Gareth R. Eaton

EPR imaging and in vivo EPR

This volume provides a detailed examination of the physical basis for EPR imaging and in vivo EPR spectroscopy, experimental arrangements, and data analysis. The EPR imaging methods described include continuous wave, spin-echo-detected and ENDOR-detected EPR with constant, stepped, modulated, and pulsed magnetic field gradients. Applications described include inhomogeneous materials, diffusion kinetics, reaction kinetics, orientation of liquid crystals, microwave distributions, magnetic field distributions, superconductors, radiation damage, and defects in solids. The book also covers other topics important to in vivo studies, including in vivo EPR spectroscopy, low-frequency EPR, state-of-the-art low-frequency EPR instruments, achievable sensitivity, and spin labels. The book will be of great interest to graduate students, researchers, and medical instrument developers who use EPR, as well as clinicians and chemists interested in the relationship between in vivo radicals (such as superoxide and diseases).

Foundations of modern epr

Historical introduction transition metals organic radicals gas phase EPR radicals formed electrochemically radicals formed by radiation applications to biological systems spin-spin interactions time-domain EPR EPR imaging development of EPR instrumentation other EPR detection methods prospects.

Spectral-spatial two-dimensional EPR imaging

Abstract EPR imaging with one spectral dimension and one spatial dimension can be achieved by obtaining spectra at a series of magnetic field gradients, followed by the use of a convoluted back-projection image reconstruction algorithm. The concept is demonstrated with samples of organic radicals, including those that exhibit hyperfine structure in the fluid solution EPR spectra.

Relaxation Times of Organic Radicals and Transition Metal Ions

Review of electron spin relaxation times for organic radicals and transition metal ions in magnetically dilute samples. Emphasis is placed on studies that have been performed as a function of temperature and that provide insight into the relaxation processes.

Dephasing of electron spin echoes for nitroxyl radicals in glassy solvents by non-methyl and methyl protons

Dephasing of two-pulse electron spin echoes for 0.1–0.4 mM solutions of nitroxyl radicals in glassy solvents at 11 K and 40 K is dominated by nuclear spins in the solvent. Below about 70 K the rate of dephasing is the same for 4-oxo-2,2,6,6-tetramethylpiperin-1-yl (tempone) and for Fremys salt (peroxylaminedisulphonate). In solvents that do not contain methyl groups the rate constant for echo dephasing increases linearly with increasing proton concentration. The shape of the echo decay curve could be simulated with a model based on dephasing due to proton flip-flops with a rate constant of 30 ± 5 Hz M−1. At the same total proton concentration the electron spin echo decay rate is faster in solvents that contain methyl groups than in solvents without methyl groups. Several observations indicate that the effect of the methyl groups in the solvent on the echo dephasing is due to quantum mechanical tunnelling: (a) the echo dephasing is approximately independent of temperature between 11 K and 40 K, (b) the ap...

Pulsed EPR spectrometer

A pulsed EPR spectrometer is described. This spectrometer is designed for the study of relaxation times of dilute solutions of samples in common solvents as a function of temperature both above and below room temperature. Resolution of pulse widths and spacings is 1 ns. Both continuous wave (cw) and pulsed electron spin‐echo studies can be done on the same sample on the same spectrometer. Details of component choices and timing synchronization are provided. Phase alternation sequences for eliminating unwanted echoes are described. Examples of performance of the spectrometer are presented.

Electron spin relaxation times of C−60 in solution

Abstract Potassium and calcium salts of C − 60 have been studied in solid and liquid solutions by ESR and optical spectroscopies. Over the temperature interval from 8 to 45 K the electron spin relaxation times T 1 and T M (the phase memory time) were measured by saturation recovery and electron spin echo spectroscopies. For both salts T 1 decreased from a few milliseconds at 8 K to less than 1 μs at 40 K. T M was 1 to 3 μm between 8 and 25 K and decreased rapidly with increasing temperature to become equal to T 1 near 40 or 50 K. Below 40 K the linewidth of the anion signal was about 5.5 G, which is much larger than the linewidth calculated from T M (0.02 G at 10 K). Above about 50 K the linewidth was relaxation time determined and increased rapidly with increasing temperature until reaching a plateau near the melting point of the solution. Analysis of the relaxation rates for C − 60 between 35 K and the melting (softening) point of the solutions gives an Arrhenius activation energy between 2.2 and 2.9 kJ/mol (180–240 cm −1 ) depending on the solvent. This measured range of activation energies for relaxation is close to and somewhat less than the lowest vibrational mode (≈ 270 cm −1 of C 60 ; in C − 60 this vibration may couple to the triply degenerate electronic ground state to produce a dynamic Jahn—Teller distortion.

Comparison of Electron Paramagnetic Resonance Methods to Determine Distances between Spin Labels on Human Carbonic Anhydrase II

Four doubly spin-labeled variants of human carbonic anhydrase II and corresponding singly labeled variants were prepared by site-directed spin labeling. The distances between the spin labels were obtained from continuous-wave electron paramagnetic resonance spectra by analysis of the relative intensity of the half-field transition, Fourier deconvolution of line-shape broadening, and computer simulation of line-shape changes. Distances also were determined by four-pulse double electron-electron resonance. For each variant, at least two methods were applicable and reasonable agreement between methods was obtained. Distances ranged from 7 to 24 A. The doubly spin-labeled samples contained some singly labeled protein due to incomplete labeling. The sensitivity of each of the distance determination methods to the non-interacting component was compared.

Interaction of spin labels with transition metals: Part 2

Abstract During the years since our previous review there has been an increasing tendency for researchers to recognize the existence of electron-electron spin-spin interactions, and to consider the possibility of both dipolar and exchange contribution to the spin-spin interaction. Despite the well-documented need, few of the studies of spin-spin interaction have included quantitation of the EPR signals or demonstration that the signal examined was due to the species of interest, few systematic studies of the factors that determine the magnitude of the exchange interaction have been carried out, and the equations most frequently used to obtain interspin distances have not been calibrated for the spin systems to which they have been applied. Paramagnetic metals have been coordinated to about 130 spin-labeled ligands: the majority of these studies have used Cu(II). Some studies have examined other metals with relatively slow electron spin relaxation rates including Ag(II), VO(IV), low spin Co(II), and Cr(III). Unfortunately in some papers it is not stated whether the EPR spectra were obtained on dilute solutions or magnetically concentrated solids. Most of the work on discrete complexes, other than from our laboratory, has been on systems in which the exchange interaction is large relative to the nuclear hyperfine splittings and the g value difference. The resulting averaged g value and reduced metal hyperfine splitting have been recognized for about 30 spin-labeled Cu(II) or vanadyl complexes. In some reports it was suggested that averaged g and reduced A values for spin-labeled copper complexes in fluid solution indicated that the nitroxyl oxygen was coordinated to the metal. Coordination of the nitroxyl oxygen gives ferromagnetic or antiferromagnetic interaction that is large enough to measure by magnetic susceptibility. In these complexes there is no signal from the singlet state and incomplete motional averaging of the large zero-field splitting of the triplet state causes severe broadening of the fluid solution spectra. Therefore it is probable that spectra in which average g and A values are observed are due to complexation of the ligand via atoms other than the nitroxyl oxygen. Studies of resolved AB splittings have examined the factors that contribute to the value of the exchange coupling constant, J . Further studies are needed to improve our understanding of the relationship of the bond pathway between the two paramagnetic centers to the magnitude of the exchange interaction. Complexes with more rapidly relaxing transition metals have not been studied extensively, but are likely to receive greater attention in the next few years as pulsed EPR techniques become accessible to more researchers. Diverse geometries have been observed for complexes in which the nitroxyl oxygen is coordinated to a transition metal. Further studies in this area are needed in order to define the relationship between geometry and the magnitude of the exchange interaction. Collision broadening of nitroxyl radical EPR spectra by paramagnetic metal ions is often stated to be due to either exchange or dipolar interactions without any indication of the basis for the judgement. Most of the efforts to understand the relative importance of the two contributions are in the Russian literature, in which it is shown that the importance of the dipolar contribution depends strongly on viscosity and metal ion. Collision interactions between paramagnetic transition metals and nitroxyl radicals have been widely used in biochemical studies to (a) distinguish between nitroxyl radicals in environments that are accessible to bulk solution and environments that are not accessible and (b) to broaden the signal from nitroxyl in bulk solution thereby facilitating study of the signal from nitroxyl that is not accessible to the bulk solution. Efforts are underway to estimate interspin distances from the effect of metal ions on nitroxyl relaxation rates. To date this work has used continuous wave EPR. Pulsed EPR techniques are likely to facilitate these studies. A disconcertingly large fraction of the applications of the Leigh theory to the measurement of metal-nitroxyl and metal-metal distances in biological systems do not pay sufficient attention to the fundamental assumptions of the model. The applications to copper-nitroxyl interactions are questionable since copper(II) relaxation rates are generally quite long. Greater attention needs to be paid to questions of chemical reversibility, quantitation of the EPR signal, and interference from small amounts of nitroxyl that are not interacting with the paramagnetic metal. More accurate measurements of electron spin relaxation rates are urgently needed to be included in these calculations. Some distance measurements have been made from changes in relaxation behavior based on continuous wave power saturation curves which yield a value for the product T 1 T 2 . Pulsed EPR measurements of relaxation time changes have not yet been applied to these estimates. Where compared, the several distance measurement techniques are in reasonable agreement, though there are some discrepancies. However, these comparisons may not reveal substantial inadequacies in the models. If the experimentally obtained parameter depends on r to the sixth power, an error by as much as a factor of 2 in an input only causes the value of r to change by 12%. Most of the distance measurements discussed in Section E rely upon the assumption that the spin-spin interaction is exclusively dipolar. The growing body of data discussed in Section B indicates that exchange interaction can be a significant factor in spin-spin interactions in a wide range of systems. This information should cause a greater use of distance measurement techniques that take into account both dipolar and exchange interactions, i.e., measurements of the intensity of half-field transitions and analysis of resolved splittings.

Saturation recovery electron paramagnetic resonance spectrometer

A versatile saturation recovery accessory based on a small, special‐purpose timing controller and an efficient mix of coaxial and waveguide microwave components has been added to a commercial electron paramagnetic resonance (EPR) spectrometer. The spectrometer was designed for study of the spin lattice relaxation time of transition metals and free radicals over a wide temperature range, such as are of interest in materials science and metallobiochemistry. A microprocessor‐based timing system was designed to provide a wide dynamic range with a simple user interface. Waveguide phase shifters and rotary vane attenuators were used for their precision and resetability, while coaxial components were used where their superior performance could be exploited. Sensitivity is provided by a low‐noise GaAs field‐effect transistor (FET) microwave preamplifier and a double‐balanced mixer (DBM) detector. A biphase modulator (±180° phase shifter) on the lo side of the DBM provided efficient addition/subtraction of success...