Decomposition study of the electron paramagnetic resonance spectrum of irradiated alanine

Recent Electron Paramagnetic Resonance (EPR) studies on alanine powders as a function of irradiation dose and temperature on the one hand and single crystal Electron Nuclear DOuble Resonance (ENDOR) studies on the other hand, showed the presence of at least three radicals contributing to the total alanine EPR spectrum. The latter spectrum obtained after irradiation at room temperature (RT), is dominated by the well-known stable-alanine-radical (SAR) CH3C*HCOO-, also denoted R1. Appropriate heating of irradiated alanine causes the relative contribution of R1 to decrease, resulting in a spectrum mainly caused by the H-abstraction radical CH3C*(NH3)COO-, denoted R2. Although the EPR spectrum of these two radicals could be satisfactorily simulated, their influence on dose reconstruction has not been reported yet. Therefore, a detailed Maximum Likelihood Common Factor Analysis (MLCFA) study has been performed on EPR spectra from polycrystalline alanine samples, after irradiation and heat treatments. Conclusions concerning the number of contributing radicals and their influence on the RT irradiated alanine EPR spectrum will be made.     

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.     

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.     

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.     

Comparative identification of irradiated herbs by the methods of electron paramagnetic resonance and thermoluminescence

Non irradiated and γ-irradiated dry herbs savoury (Savoury), wild thyme (Thymus serpollorium) and marjoram (Origanum) with absorbed dose of 8 kGy have been investigated by the methods of elecrtron paramagnetic resonance (EPR) and thermoluminescence (TL). Non-irradiated herbs exhibit only one weak siglet EPR signal whereas in irradiated samples its intensity increase and in addition two satelite lines are recorded. This triplet EPR spectrum is attributed to cellulose free radical generated by irradiation. It has been found that upon keeping the samples under the normal stock conditions the life-time of the cellulose free radical in the examined samples is ∼60–80 days. Thus the conclusion has been made that the presence of the EPR signal of cellulose free radical is unambiguous indication that the sample under study has been irradiated but its absence can not be considered as the opposite evidence. In the case when EPR signal was absent the method of TL has been used to give the final decision about the previous radiation treatment of the sample.     

Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.     

Comparative investigation of irradiated meat by the methods of electron paramagnetic resonance and gas chromatography

A comparative study of irradiated pork meat containing bone was made by the methods of electron paramagnetic resonance (EPR) and gas chromatography (GC). In this investigation EPR has the advantage to be a very fast and unambiguous method even in the cases of thermal treatment of bones. On the other hand, GC analysis is a time consuming procedure however, it becomes very valuable for meat samples that contain no bones.     

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.     

Investigations on the electron paramagnetic resonance parameters for Mn2+ in the ABF3 fluoroperovskites

The electron paramagnetic resonance (EPR) parameters (g factor, the hyperfine structure constant A and the superhyperfine parameters A' and B') for Mn(2+) in the fluoroperovskites ABF(3) (A=K and Cs; B=Zn, Mg, Cd and Ca) are theoretically investigated from the perturbation formulas of these parameters for a 3d(5) ion under ideal octahedra. In the above treatments, not only the crystal-field mechanism but also the charge transfer mechanism is considered uniformly on the basis of the cluster approach. The theoretical EPR parameters are in good agreement with the experimental data. The charge transfer contribution to the g-shift Δg (≈g-g(s), where g(s)≈2.0023 is the spin-only value) is opposite (positive) in sign and comparable in magnitude to the crystal-field one. Nevertheless, the charge transfer contribution to the hyperfine structure constant shows the same sign and about 10% that of the crystal-field one. So, the conventional argument that the charge transfer contributions to the zero-field splittings are negligible for 3d(5) ions under low symmetrically distorted fluorine octahedra is proved no longer valid for the Δg analysis of ABF(3):Mn(2+) in view of the dominant second-order charge transfer perturbation terms. The unpaired spin densities of the fluorine 2s, 2p σ and 2p π orbitals are determined from the quantitative dependences upon the related molecular orbital coefficients, rather than obtained by fitting the observed superhyperfine parameters in the previous works.     

A study of Roman glass by reflectance and electron paramagnetic resonance spectroscopies

Reflectance and electron paramagnetic resonance (EPR) spectroscopies were used to study 25 fragments of Roman glass. Colour coordinates were used for an unbiased classification of the glasses in colour groups, which accounted for the presence of blue, blue-green, green, yellow-green, yellow and purple samples. Reflectance spectra were recorded in the 250–2500 nm wavelength range and showed absorption bands characteristic of Fe^II, Fe^III and Mn^III ions; furthermore, Co^II and Cu^II bands were observed in the spectra of the blue glasses. A decrease of the absorbance ratio of Fe^II to Fe^III ions was observed moving from blue-green to green and yellow-green glasses; however, yellow fragments still proved to be reduced glasses. EPR spectra displayed the characteristic patterns of Fe^III and Mn^II ions, with g-values in the 2–5 interval and spectral features depending on the relative content of the two elements. The characteristic pattern of the V^IV ion (g ≈ 2) and signals due to the formation of iron-sulphur complexes (g ≈ 6) appeared in the spectrum of a dark yellow glass, recorded at 77 K.     

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.     

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.     

Transition metal ions in black tea: an electron paramagnetic resonance study

Tea (Camellia Sinensis) is the most widely consumed beverage in the world and is known to have therapeutic, antioxidant and nutritional effects. It contains dimeric flavanols and polyphenols which are known as the most important organic compounds in tea infusions, and can make strong and stable complexes with metal ions. In this study, we carried out a series of electron paramagnetic resonance experiments on well-known paramagnetic transition metal ions, namely Mn²⁺, Fe³⁺, Cu²⁺, VO²⁺, and Cr³⁺ doped in black tea cultivated along the shore of Black Sea, Turkey, to see the effects and structures formed.     

High-field 285 GHz electron paramagnetic resonance study of indigenous radicals of humic acids

Humic substances, the largest source of carbon on Earth, contain indigenous stable free radicals that are involved in important biogeochemical environmental processes occurring in soil and water systems. Here, we present the first high-magnetic-field 285GHz electron paramagnetic resonance spectra for humic acids from various geographical origins. All humic acids irrespective of their origin contain two limiting types of indigenous stable radicals, types I and II, with distinct electronic structure. Type I, which prevails at acidic pH 5, is characterized by a g tensor with principal values gIx = 2.0032, gIy = 2.0032, and gIz = 2.0023. Type II, which prevails at alkaline pH 12, is characterized by gIIx = 2.0057, gIIy = 2.0055, and gIIz = 2.0023. The two limiting types are correlated in a unified reversible manner with pH, irrespective of the geographic origin of the HA. Both types of radical centers are consistent with pi-type radicals. They persist not only in liquid solutions but also in humic acid powders.     

Investigation of antioxidant radicals by the electron paramagnetic resonance method

Journal of Structural Chemistry -     

An electron paramagnetic resonance study of Cu(II) sorbed on quartz

The nature of the interaction among Cu(II), adsorbed water, and quartz surface was studied using electron paramagnetic resonance (EPR) spectroscopy. The EPR lineshape gave information concerning the motional status of sorbed Cu(II) that revealed its binding strength at the surface. Two distinct absorption lines of sorbed Cu(II), namely, the liquid-type and the solid-type signal, were simultaneously observed at the fully hydrated surface at room temperature. The absorption lines and the variation of their intensity with experimental and measurement conditions such as degree of hydration, pH, ionic strength, and surface coverage indicated that there exist three kinds of Cu(II) entities, the inner-sphere surface complex, the outer-sphere surface complex, and the surface precipitate on the quartz surface, and that their concentrations change with experimental conditions. The reversible conversion of the liquid-type signal to the solid-type one during the drying-wetting or freezing-melting of the surface suggested the development of multiple layers of adsorbed water molecules on the quartz surface. It is assumed that the innermost layer of the water layers contains the inner-sphere Cu(II) surface complexes, while the outer layers contain the outer-sphere complexes whose binding strength decreases outward with increasing distance from the surface. The result of this work suggests that the sorption mechanism of a metal cation on a given mineral surface; hence its mobility in the environment may change significantly with the solution pH, the ionic strength, and the surface coverage.