagnetic resonance (EPR) and echo-detected (ED) EPR were applied to study molecular motions of nitroxide spin probes in glassy glycerol and o-terphenyl. A linear decrease with increasing temperature of the total splitting in the cw EPR line shape was observed at low temperatures in both solvents. Above some temperature points the temperature dependencies become sharper. Within the model of molecular librations, this behavior is in qualitative and quantitative agreement with the numerical data on neutron scattering and Mossbauer absorption for molecular glasses and biomolecules, where temperature dependence of the mean-squared amplitude of the vibrational motion was obtained. In analogy with these data the departure from linear temperature dependence in cw EPR may be ascribed to the transition from harmonic to anharmonic motion (this transition is called dynamical transition). ED EPR spectra were found to change drastically above 195 K in glycerol and above 245 K in o-terphenyl, indicating the appearance of anisotropic transverse spin relaxation. This appearance may also be attributed to the dynamical transition as an estimation shows the anisotropic relaxation rates for harmonic and anharmonic librational motions and because these temperature points correspond well to those known from neutron scattering for these solvents. The low sensitivity of ED EPR to harmonic motion and its high sensitivity to the anharmonic one suggests that ED EPR may serve as a sensitive tool to detect dynamical transition in glasses and biomolecules.

Variations ofg-tensor principal values in reduced [2Fe−2S] cluster of iron-sulfur proteins

This article discusses the present status of the theoretical and experimental studies of the electronic structure of the reduced [2Fe−2S] cluster with a special emphasis on theg-tensor variations in the Rieske-type proteins. The necessity of this analysis is dictated by the fact that the changes in the electron paramagnetic resonance line shape of the reduced cluster are broadly exploited as the basis for different mechanistic conclusions in the structure-function studies of the bc1/b6f families.

Quantum critical behavior induced by Mn impurity in CuGeO 3

The results of high frequency (60–315 GHz) studies of the electron spin resonance (ESR) in CuGeO 3 single crystals containing 0.9% of the Mn impurity are reported. The quantitative ESR line shape analysis shows that the low-temperature ( T<40 K) magnetic susceptibility of Cu 2+ chains diverges as χ∼1/ T α with the critical exponent α = 0.81 ± 0.03 and therefore indicates an onset of a quantum critical (QC) regime. The scenario, in which disorder caused by the Mn impurity in quantum spin chains in CuGeO 3 may lead to the co-existence of the QC regime and the spin-Peierls dimerisation, is discussed.