Loop-gap Resonator Research Articles

Electrochemical electron paramagnetic resonance utilizing loop gap resonators and micro-electrochemical cells.

A miniaturised electrochemical cell design for Electron Paramagnetic Resonance (EPR) studies is reported. The cell incorporates a Loop Gap Resonator (LGR) for EPR investigation of electrochemically generated radicals in aqueous (and other large dielectric loss) samples and achieves accurate potential control for electrochemistry by using micro-wires as working electrodes. The electrochemical behaviour of the cell is analysed with COMSOL finite element models and the EPR sensitivity compared to a commercial TE011 cavity resonator using 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) as a reference. The electrochemical EPR performance is demonstrated using the reduction of methyl viologen as a redox probe in both water and acetonitrile. The data reported herein suggest that sub-micromolar concentrations of radical species can be detected in aqueous samples with accurate potential control, and that subtle solution processes coupled to electron transfer, such as comproportionation reactions, can be studied quantitatively using EPR.

General expressions for the coupling coefficient, quality and filling factors for a cavity with an insert using energy coupled mode theory

A cavity (CV) with a dielectric resonator (DR) insert forms an excellent probe for the use in electron paramagnetic resonance (EPR) spectrometers. The probe’s coupling coefficient, κ, the quality factor, Q, and the filling factor, η are vital in assessing the EPR spectrometer’s performance. Coupled mode theory (CMT) is used to derive general expressions for these parameters. For large permittivity the dominating factor in κ is the ratio of the DR and CV cross sectional areas rather than the dielectric constant. Thus in some cases, resonators with low dielectric constant can couple much stronger with the cavity than do resonators with a high dielectric constant. When the DR and CV frequencies are degenerate, the coupled η is the average of the two uncoupled ones. In practical EPR probes the coupled η is approximately half of that of the DR. The Q of the coupled system generally depends on the eigenvectors, uncoupled frequencies (ω1,ω2) and the individual quality factors (Q1,Q2). It is calculated for different probe configurations and found to agree with the corresponding HFSS® simulations. Provided there is a large difference between the Q1, Q2 pair and the frequencies of DR and CV are degenerate, Q is approximately equal to double the minimum of Q1 and Q2. In general, the signal enhancement ratio, Iwithinsert/Iempty, is obtained from Q and η. For low loss DRs it only depends on η1/η2. However, when the DR has a low Q, the uncoupled Qs are also needed. In EPR spectroscopy it is desirable to excite only a single mode. The separation between the modes, Φ, is calculated as a function of κ and Q. It is found to be significantly greater than five times the average bandwidth. Thus for practical probes, it is possible to excite one of the coupled modes without exciting the other. The CMT expressions derived in this article are quite general and are in excellent agreement with the lumped circuit approach and finite numerical simulations. Hence they can also be applied to a loop-gap resonator in a cavity. For the design effective EPR probes, one needs to consider the κ, Q and η parameters.

Orthogonal resonators for pulse in vivo electron paramagnetic imaging at 250 MHz

A 250MHz bimodal resonator with a 19mm internal diameter for in vivo pulse electron paramagnetic resonance (EPR) imaging is presented. Two separate coaxial cylindrical resonators inserted one into another were used for excitation and detection. The Alderman–Grant excitation resonator (AGR) showed the highest efficiency among all the excitation resonators tested. The magnetic field of AGR is confined to the volume of the detection resonator, which results in highly efficient use of the radio frequency power. A slotted inner single loop single gap resonator (SLSG LGR), coaxial to the AGR, was used for signal detection. The resulting bimodal resonator (AG/LGR) has two mutually orthogonal magnetic field modes; one of them has the magnetic field in the axial direction. The resonator built in our laboratory achieved 40dB isolation over 20MHz bandwidth with quality factors of detection and excitation resonators of 36 and 11 respectively. Considerable improvement of the B1 homogeneity and EPR image quality in comparison with reflection loop-gap resonator of similar size and volume was observed.

Structure and Dynamics of Lens Lipid Membranes Derived from a Single Porcine Donor: High Field EPR Study

To obtain correct EPR line shapes, spin-lattice relaxation times, and bimolecular collision rates between spin labels and oxygen, samples must be thoroughly deoxygenated or equilibrated with a controlled oxygen partial pressure. These measurements are conveniently carried out using a gas permeable plastic sample tube of small diameter that fits in a loop-gap resonator. Flow of gas over the tube allows easy deoxygenation or controlled oxygenation of the sample. In the initial design of the W-band (94 GHz) loop-gap resonator, samples were equilibrated with gas at room temperature outside the resonator, transferred to a quartz capillary, and positioned in the resonator. In a recently designed W-band loop-gap resonator, the sample is positioned in a gas-permeable Teflon tubing inside the resonator, which allows measurements of extremely small volume (∼30 nL) of sample. We used this new design to measure properties of lens lipid membranes derived from total lipids extracted from both lenses (single donor) of a 2-year-old porcine cortex and nucleus. Detailed profiles of membrane fluidity and oxygen transport parameter were obtained from saturation recovery EPR. Analysis of conventional spectra using the microscopic-order macroscopic-disorder (MOMD) model provided rotational diffusion coefficients (R(perpendicular) and R(parallel)) and order parameters. Three different types of motion of lipid spin labels n-PC, T-PC, and CSL (ASL) with, respectively, nitroxide z-axis, x-axis, and y-axis parallel to the bilayer normal, are discussed. Results demonstrate that EPR at W-band has the potential to be a powerful tool for studying samples of small volume, ∼30 nL, obtained from eye lenses of a single human donor.

High‐permittivity solid ceramic resonators for high‐field human MRI

The properties of a ceramic-based annular dielectric resonator designed for 7 T MRI have been examined. Electromagnetic simulations and experimentally determined modal frequencies agree to within ~1%. The dependence of the resonance frequency of the degenerate quadrature HEM11 modes on hole diameter and shield diameter was also investigated. The constructed coil, with a 2.5 cm diameter hole, had an unloaded Q value of 400, which was reduced to 150 when loaded with a human finger. Simulated and experimental B1(+) maps show a high degree of homogeneity with a sensitivity of ~11.5 μT/√W at the centre. A comparison with a loop gap resonator showed an approximately 25% higher sensitivity for the dielectric resonator. High-resolution images of the digital interphalangeal (DIP) and proximal interphalangeal (PIP) joints of volunteers were acquired in imaging times of less than 2 min. Finally, novel methods of double tuning such ceramic resonators to two relatively close frequencies, e.g. proton and fluorine, have been shown.

ChemInform Abstract: Probe Development for Biosolids NMR Spectroscopy

AbstractReview: 62 refs.

Compact microwave cavity for high performance rubidium frequency standards

The design, realization, and characterization of a compact magnetron-type microwave cavity operating with a TE(011)-like mode are presented. The resonator works at the rubidium hyperfine ground-state frequency (i.e., 6.835 GHz) by accommodating a glass cell of 25 mm diameter containing rubidium vapor. Its design analysis demonstrates the limitation of the loop-gap resonator lumped model when targeting such a large cell, thus numerical optimization was done to obtain the required performances. Microwave characterization of the realized prototype confirmed the expected working behavior. Double-resonance and Zeeman spectroscopy performed with this cavity indicated an excellent microwave magnetic field homogeneity: the performance validation of the cavity was done by achieving an excellent short-term clock stability as low as 2.4 × 10(-13) τ(-1/2). The achieved experimental results and the compact design make this resonator suitable for applications in portable atomic high-performance frequency standards for both terrestrial and space applications.

Dielectric Resonator for K a-Band Pulsed EPR Measurements at Cryogenic Temperatures: Probehead Construction and Applications

The construction and performance of a Ka-band pulsed electron paramagnetic resonance (EPR) cryogenic probehead that incorporates dielectric resonator (DR) is presented. We demonstrate that the use of DR allows one to optimize pulsed double electron-electron resonance (DEER) measurements utilizing large resonator bandwidth and large amplitude of the microwave field B1 . In DEER measurements of Gd-based spin labels, use of this probe finally allows one to implement the potentials of Gd-based labels in distance measurements. Evidently, this DR is well suited to any applications requiring large B1-fields and resonator bandwidths, such as electron spin echo envelope modulation spectroscopy of nuclei having low magnetic moments and strong hyperfine interactions and double quantum coherence dipolar spectroscopy as was recently demonstrated in the application of a similar probe based on an loop-gap resonator and reported by Forrer et al. (J Magn Reson 190:280, 2008).

Rectangular loop-gap resonator with the light access to the sample

A modified rectangular loop-gap resonator for X-band electron paramagnetic resonance (EPR) studies of aqueous samples, enabling the light access, is described. Changes introduced into rectangular resonator geometry, previously presented in Piasecki et al. (1998) [1], and redesigned coupling structure lead to the better thermal and mechanical stability. The modified structure makes provision for the controlled light access to the sample placed in a flat cell during an EPR experiment. The sensitivity of the resonator for aqueous samples as well as an experimentally tested microwave magnetic field homogeneity are presented. Results of simulations and experimental tests indicate that the presence of light access holes in the resonator’s front side does not disturb the uniformity of microwave magnetic field distribution in the nodal plane. The optimal flat cell thickness for unsaturable and saturable aqueous samples has been calculated for this new structure. A modified rectangular geometry of the loop-gap resonator ensures a good performance for aqueous samples allowing its convenient and efficient light illumination during EPR signal recording .

Discrimination and Characterization of Cholesterol Crystalline Domains using EPR Spin-Labeling: Application to Lens Lipid Membranes

Saturation-recovery EPR at X-band (9.4 GHz) and W-band (94 GHz) was used to discriminate cholesterol crystalline domains (CCDs) in model phospholipid membranes and membranes made of pigs lens lipids. We used dual-probe saturation-recovery EPR approach with cholesterol analogue spin labels, CSL or ASL, and oxygen (hydrophobic) or NiEDDA (polar) relaxation agents. The CCD was present in all of the phospholipid membranes when the cholesterol-to-phospholipid mixing ratio exceeded the cholesterol solubility threshold. Saturation-recovery curves show that spin labels detect a single homogenous environment in membranes made from cortical lipids. However, in membranes made from nuclear lipids, two domains were detected and were assigned to the bulk phospholipid-cholesterol domain and the CCD, respectively. Profiles of the oxygen transport parameter (oxygen diffusion-concentration product) across these domains contain significant structural information. These profiles also allow calculating the oxygen permeability coefficient across domain. The evaluated upper limit of the oxygen permeability coefficient across the CCD at 37°C (34.4 cm/s) is significantly lower than across the water layer of the same thickness (85.9 cm/s), indicating that the CCD can form a barrier for oxygen transport in the lens nucleus. The new capabilities in measurements of the oxygen transport parameter at W-band are demonstrated and compared with results obtained at X-band. The loop-gap resonators used for W-band measurements have a sample volume of 30 nL, while the sample volume of the loop-gap resonator used for X-band is 3µL. These specifications ensure feasibility of experiments planned for samples with a limited sample volume. For example, the oxygen transport parameter profiles measured at X-band for lens lipid membranes were obtained for samples prepared from 50 - 100 eyes. Reliable profiles at W-band can be obtained based on samples prepared from one eye.

Structure of Apolipoprotein A-I N Terminus on Nascent High Density Lipoproteins

Apolipoprotein A-I (apoA-I) is the major protein component of high density lipoproteins (HDL) and a critical element of cholesterol metabolism. To better elucidate the role of the apoA-I structure-function in cholesterol metabolism, the conformation of the apoA-I N terminus (residues 6-98) on nascent HDL was examined by electron paramagnetic resonance (EPR) spectroscopic analysis. A series of 93 apoA-I variants bearing single nitroxide spin label at positions 6-98 was reconstituted onto 9.6-nm HDL particles (rHDL). These particles were subjected to EPR spectral analysis, measuring regional flexibility and side chain solvent accessibility. Secondary structure was elucidated from side-chain mobility and molecular accessibility, wherein two major α-helical domains were localized to residues 6-34 and 50-98. We identified an unstructured segment (residues 35-39) and a β-strand (residues 40-49) between the two helices. Residues 14, 19, 34, 37, 41, and 58 were examined by EPR on 7.8, 8.4, and 9.6 nm rHDL to assess the effect of particle size on the N-terminal structure. Residues 14, 19, and 58 showed no significant rHDL size-dependent spectral or accessibility differences, whereas residues 34, 37, and 41 displayed moderate spectral changes along with substantial rHDL size-dependent differences in molecular accessibility. We have elucidated the secondary structure of the N-terminal domain of apoA-I on 9.6 nm rHDL (residues 6-98) and identified residues in this region that are affected by particle size. We conclude that the inter-helical segment (residues 35-49) plays a role in the adaptation of apoA-I to the particle size of HDL.

RF coil considerations for short‐T2 MRI

With continuing hardware and pulse sequence advancements, modern MRI is gaining sensitivity to signals from short-T(2) (1)H species under practical experimental conditions. However, conventional MRI coils are typically not designed for this type of application, as they often contain proton-rich construction materials that may contribute confounding (1)H background signal during short-T(2) measurements. An example of this is shown herein. Separately, a loop-gap style coil was used to compare different coil construction materials and configurations with respect to observed (1)H background signal sizes in a small animal imaging system. Background signal sources were spatially identified and quantified in a number of different coil configurations. It was found that the type and placement of structural coil materials around the loop-gap resonator, as well as the coil's shielding configuration, are critical determinants of the coil's background signal size. Although this study employed a loop-gap resonator design, these findings are directly relevant to standard volume coils commonly used for MRI.

Electron paramagnetic resonance oxygen imaging of a rabbit tumor using localized spin probe delivery

Application of in vivo electron paramagnetic resonance (EPR) oxygen imaging (EPROI) to tumors larger than those of mice requires development of both instrumental and medical aspects of imaging. 250 MHz EPR oxygen imaging was performed using a loop-gap resonator with a volume exceeding 100 cm3. The paramagnetic spin probe was injected directly into the femoral artery feeding the rabbit leg/tumor. The authors present continuous wave and electron spin echo EPR oxygen images of a large size (4 cm) VX-2 tumor located on the leg of a New Zealand white rabbit. This study demonstrates the feasibility of continuous wave and electron spin echo oxygen imaging modalities for investigation of volumes of tumor and normal tissue relevant to large animals. The injection of the spin probe directly into the artery feeding a rabbit leg will allow one to reduce, by over one order of magnitude, the amount of spin probe used as compared to whole animal i.v. injection.

Lipid Composition Regulates the Orientation of Transmembrane Helices in HorA, an ABC Multidrug Transporter

ATP-binding cassette (ABC) transporters constitute a large class of molecular pumps whose central role in chemotherapy resistance has highlighted their clinical relevance. We investigated whether the lipid composition of the membrane affects the function and structure of HorA, a bacterial ABC multidrug transporter. When the transporter was reconstituted in a bilayer where phosphatidylethanolamine (PE), the main lipid of the bacterial membrane, was replaced with phosphatidylcholine (PC), ATP hydrolysis and substrate transport became uncoupled. Although ATPase activity was maintained, HorA lost its ability to extrude the prototypical substrate Hoechst33342. Attenuated Total Reflection-Fourier Transform Infrared spectroscopy (ATR-FTIR) revealed that, although the secondary structure of the protein was unaffected, the orientation of the transmembrane helices (TM) was modified by the change in lipid composition. The orientation of the backbone carbonyls indicated that the helices opened wider in PE versus PC-containing liposomes, with 10 degrees difference. This was supported by hydrogen/deuterium exchange studies showing increased protection of the backbone from the solvent in PC-containing liposomes. Electron Paramagnetic Resonance was used to further probe the structural change. In the PC-containing liposomes we observed increased mobility of the spin label in TM4, along with increased exposure to molecular oxygen, used as a hydrophobic quencher. This indicates that the lipid change induced modification of the orientation of TM4, exposing Cys-180 to the lipid phase. The lipid composition of the bilayer thus modulates the structure of HorA, and in turn its ability to extrude its substrates.

In vivo temporal EPR study using a region-selected intensity determination method to estimate cerebral reducing ability in rats treated with olanzapine

Region-selected intensity determination (RSID) is a method for obtaining the temporal changes in electron paramagnetic resonance (EPR) signal intensity from a target region, without the use of complicated procedures employed in the conventional imaging methods. An in vivo 700-MHz radio frequency EPR spectrometer equipped with a bridged loop-gap resonator was used with the RSID method to estimate intracerebral reducing ability in the rat following acute administration of olanzapine (OZP) or haloperidol (HPD). To this end, temporal changes in EPR signal intensity of target regions (the striatum and the prefrontal cortex) of rats which had received a blood-brain-barrier-permeable nitroxide radical (3-hydroxymethyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl) via an intravenous route were observed. The half-lives of EPR signal intensity in both regions of OZP- or HPD-treated rats were significantly longer than in control animals. This indicated that reducing abilities of the striatum and cerebral cortex decreased in the rats to which either OZP or HPD had been acutely administered.

W-band frequency-swept EPR

This paper describes a novel experiment on nitroxide radical spin labels using a multiarm EPR W-band bridge with a loop-gap resonator (LGR). We demonstrate EPR spectroscopy of spin labels by linear sweep of the microwave frequency across the spectrum. The high bandwidth of the LGR, about 1 GHz between 3 dB points of the microwave resonance, makes this new experiment possible. A frequency-tunable yttrium iron garnet (YIG) oscillator provides sweep rates as high as 1.8 × 10 5 GHz/s, which corresponds to 6.3 kT/s in magnetic field-sweep units over a 44 MHz range. Two experimental domains were identified. In the first, linear frequency sweep rates were relatively slow, and pure absorption and pure dispersion spectra were obtained. This appears to be a practical mode of operation at the present level of technological development. The main advantage is the elimination of sinusoidal magnetic field modulation. In the second mode, the frequency is swept rapidly across a portion of the spectrum, and then the frequency sweep is stopped for a readout period; FID signals from a swept line oscillate at a frequency that is the difference between the spectral position of the line in frequency units and the readout position. If there is more than one line, oscillations are superimposed. The sweep rates using the YIG oscillator were too slow, and the portion of the spectrum too narrow to achieve the full EPR equivalent of Fourier transform (FT) NMR. The paper discusses technical advances required to reach this goal. The hypothesis that trapezoidal frequency sweep is an enabling technology for FT EPR is supported by this study.

To Determine the Structure of Vimentin Head Domain Using SDSL-EPR Approach

Cysteines were placed in vimentin head spanning 1-108 positions using basic genetic engineering approaches. The mutants were expressed in bacteria, purified by FPLC, and spin-labeled using O-87500 [MTSL-d15]. Labeled proteins were assembled into filaments by dialysis from 8M urea and filament assembly was checked by electron microscopy. EPR measurements were carried out in JEOL X-band spectrometer fitted with a loop-gap resonator. The SDSL EPR spectral data indicated that 48/50 positions tested displayed spectral broadening indicating the close proximity of these positions (<2.1 nm). Mixing protein spin labeled at a given residue, with protein labeled at sites slightly “up”- or downstream, decreased the level of dipolar interaction in all cases. This data provide strong evidence that the polypeptide backbones of head domains remain in close proximity, forming a symmetric structure which folds back on the rod domain, bringing residues 17 of the head and 137 of the rod into close proximity. By monitoring in vitro assembly process, we show that head-head interactions occur early in the assembly process. We show that spin labels in the region of residues 50 and 103 display high level of mobility at protofilament stage of assembly but becoming more compacted and motionally constrained upon assembly into intact filaments, suggesting that head domain structure is dynamic and changes during assembly. Finally, because the vimentin head domain is the major site of phosphorylation, we compared specific spin-spin interactions in the phosphorylated state, and have identified distinct structural changes resulting from phosphorylation of head domain. This report presents the first evidence-based structural model for the head domain of any IF protein. We provide data which shows that head domain structure is dynamic, changing with both assembly into filaments, but also with phosphorylation, a physiologic regulator of vimentin assembly/disassembly.

Determination of Quality Factor for Highly Overcoupled EPR Resonators

We reported determination of the loaded quality factor (Q) of highly overcoupled (dielectric, loop-gap, and cavity) resonators used in time-domain electron paramagnetic resonance. We introduced a microwave absorber into resonators and achieved critical-coupling. Due to the deep “Q-dip” of critical-coupling, we can easily determine the loaded Q as low as 10. The loaded Q of resonators with and without the microwave absorber was examined under various overcoupling conditions. We found that the radiation Q (Q r) can be calculated from the loaded Q of the resonator that contains the microwave absorber. We proposed a simple model that represents the loaded Q of the overcoupled resonator in terms of two parameters, Q 0 and Q r. Q 0 is the effective unloaded Q of the resonator determined for the critically coupled resonator without the microwave absorber and is independent of a degree of coupling. The model can be applied to overcoupling in which the coupling parameter (Q 0/Q r) is in the range of 1 to ca. 20.

Coarse and Fine Control of the EPR Frequency of a Loop-Gap Resonator Using a Single-Turn Coil with a Varactor Diode Attached

Coarse control and fine control of the resonant frequency of a loop-gap resonator (LGR) operating at an electron paramagnetic resonance (EPR) frequency of ca. 650 MHz were achieved using a single-turn coil with a varactor diode attached (a frequency shift coil). When the distance between the LGR and the frequency shift coil was changed from 15 to 10 mm under the condition of constant voltage to the varactor diode (0 V), a shift of the resonant frequency of the LGR of ca. 20 MHz was observed (coarse frequency control). When the voltage applied to the varactor diode was changed from 0 to 15 V at the same distance between the LGR and the frequency shift coil (10 mm), a shift of the resonant frequency of the LGR of ca. 200 kHz was observed (fine frequency control). There were no significant changes in sensitivity of EPR measurements of a phantom (comprised of agar with a nitroxide radical and physiological saline solution) without and with the frequency shift coil. The EPR sensitivity did not change discernibly when the resonant frequency was shifted by the frequency shift coil. Furthermore, radio-frequency phase adjustment for homodyne detection could be performed by using the frequency shift coil without applying frequency modulation to the carrier wave.

High Q calcium titanate cylindrical dielectric resonators for magnetic resonance microimaging

At high magnetic fields radiation losses, wavelength effects, self-resonance, and the high resistance of typical components all contribute to increased losses in conventional RF coil designs. High permittivity ceramic dielectric resonators create strong uniform magnetic fields in a compact structure at high frequencies and can potentially solve some of the challenges of high field coil design. In this study an NMR probe was constructed for operation at 600 MHz (14.1 T) using an inductively fed CaTiO 3 (relative permittivity of 156) cylindrical hollow bore dielectric resonator. The design has an unmatched Q value greater than 2000, and the electric field is largely confined to the dielectric itself, with near zero values in the hollow bore which accommodates the sample. Experimental and simulation mapping of the RF field show good agreement, with the ceramic resonator giving a pulse width approximately 25% less than a loop gap resonator of similar inner dimensions. High resolution images, with voxel dimensions less than 50 μm 3, have been acquired from fixed zebrafish samples, showing excellent delineation of several fine structures.