One- and Two-Dimensional Electron Paramagnetic Resonance Imaging in Skin

EPR imaging with modulated field gradients provides the possibility for obtaining an EPR spectrum in a selected volume We demonstrate the feasibility of X-band (9.5GHz) electron paramagnetic resonance (EPR) imaging in skin biopsies of hairless mice. One- (ID) and two-dimensional (2D) EPR images of the persistent free radical di-tertiary-butyl-nitroxide are measured. At a microwave frequency of 9.5 GHz (X-band), 2D images are obtained in skin biopsies with an actual point distinction resolution of 25 μm. In a biological model system. 2D images are measured at L-band frequency (2.0 GHz) with a pixel resolution of 61 μm. and a theoretical spatial resolution of 12.5 μm. In combination with the spin labeling and spin trapping technique. EPR imaging is the most direct approach to analyzing spatial distribution of physico-chemical properties in skin, such as membrane fluidity and polarity. as well as detection of free radicals.     

One- and Two-Dimensional Electron Paramagnetic Resonance Imaging in Skin

EPR imaging with modulated field gradients provides the possibility for obtaining an EPR spectrum in a selected volume We demonstrate the feasibility of X-band (9.5GHz) electron paramagnetic resonance (EPR) imaging in skin biopsies of hairless mice. One- (ID) and two-dimensional (2D) EPR images of the persistent free radical di-tertiary-butyl-nitroxide are measured. At a microwave frequency of 9.5 GHz (X-band), 2D images are obtained in skin biopsies with an actual point distinction resolution of 25 μm. In a biological model system. 2D images are measured at L-band frequency (2.0 GHz) with a pixel resolution of 61 μm. and a theoretical spatial resolution of 12.5 μm. In combination with the spin labeling and spin trapping technique. EPR imaging is the most direct approach to analyzing spatial distribution of physico-chemical properties in skin, such as membrane fluidity and polarity. as well as detection of free radicals.     

Measurement of oxygen concentrations in the intact beating heart using electron paramagnetic resonance spectroscopy: A technique for measuring oxygen concentrationsin situ

Electron paramagnetic resonance (EPR) spectroscopy can be applied to measure oxygen concentrations in cells and tissues. Oxygen is paramagnetic, and thus it interacts with a free radical label resulting in a broadening of the observed linewidth. Recently we have developed instrumentation in order to enable the performance of EPR spectroscopy and EPR oximetry in the intact beating heart. This spectrometer consists of 1–2-GHz microwave bridge with the source locked to the resonant frequency of a specially designed lumped circuit resonator. This technique is applied to measure the kinetics of the uptake and clearance of different free radical labels. It is demonstrated that this technique can be used to noninvasively measure tissue oxygen concentration. In addition, rapid scan EPR measurements can be performed enabling gated millisecond measurements of oxygen concentrations to be performed over the cardiac cycle. Thus, low-frequency EPR spectroscopy offers great promise in the study of tissue oxygen concentrations and the role of oxygen in metabolic control.     

Use of Rapid-Scan EPR to Improve Detection Sensitivity for Spin-Trapped Radicals

The short lifetime of superoxide and the low rates of formation expected in vivo make detection by standard continuous wave (CW) electron paramagnetic resonance (EPR) challenging. The new rapid-scan EPR method offers improved sensitivity for these types of samples. In rapid-scan EPR, the magnetic field is scanned through resonance in a time that is short relative to electron spin relaxation times, and data are processed to obtain the absorption spectrum. To validate the application of rapid-scan EPR to spin trapping, superoxide was generated by the reaction of xanthine oxidase and hypoxanthine with rates of 0.1–6.0 μM/min and trapped with 5-tert-butoxycarbonyl-5-methyl-1-pyrroline-N-oxide (BMPO). Spin trapping with BMPO to form the BMPO-OOH adduct converts the very short-lived superoxide radical into a more stable spin adduct. There is good agreement between the hyperfine splitting parameters obtained for BMPO-OOH by CW and rapid-scan EPR. For the same signal acquisition time, the signal/noise ratio is >40 times higher for rapid-scan than for CW EPR. Rapid-scan EPR can detect superoxide produced by Enterococcus faecalis at rates that are too low for detection by CW EPR.     

Sensitive detection and localization of hydroxyl radical production in cucumber roots and Arabidopsis seedlings by spin trapping electron paramagnetic resonance spectroscopy

As reactive oxygen species are important for many fundamental biological processes in plants, specific and sensitive techniques for their detection in vivo are essential. In particular, the analysis of hydroxyl radical (OH*) formation in biological reactions has rarely been attempted. Here, it is shown that spin trapping electron paramagnetic resonance (EPR) spectroscopy allows the detection and quantitative estimation of OH* production in vivo in one single cucumber seedling root. It is possible to localize the OH* production site to the growth zone of the root by varying the position of the intact seedling inside the resonator cavity of the EPR spectrometer. Moreover, the demonstration of impaired OH* formation in the root of the Arabidopsis mutant rhd2 impaired in a superoxide-producing Nicotimamide adenine dinucleotide phosphate (NADPH) oxidase has been accomplished. Spin trapping EPR provides a valuable tool for analyzing the production of OH*in vivo with high resolution in small tissue samples.     

Multi-frequency and high-field EPR study of manganese(III) protoporphyrin IX reconstituted myoglobin with an S=2 integer electron spin

We investigate the electronic state of Mn(III) center with an integer electron spin S=2 in the manganese(III) protoporphyrin IX reconstituted myoglobin, Mn(III)Mb, by means of multi-frequency electron paramagnetic resonance (MFEPR) spectroscopy. Using a bimodal cavity resonator, X-band EPR signal from Mn(III) center in the Mn(III)Mb was observed near zero-field region. The temperature dependence of this signal indicates a negative axial zero-field splitting value, D<0. The EPR analysis shows that this signal is attributed to the transition between the closely spaced M(s)=+/-2 energy levels for the z-axis, corresponding to the heme normal. To determine the zero-field splitting (ZFS) parameters, EPR experiments on the Mn(III)Mb were performed at various temperatures for some frequencies between 30GHz and 130GHz and magnetic fields up to 14T. We observed several EPR spectra which are analyzed with a spin Hamiltonian for S=2, yielding highly accurate ZFS parameters; D=-3.79cm(-1) and |E|=0.08cm(-1) for an isotropic g=2.0. These ZFS parameters are compared with those in some Mn(III) complexes and Mn(III) superoxide dismutase (SOD), and effects on these parameters by the coordination and the symmetry of the ligands are discussed. To the best of our knowledge, these EPR spectra in the Mn(III)Mb are the very first MFEPR spectra at frequencies higher than Q-band in a metalloprotein with an integer spin.     

Electron paramagnetic resonance imaging of nitric oxide organ distribution in lipopolysuccaride treated mice

The recent development of electron paramagnetic resonance (EPR) permits its application for in vivo studies of nitric oxide (NO). In this study, we tried to obtain 3D EPR images of endogenous NO in the abdominal organs of lipopolysuccaride (LPS) treated mice. Male ICR mice, each weighing about 30 g, received 10 mg/kg of LPS intraperitoneally. Six hours later, a spin trapping reagent comprised of iron and an N-dithiocarboxy sarcosine complex (Fe(DTCS)₂, Fe 200 mM, DTCS/Fe = 3) were injected subcutaneously. Two hours after this treatment, the mice were fixed in a plastic holder and set in the EPR system, equipped with a loop-gap resonator and a 1 GHz microwave. NO was detected as an NO-Fe(DTCS)₂ complex, which had a characteristic 3-line EPR spectrum. NO-Fe(DTCS)₂ complexes in organ homogenates were also measured using a conventional X-band EPR system. NO-Fe(DTCS)₂ spectra were obtained in the upper abdominal area of LPS treated mice at 8 h after the LPS injection. 3D EPR tiled and stereoscopic images of the NO distribution in the hepatic and renal areas were obtained at the same time. The NO-Fe(DTCS)₂ distribution in abdominal organs was confirmed in each organ homogenate using conventional X-band EPR. This is the first known EPR image of NO in live mice kidneys.     

The Effect of Oxygen at Physiological Levels on the Detection of free Radical Intermediates by Electron Paramagnetic Resonance

It is well known that oxygen enhances Che relaxation of free radical EPR probes through spin lattice and Heisenberg spin-spin interactions with consequent effect on the line height and width. The two relaxation processes have opposing effects on the signal heights and depend on the concentration of oxygen, the incident microwave power, and the presence of other paramagnetic species. During EPR studies of chemical, biochemical, and cellular processes involving free radicals, molecular oxygen has significant magnetic influence on the EPR signal intensity of the free radical species under investigation in addition to affecting the rates of production of the primary species and the stability of the spin adduct nitroxides. These effects are often overlooked and can cause artifacts and lead to erroneous interpretation. In the present study, the effects of oxygen and ferricyanide on the EPR signal height of stable and persistent spin adduct nitroxides at commonly employed microwave powers were examined. The results show that under commonly adopted EPR spectrometer instrumental conditions, artifactual changes in the EPR signal of spin adducts occur and the best way to avoid them is by keeping the oxygen level constant using a gas-permeable cell.     

Narrow-Band Generation of Plasma Relativistic Microwave Generator

Plasma Physics Reports - Operation of the plasma relativistic microwave generator is considered in a frequency range of 1‒5&nbsp;GHz. From data on the generation frequencies and resonator...     

High field/high frequency saturation transfer electron paramagnetic resonance spectroscopy: increased sensitivity to very slow rotational motions

Saturation transfer electron paramagnetic resonance (ST-EPR) spectroscopy has been employed to characterize the very slow microsecond to millisecond rotational dynamics of a wide range of nitroxide spin-labeled proteins and other macromolecules in the past three decades. The vast majority of this previous work has been carried out on spectrometers that operate at X-band ( approximately 9 GHz) microwave frequency with a few investigations reported at Q-band ( approximately 34 GHz). EPR spectrometers that operate in the 94-250-GHz range and that are capable of making conventional linear EPR measurements on small aqueous samples have now been developed. This work addresses potential advantages of utilizing these same high frequencies for ST-EPR studies that seek to quantitatively analyze the very slow rotational dynamics of spin-labeled macromolecules. For example, the uniaxial rotational diffusion (URD) model has been shown to be particularly applicable to the study of the rotational dynamics of integral membrane proteins. Computational algorithms have been employed to define the sensitivity of ST-EPR signals at 94, 140, and 250 GHz to the correlation time for URD, to the amplitude of constrained URD, and to the orientation of the spin label relative to the URD axis. The calculations presented in this work demonstrate that these higher microwave frequencies provide substantial increases in sensitivity to the correlation time for URD, to small constraints in URD, and to the geometry of the spin label relative to the URD axis as compared with measurements made at X-band. Moreover, the calculations at these higher frequencies indicate sensitivity to rotational motions in the 1-100-ms time window, particularly at 250 GHz, thereby extending the slow motion limit for ST-EPR by two orders of magnitude relative to X- and Q-bands.     

A Study of Dielectric Properties of Proteinuria between 0.2 GHz and 50 GHz

This paper investigates the dielectric properties of urine in normal subjects and subjects with chronic kidney disease (CKD) at microwave frequency of between 0.2 GHz and 50 GHz. The measurements were conducted using an open-ended coaxial probe at room temperature (25°C), at 30°C and at human body temperature (37°C). There were statistically significant differences in the dielectric properties of the CKD subjects compared to those of the normal subjects. Statistically significant differences in dielectric properties were observed across the temperatures for normal subjects and CKD subjects. Pearson correlation test showed the significant correlation between proteinuria and dielectric properties. The experimental data closely matched the single-pole Debye model. The relaxation dispersion and relaxation time increased with the proteinuria level, while decreasing with the temperature. As for static conductivity, it increased with proteinuria level and temperature.     

Exploring the Structure of the 100 Amino-Acid Residue Long N-Terminus of the Plant Antenna Protein CP29

The structure of the unusually long (∼100 amino-acid residues) N-terminal domain of the light-harvesting protein CP29 of plants is not defined in the crystal structure of this membrane protein. We studied the N-terminus using two electron paramagnetic resonance (EPR) approaches: the rotational diffusion of spin labels at 55 residues with continuous-wave EPR, and three sets of distances with a pulsed EPR method. The N-terminus is relatively structured. Five regions that differ considerably in their dynamics are identified. Two regions have low rotational diffusion, one of which shows α-helical character suggesting contact with the protein surface. This immobile part is flanked by two highly dynamic, unstructured regions (loops) that cover residues 10–22 and 82–91. These loops may be important for the interaction with other light-harvesting proteins. The region around residue 4 also has low rotational diffusion, presumably because it attaches noncovalently to the protein. This section is close to a phosphorylation site (Thr-6) in related proteins, such as those encoded by the Lhcb4.2 gene. Phosphorylation might influence the interaction with other antenna complexes, thereby regulating the supramolecular organization in the thylakoid membrane.     

A biresonant plasma source based on a gapped linear microwave vibrator

The operating principle of a novel microwave plasma source—a linear microwave vibrator with a gap—is discussed. The source is placed on a microwave-transparent window of a chamber filled with a plasma-forming gas (argon or methane). The device operation is based on the combination of two resonances—geometric and plasma ones. The results of experimental tests of the source are presented. For a microwave frequency of 2.45 GHz, microwave power of ≤1 kW, and plasma-forming gas pressure in the range 5 × 10⁻²–10⁻¹ Torr, the source is capable of filling the reactor volume with a plasma having an electron density of about 10¹² cm⁻³ and electron temperature of a few electronvolts.     

Folding of the cocaine aptamer studied by EPR and fluorescence spectroscopies using the bifunctional spectroscopic probe Ç

The cocaine aptamer is a DNA molecule that binds cocaine at the junction of three helices. The bifunctional spectroscopic probe Ç was incorporated independently into three different positions of the aptamer and changes in structure and dynamics upon addition of the cocaine ligand were studied. Nucleoside Ç contains a rigid nitroxide spin label and can be studied directly by electron paramagnetic resonance (EPR) spectroscopy and fluorescence spectroscopy after reduction of the nitroxide to yield the fluoroside Ç^f. Both the EPR and the fluorescence data for aptamer 2 indicate that helix III is formed before cocaine binding. Upon addition of cocaine, increased fluorescence of a fully base-paired Ç^f, placed at the three-way junction in helix III, was observed and is consistent with a helical tilt from a coaxial stack of helices II and III. EPR and fluorescence data clearly show that helix I is formed upon addition of cocaine, concomitant with the formation of the Y-shaped three-way helical junction. The EPR data indicate that nucleotides in helix I are more mobile than nucleotides in regular duplex regions and may reflect increased dynamics due to the short length of helix I.     

Site-directed spin labeling measurements of nanometer distances in nucleic acids using a sequence-independent nitroxide probe

In site-directed spin labeling (SDSL), local structural and dynamic information is obtained via electron paramagnetic resonance (EPR) spectroscopy of a stable nitroxide radical attached site-specifically to a macromolecule. Analysis of electron spin dipolar interactions between pairs of nitroxides yields the inter-nitroxide distance, which provides quantitative structural information. The development of pulse EPR methods has enabled such distance measurements up to 70Å in bio-molecules, thus opening up the possibility of SDSL global structural mapping. This study evaluates SDSL distance measurement using a nitroxide (designated as R5) that can be attached, in an efficient and cost-effective manner, to a phosphorothioate backbone position at arbitrary DNA or RNA sequences. R5 pairs were attached to selected positions of a dodecamer DNA duplex with a known NMR structure, and eight distances, ranging from 20 to 40Å, were measured using double electron-electron resonance (DEER). The measured distances correlated strongly (R² = 0.98) with the predicted values calculated based on a search of sterically allowable R5 conformations in the NMR structure, thus demonstrating accurate distance measurements using R5. Furthermore, distance measurement in a 42 kD DNA was demonstrated. The results establish R5 as a sequence-independent probe for global structural mapping of DNA and DNA–protein complexes.     

Two sub-states of the red2 state of methyl-coenzyme M reductase revealed by high-field EPR spectroscopy

Methyl-coenzyme M reductase (MCR) catalyzes the formation of methane from methyl-coenzyme M and coenzyme B in methanogenic archaea. The enzyme has two structurally interlinked active sites embedded in an α₂β₂γ₂ subunit structure. Each active site has the nickel porphyrinoid F₄₃₀ as a prosthetic group. In the active state, F₄₃₀ contains the transition metal in the Ni(I) oxidation state. The active enzyme exhibits an axial Ni(I)-based continuous wave (CW) electron paramagnetic resonance (EPR) signal, called red1a in the absence of substrates or red1c in the presence of coenzyme M. Addition of coenzyme B to the MCR-red1 state can partially and reversibly convert it into the MCR-red2 form, which shows a rhombic Ni(I)-based EPR signal (at X-band microwave frequencies of approximately 9.4 GHz). In this report we present evidence from high-field/high-frequency CW EPR spectroscopy (W-band, microwave frequency of approximately 94 GHz) that the red2 state consists of two substates that could not be resolved by EPR spectroscopy at X-band frequencies. At W-band it becomes apparent that upon addition of coenzyme B to MCR in the red1c state, two red2 EPR signals are induced, not one as was previously believed. The first signal is the well-characterized (ortho)rhombic EPR signal, thus far called red2, while the second previously unidentified signal is axial. We have named the two substates MCR-red2r and MCR-red2a after their rhombic and axial signals, respectively. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Examples of high-frequency EPR studies in bioinorganic chemistry

Low-temperature EPR spectroscopy with frequencies between 95 and 345 GHz and magnetic fields up to 12 T has been used to study metal sites in proteins or inorganic complexes and free radicals. The high-field EPR method was used to resolve g-value anisotropy by separating it from overlapping hyperfine couplings. The presence of hydrogen bonding interactions to the tyrosyl radical oxygens in ribonucleotide reductases were detected. At 285 GHz the g-value anisotropy from the rhombic type 2 Cu(II) signal in the enzyme laccase has its g-value anisotropy clearly resolved from slightly different overlapping axial species. Simple metal site systems with S>1/2 undergo a zero-field splitting, which can be described by the spin Hamiltonian. From high-frequency EPR, the D values that are small compared to the frequency (high-field limit) can be determined directly by measuring the distance of the outermost signal to the center of the spectrum, which corresponds to (2 S-1)* mid R: Dmid R: For example, D values of 0.8 and 0.3 cm(-1) are observed for S=5/2 Fe(III)-EDTA and transferrin, respectively. When D values are larger compared to the frequency and in the case of half-integer spin systems, they can be obtained from the frequency dependence of the shifts of g(eff), as observed for myoglobin in the presence ( D=5 cm(-1)) or absence ( D=9.5 cm(-1)) of fluoride. The 285 and 345 GHz spectra of the Fe(II)-NO-EDTA complex show that it is best described as a S=3/2 system with D=11.5 cm(-1), E=0.1 cm(-1), and g(x)= g(y)= g(z)=2.0. Finally, the effects of HF-EPR on X-band EPR silent states and weak magnetic interactions are demonstrated.     

Structure and dynamics of the force-generating domain of myosin probed by multifrequency electron paramagnetic resonance

Spin-labeling and multifrequency EPR spectroscopy were used to probe the dynamic local structure of skeletal myosin in the region of force generation. Subfragment 1 (S1) of rabbit skeletal myosin was labeled with an iodoacetamide spin label at C707 (SH1). X-and W-band EPR spectra were recorded for the apo state and in the presence of ADP and nucleotide analogs. EPR spectra were analyzed in terms of spin-label rotational motion within myosin by fitting them with simulated spectra. Two models were considered: rapid-limit oscillation (spectrum-dependent on the orientational distribution only) and slow restricted motion (spectrum-dependent on the rotational correlation time and the orientational distribution). The global analysis of spectra obtained at two microwave frequencies (9.4 GHz and 94 GHz) produced clear support for the second model and enabled detailed determination of rates and amplitudes of rotational motion and resolution of multiple conformational states. The apo biochemical state is well-described by a single structural state of myosin (M) with very restricted slow motion of the spin label. The ADP-bound biochemical state of myosin also reveals a single structural state (M*, shown previously to be the same as the post-powerstroke ATP-bound state), with less restricted slow motion of the spin label. In contrast, the extra resolution available at 94 GHz reveals that the EPR spectrum of the S1.ADP.V_i-bound biochemical state of myosin, which presumably mimics the S1.ADP.P_i state, is resolved clearly into three spectral components (structural states). One state is indistinguishable from that of the ADP-bound state (M*) and is characterized by moderate restriction and slow motion, with a mole fraction of 16%. The remaining 84% (M**) contains two additional components and is characterized by fast rotation about the x axis of the spin label. After analyzing EPR spectra, myosin ATPase activity, and available structural information for myosin II, we conclude that post-powerstroke and pre-powerstroke structural states (M* and M**) coexist in the S1.ADP.V_i biochemical state. We propose that the pre-powerstroke state M** is characterized by two structural states that could reflect flexibility between the converter and N-terminal domains of myosin.     

Progress towards a DNA-based model system for the study of electron spin-spin interactions

A DNA-based model system is described for studying electron spin-spin interactions between a paramagnetic metal ion and a nitroxide spin label. The modified base deoxythymidine-EDTA (dT-EDTA) chelates the divalent or trivalent metal ion and produces a new feature in the circular dichroism (CD) spectra that makes it possible to monitor local DNA melting. Based on the results of optical and electron paramagnetic resonance (EPR) experiments, we find that the terminus of the DNA duplex that incorporates dT-EDTA and the spin-label melts at a higher temperature than the rest of the DNA duplex. EPR microwave progressive power saturation experiments performed at 77 K are consistent with the specific binding of Dy(III) at the EDTA site and an intramolecular dipole-dipole interaction between the nitroxide spin-label and the chelated Dy(III). This model system should be suitable for studying the relaxation properties of metal ions by saturation-recovery EPR.     

Structural investigation of oxidized chlorosomes from green bacteria using multifrequency electron paramagnetic resonance up to 330 GHz

Chemical oxidation of the chlorosomes from Chloroflexus aurantiacus and Chlorobium tepidum green bacteria produces bacteriochlorophyll radicals, which are characterized by an anomalously narrow EPR signal compared to in vitro monomeric BChl c ^.+ [Van Noort PI, Zhu Y, LoBrutto R and Blankenship RE (1997) Biophys J 72: 316–325]. We have performed oxidant concentration and temperature-dependent X-band EPR measurements in order to elucidate the line narrowing mechanism. The linewidth decreases as the oxidant concentration is increased only for Chloroflexus indicating that for this system Heisenberg spin exchange is at least partially responsible for the EPR spectra narrowing. For both species the linewidth is decreasing on increasing the temperature. This indicates that temperature-activated electron transfer is the main narrowing mechanism for BChl radicals in chlorosomes. The extent of the electron transfer process among different BChl molecules has been evaluated and a comparison between the two species representative of the two green bacteria families has been made. In parallel, high frequency EPR experiments have been performed on the oxidized chlorosomes of Chloroflexus and Chlorobium at 110 and 330 GHz in the full temperature range investigated at X-band. The g-tensor components obtained from the simulation of the 330 GHz EPR spectrum from Chlorobium show the same anisotropy as those of monomeric Chl a ^.+ [Bratt PJ, Poluektov OG, Thurnauer MC, Krzystek J, Brunel LC, Schrier J, Hsiao YW, Zerner M and Angerhofer A (2000) J Phys Chem B 104: 6973–6977]. The spectrum of Chloroflexus has a nearly axial g-tensor with reduced anisotropy compared to Chlorobium and monomeric Chl a in vitro. g-tensor values and temperature dependence of the linewidth have been discussed in terms of the differences in the local structure of the chlorosomes of the two families.This revised version was published online in October 2005 with corrections to the Cover Date.