Electron paramagnetic resonance for everybody--MICROspec-X--a new class of electron paramagnetic resonance spectrometer

The electron paramagnetic resonance spectroscopy is the only method for detecting free radicals. Free radicals have an increased importance in our daily life. A small transportable EPR spectrometer is presented for the popularisation of the EPR method. The technical construction and some applications are illustrated which show the usability of the spectrometer.     

MoV electron paramagnetic resonance of sulfite oxidase revisited: the low-pH chloride signal

Valuable information on the active sites of molybdenum enzymes has been provided by Mo(V) electron paramagnetic resonance (EPR) spectroscopy. In recent years, multiple resonance techniques have been extensively used to examine details of the active-site structure, but basic continuous-wave (CW) EPR has not been re-evaluated in several decades. Here, we present a re-examination of the CW EPR spectroscopy of the sulfite oxidase low-pH chloride species and provide evidence for direct coordination of molybdenum by chloride.     

Hyperfine artifacts in electron paramagnetic resonance imaging

Low-molecular weight nitroxide labels (nitroxides) are commonly used as probes in electron paramagnetic resonance (EPR) spectroscopy. The nitroxides exhibit multiple lines in their EPR spectrum due to hyperfine coupling of the unpaired electron with the nitrogen nucleus. In EPR imaging, these hyperfine lines cause either hyperfine-based limitations in the maximum obtainable image resolution or hyperfine-based artifacts in the reconstructed image. In this article we discuss the effect of hyperfine artifacts on the quality of the image and report the application of a numerical method based on forward-subtraction principles for removing hyperfine artifacts in the measured projections. We demonstrate using computer simulations and imaging phantoms that marked enhancement in image quality and resolution can be obtained by removing the hyperfine-imposed limit on the gradient magnitude and performing post-acquisition corrections for removing hyperfine artifacts in the image.     

Thermal and electron paramagnetic resonance studies on pyridine perchromate

A number of physical techniques including scanning calorimetry and electron paramagnetic resonance have been used in an attempt to elucidate the course of the decomposition reaction of pyridine perchromate.     

Exchange narrowing in electron paramagnetic resonance of ruby

Previous theories of exchange narrowing of electron paramagnetic resonance spectra are re-examined. In the case of strong exchange interactions the diagonal matrix elements of the dipolar and fine structure interactions are time independent and symmetrization and narrowing of the electron resonance spectra result from a reordering of the magnetic energy levels by the strong exchange interaction. Lineshape calculations are given for concentrated ruby (10 mole % Cr₂O₃) and reasonable agreement with experiment is obtained.     

Uses of electron paramagnetic resonance in studying fracture

The uses of electron paramagnetic resonance (EPR) in studying aspects of polymer fracture are discussed. The sensitivity of EPR is such that all phases of fracture are not amenable to investigation by these means. This paper attempts to define those areas where the authors' experience would indicate that success might or might not be expected. A discussion of the difference between the tensile fracture of drawn polymer fibers, in which strong signals are obtained, and cast and molded materials is given.     

Pulse annealing electron paramagnetic resonance with probing transition ions

The analysis of the sequence of electron paramagnetic resonance (EPR) spectra of trace amounts of substitutional probing paramagnetic ions incorporated in (nano)crystalline samples submitted to isothermal and isochronal pulse annealing treatments can offer a wealth of information on the thermally induced compositional and structural changes of the host material. The potential of this new thermal analysis method is illustrated here with results of such investigations on the thermal decomposition of crystalline zinc hydroxide (Zn(OH)₂) and anhydrous zinc carbonate basic (Zn₅(CO₃)₂(OH)₆) precursors containing trace amounts of substitutional Mn²⁺ probing ions into nanostructured zinc oxide-ZnO. The quantitative analysis of the sequence of isochronal pulse annealing EPR spectra could provide, besides the thermal decomposition curves of the two precursors, additional information about the structure of the resulting nanostructured ZnO, some of it hard to get by standard structural diffraction techniques. The analysis of both isochronal and isothermal pulse annealing EPR data was further used to investigate the crystallization mechanism of the initially formed nanostructured disordered ZnO and to quantitatively describe the further growth of the resulting ZnO nanocrystals with the increasing annealing temperature and duration.     

A rapid-scanning electron paramagnetic resonance spectrometer with stopped-flow mixing

Modifications to a commercial electron paramagnetic resonance (e.p.r.) spectrometer are described which allow data to be obtained from 64 e.p.r, spectra (0.1–100 gauss per scan) with scan times of 0.010–0.900 s and with 0.020–900 s delay times between scans. Reagents are delivered into an e.p.r, cell from a stopped-flow mixer that triggers the generation of a microprocessor-controlled waveform to drive a rapid-scan unit. This digitally synthesized waveform is designed to correct inherent imperfections in the Helmholtz-type sweep coil circuit in the rapid-scan unit. The spectra are digitized (250 points per scan) and can be processed to determine rates of reaction. Performance of the system is demonstrated by the determination of the kinetics of rearrangement of a bis(di- peptide)nickelate(III) complex.     

Filling factor in a pulsed electron paramagnetic resonance experiment

A definition and mathematical treatment to calculate the filling factor in a pulsed electron paramagnetic resonance (EPR) experiment are presented. The differences between filling factors in traditional, continuous wave (CW)-EPR experiments (eta), and in pulsed-EPR experiments (eta(p)), are discussed. We present some examples to demonstrate how eta(p) depends upon the particular pulse sequence and sample characteristics.     

The determination of tungsten by electron paramagnetic resonance spectrometry

Electron paramagnetic resonance is used to determine tungsten in the range 5–400 μg ml^-1. Tungstate is reduced to tungsten(V) in the presence of thiocyanate in acidic medium and detected as the tungsten(V)—thiocyanate complex in amyl acetate after extraction. Molybdenum does not interfere; vanadium (5 mg) interferes. The relative standard deviation for mid-range concentrations is about 3%.     

Multifrequency electron paramagnetic resonance study on deproteinized human bone

Irradiated samples of deproteinized powdered human bone (femur) have been examined by electron paramagnetic resonance (EPR) spectroscopy in X, Q and W bands. In the bone powder sample only one type of CO2- radical ion is stabilized in the hydroxyapatite structure in contrast to powdered human tooth enamel, a material also containing hydroxyapatite, widely used for EPR dosimetry and in which a few radicals are stable at room temperature. It is suggested that the use of deproteinized bone for EPR dosimetry could improve the accuracy of dose determination.     

Radiation-induced radicals in glucose-1-phosphate. I. Electron paramagnetic resonance and electron nuclear double resonance analysis of in situ X-irradiated single crystals at 77 K

Electron magnetic resonance analysis of radiation-induced defects in dipotassium glucose-1-phosphate dihydrate single crystals in situ X-irradiated and measured at 77 K shows that at least seven different carbon-centered radical species are trapped. Four of these (R1-R4) can be fully or partly characterized in terms of proton hyperfine coupling tensors. The dominant radical (R2) is identified as a C1-centered species, assumedly formed by a scission of the sugar-phosphate junction and the concerted formation of a carbonyl group at the neighboring C2 carbon. This structure is chemically identical to a radical recently identified in irradiated sucrose single crystals. Radical species R1 and R4 most likely are C3- and C6-centered species, respectively, both formed by a net hydrogen abstraction. R3 is suggested to be chemically similar to but geometrically different from R4. Knowledge of the identity of the sugar radicals present at 77 K provides a first step in elucidating the formation mechanism of the phosphoryl radicals previously detected after X-irradiation at 280 K. In paper II, the chemical identity, precise conformation, and possible formation mechanisms of these radical species are investigated by means of DFT calculations and elementary insight into the radiation chemistry of sugar and sugar derivatives is obtained.     

Multi-frequency electron paramagnetic resonance study of irradiated human finger phalanxes

Electron paramagnetic resonance (EPR) is often used in dosimetry using biological samples such as teeth and bones. It is generally assumed that the radicals, formed after irradiation, are similar in both tissues as the mineral part of bone and tooth is carbonated hydroxyapatite. However, there is a lack of experimental evidence to support this assumption. The aim of the present study was to contribute to that field by studying powder and block samples of human finger phalanxes that were irradiated and analyzed by multi-frequency EPR. The results obtained from bones are different from the ones obtained in enamel by several respects: the ordering of the apatite crystallites is much smaller in bone, complicating the assignment of the observed CO2- radicals to a specific location, and one type of CO3(3-) radical was only found in enamel. Moreover, a major difference was found in the non-CO2- and non-CO3(3-) signals. The elucidation of the nature of these native signals (in bone and tooth enamel) still represents a big challenge.     

A point about electron paramagnetic resonance detection of irradiated foodstuffs

This paper makes a point about the identification of irradiated foodstuffs by means of electron paramagnetic resonance (EPR) or electron spin resonance (ESR). EPR is the most accurate method for such routine applications since radicals are stabilised for a long time in all (or part of) foods that are in solid and dry states; consequently, EPR can be applied to meat and fish bones, fruit and relative products (from vegetal origin). More details are given for mollusc shells, such as oysters and mussels.     

Distance measurements on spin-labelled biomacromolecules by pulsed electron paramagnetic resonance

The biological function of protein, DNA, and RNA molecules often depends on relative movements of domains with dimensions of a few nanometers. This length scale can be accessed by distance measurements between spin labels if pulsed electron paramagnetic resonance (EPR) techniques such as electron-electron double resonance (ELDOR) and double-quantum EPR are used. The approach does not require crystalline samples and is well suited to biomacromolecules with an intrinsic flexibility as distributions of distances can be measured. Furthermore, oligomerization or complexation of biomacromolecules can also be studied, even if it is incomplete. The sensitivity of the technique and the reliability of the measured distance distribution depend on careful optimization of the experimental conditions and procedures for data analysis. Interpretation of spin-to-spin distance distributions in terms of the structure of the biomacromolecules furthermore requires a model for the conformational distribution of the spin labels.     

Investigation of antioxidant radicals by the electron paramagnetic resonance method

Journal of Structural Chemistry -     

New molybdenum compound suitable for determination by electron paramagnetic resonance

Molybdenum reacted with diethyldithiocarbamate showed a single line at g=1.980 in electron paramagnetic resonance spectrometry at room temperature after reduction by oxalic acid. Molybdenum in 10 μl of urine could be quantitated within 5 min with the detection limit of 50 pg.