Electron Paramagnetic Resonance Signal Amplitude Research Articles

Paramagnetic Centers of Mineral Component in Annealed Bone Tissue

The mineral components of powdered samples of cortical dense bone tissue of domestic animals (pig and cow) were studied by electron paramagnetic resonance (EPR). Bone tissue was heated in a temperature range 600-1000 ºС in an oven and then held for more than one year at room temperature. This was done to allow short-lived defects to decay and to let the mineral structures stabilize. In these heat-treated samples, within the limits of the sensitivity of the experiments, no EPR signals were detected, but after irradiation with X-rays, multicomponent EPR signals appeared. Spectra vary depending on the heating temperatures. Paramagnetic centers related to РО32–, NO42–, CO2–, CO33– and O– groups are observed. The g-factor of spectroscopic splitting and amplitude of ultrafine interaction of the indicated paramagnetic centers in the spectra were determined. Due to different relaxation behavior of the paramagnetic centers, and hence their saturation effects, the form of the total EPR signals significantly depends on the microwave power level at which the spectra are recorded. Therefore, EPR signals were recorded at high (5 mW) and low (0.13 mW) microwave power levels. The temperature dependency of EPR signal amplitude was determined for some paramagnetic centers in the range of 600-1000 ºС. The EPR signal properties of the heated bone samples and synthetic hydroxylapatites were compared. The spectra indicate that phosphates in bone tissue have a more complex structure than simple synthetic hydroxylapatite. The results of this work can be used to create synthetic analogs of bone tissue, to help in the manufacturing of implants that are used to treat bone tissue, and to study the processes related to the assimilation of mineralogic-based implants by living bone tissue.

In Vivo Electron Paramagnetic Resonance Tooth Dosimetry: Dependence of Radiation-Induced Signal Amplitude on the Enamel Thickness and Surface Area of Ex Vivo Human Teeth.

In vivo L-band electron paramagnetic resonance tooth dosimetry is a newly developed and very promising method for retrospective biodosimetry in individuals who may have been exposed to significant levels of ionizing radiation. The present study aimed to determine the relationships among enamel thickness, enamel area, and measured electron paramagnetic resonance signal amplitude with a view to improve the quantitative accuracy of the dosimetry technique. Ten isolated incisors were irradiated using well-characterized doses, and their radiation-induced electron paramagnetic resonance signals were measured. Following the measurements, the enamel thickness and area of each tooth were measured using micro-focus computed tomography. Linear regression showed that the enamel area at each measurement position significantly affected the radiation-induced electron paramagnetic resonance signal amplitude (p < 0.001). Simulation data agreed well with this result. These results indicate that it is essential to properly consider enamel thickness and area when interpreting electron paramagnetic resonance tooth dosimetry measurements to optimize the accuracy of dose estimation.

EPR study of table sugar rod and powder as high dose dosimeters

Abstract In order to improve the ease of sample handling, the reproducibility of signal detection and quantification, simple methods of incorporating a homogeneous mixture containing sugar powder (30%) with wax (35%) and rubber (35%) into rods has been adopted. The dose response, the time stability of the free radicals produced in table sugar dosimeters (in both rod and powder form) by gamma radiation and the effect of the temperature during irradiation were studied by EPR (Electron Paramagnetic Resonance). The peak-to-peak height (PPH) measurement of the EPR signal is studied as a function of the absorbed gamma dose in the range 0.1–58 kGy. For the two forms, a linear dependency is found between 0.1 and 26 kGy. At higher doses the EPR signal amplitude continues to grow but non-linear up to 58 kGy. The dependence of temperature during irradiation has been investigated in the temperature range 25–40 °C and the calculated correction coefficients were found (2.7 ± 0.2)% °C −1 and (1.5 ± 0.3)% °C −1 , respectively, for powder and rod forms. The time stability of the stored sugar samples was investigated for 34 days at room temperature, a rapid decrease of EPR signal was showed immediately after irradiation followed by slowly decrease.

Probing the Catalytic Center of TiO2/SO4 2− Solid Superacid Catalyst by X-Band In Situ High-Temperature EPR Spectroscopy

Paramagnetic centers of the solid superacid catalyst in the sulfated TiO2 are prone to the electron paramagnetic resonance (EPR) spectroscopy. The induction of the catalytic-active sites in TiO2 powder presubmerged in H2SO4 solution as a function of the calcinated temperature of 293–873 K is investigated by X-band in situ continuous-wave EPR measurements. Sulfated-acid sites composed of the Ti3+ ion are formed upon calcination. The overall experimental results show that the population of these sites goes uphill with the elevating temperature, reaches a maximum at ~623 K and decreases afterward to close zero. During the process, the decomposition of the TiO2/SO4 2− leads to the formation of Ti3+ species and then to the increasing EPR signal amplitude, and the consecutive decomposition of the sulfur at higher temperature (>650 K) to the diminishing signal. The X-ray diffraction indicates that the introduction of SO4 2− stabilizes the geometric structure in the anatase phase.

Synthesis of 14N- and 15N-labeled Trityl-nitroxide Biradicals with Strong Spin−Spin Interaction and Improved Sensitivity to Redox Status and Oxygen

Simultaneous evaluation of redox status and oxygenation in biological systems is of great importance for the understanding of biological functions. Electron paramagnetic resonance (EPR) spectroscopy coupled with the use of the nitroxide radicals have been an indispensable technique for this application but are still limited by low oxygen sensitivity and low EPR resolution in part due to the moderately broad EPR triplet and spin quenching through bioreduction. In this study, we showed that these drawbacks can be overcome through the use of trityl-nitroxide biradicals allowing for the simultaneous measurement of redox status and oxygenation. A new trityl-nitroxide biradical TNN14 composed of a pyrrolidinyl-nitroxide and a trityl and its isotopically labeled (15)N analogue TNN15 were synthesized and characterized. Both biradicals exhibited much stronger spin-spin interaction with J > 400 G compared with that of the previous synthesized trityl-nitroxide biradicals TN1 (∼160 G) and TN2 (∼52 G) with longer linker chain length. The enhanced stability of TNN14 was evaluated using ascorbate as reductant, and the effect of different types of cyclodextrins on its stability in the presence of ascorbate was also investigated. Both biradicals are sensitive to redox status, and their corresponding trityl-hydroxylamines resulting from the reduction of the biradicals by ascorbate share the same oxygen sensitivity. Of note is that the (15)N-labeled TNN15-H with an EPR doublet exhibits improved EPR signal amplitude as compared with that of TNN14-H with an EPR triplet. In addition, cyclic voltammetric studies verify the characteristic electrochemical behaviors of the trityl-nitroxide biradicals.

Electron Paramagnetic Resonance Method for the Quantitative Assay of Ketoconazole in Pharmaceutical Preparations

In this study, electron paramagnetic resonance (EPR) is used, for the first time, as an analytical tool for the quantitative assay of ketoconazole (KTZ) in drug formulations. The drug was successfully characterized by the prominent signals by two radical species produced as a result of its oxidation with 400 microg/mL cerium(IV) in 0.10 mol dm(-3) sulfuric acid. The EPR signal of the reaction mixture was measured in eight capillary tubes housed in a 4 mm EPR sample tube. The radical stability was investigated by obtaining multi-EPR scans of each KTZ sample solution at time intervals of 2.5 min of the reaction mixing time. The plot of the disappearance of the radical species show that the disappearance is apparently of zero order. The zero-time intercept of the EPR signal amplitude, which should be proportional to the initial radical concentration, is linear in the sample concentration in the range between 100 and 400 microg/mL, with a correlation coefficient, r, of 0.999. The detection limit was determined to be 11.7 +/- 2.5 microg/mL. The method newly adopted was fully validated following the United States Pharmacopeia (USP) monograph protocol in both the generic and the proprietary forms. The method is very accurate, such that we were able to measure the concentration at confidence levels of 99.9%. The method was also found to be suitable for the assay of KTZ in its tablet and cream pharmaceutical preparations, as no interferences were encountered from excipients of the proprietary drugs. High specificity, simplicity, and rapidity are the merits of the present method compared to the previously reported methods.

Numeric experiment for analysis of uncertainty of spectra processing in EPR dosimetry

In dosimetry based on Electron Paramagnetic Resonance (EPR) spectroscopy, radiation doses are evaluated from the analysis of the EPR spectrum. Usually investigators use their own specially designed computer software for extraction of radiation induced signal from the total EPR spectrum and for determination of its intensity. This process is able to discriminate the total spectrum of irradiated material into the radiation induced signal and the non-radiation induced signals. However even after separation of these spectral components the result represents the superposition of true radiation line and a residual noise. The present study is aimed at investigating the process of EPR signal amplitude reconstruction in tooth enamel, one of the most radiation sensitive materials among those used in EPR dosimetry. The specific scope of this paper is the understanding of the influence of the selected model for description of radiation signal, of the fitting method, and of noise variation on the invert restoration of primary signal. Three numerical experiments for simulation of the amplitude reconstruction were performed: 1. Test of models for description of radiation signal: Influence of the complexity of the model for signal reconstruction on the uncertainty of spectra processing was tested. 2. Test of fitting method of radiation signal: Influence of fitting method on the uncertainty of spectra processing was tested. 3. Test on noise and deconvolution influence on specific method of amplitude reconstruction used in DOSEREC software. Modeled cumulative signal was obtained by superposition of modeled radiation signals and simulated random noise. Model of random noise were constructed based on EPR measurements of 156 spectra of tooth samples of Urals rural population. The obtained noise amplitudes include not only white noise but also individual variations of chemical composition of samples and error of preliminary deconvolution. Variation of reconstructed spectra versus simulated spectra gives the information about accuracy of signal processing and about main sources of errors and uncertainties. In this report the results of numeric experiments will be discussed in context of possibility of improvement of spectral analysis. Acknowledgment - This work was supported by project “Southern Urals Radiation Risk Research – SOUL”.

Estimation of X-ray beam quality by electron paramagnetic resonance (EPR) spectroscopy

A novel dosimetry-based technique using EPR spectroscopy to determine X-ray beam quality is proposed. The radiation-sensitive material is made of a mixture of two polycrystalline substances with different X-ray absorption properties. The composite samples, consisting of polycrystalline lithium formate monohydrate and calcium formate, were prepared as pellets, X-irradiated, and analyzed with EPR spectroscopy. The ratio of the EPR signal amplitudes of the two constituents can serve as a measure of the X-ray beam quality given by the equivalent photon energy. The calculation of the signal amplitude ratio involves a reconstruction of the composite EPR spectrum. The logarithm of the signal amplitude ratio appears to be linearly correlated with the logarithm of the equivalent photon energy. The linear relationship can be used as a calibration for estimating the equivalent photon energy from the composite EPR spectrum. The composite material was used to investigate the changes in the equivalent photon energy in a Perspex phantom with increasing depth. When a 220 kV X-ray beam with an equivalent photon energy of about 100 keV was used, changes in the EPR signal amplitude ratio revealed a buildup of scattered photons with increasing depth in the phantom. This change could be related to the equivalent photon energy using the logarithmic calibration curve. It was found that the equivalent photon energy at the depth of 13 cm in the phantom was 25% lower than on its surface. The proposed method can be used for estimating equivalent photon energy in both standardized and non-standardized situations, the latter corresponding to beam setups where use of filters and ionization chambers is difficult or impossible. Also, the system can provide a means for measuring photon energy in X-irradiated phantoms.

Effectiveness of chemical etching for background electron paramagnetic resonance signal reduction in tooth enamel.

A method of preparing enamel samples for electron paramagnetic resonance retrospective dosimetry that leads to a reduction of the non-radiation induced signals (background) is proposed. The method is effective in removing residual dentin and in reducing surface paramagnetic centers both native and induced by the sample preparation procedure. The main steps in the method are the use of longwave length ultraviolet light for distinguishing dentin from enamel and of chemical etching with phosphoric acid for removal of surface paramagnetic centers. Several acid concentrations and etching times were tested and the optimization of the method is illustrated in regard to both sample mass loss and background signal amplitude reduction. The best acid concentration was determined to be 42%, leading to a 30% reduction in the background electron paramagnetic resonance signal amplitude per unit mass.

Electron paramagnetic resonance and magnetically modulated microwave absorption experiments on κ-(BEDT—TTF) 2Cu[N(CN) 2]Cl

Electron paramagnetic resonance (EPR) and magnetically modulated microwave absorption (MMMA) measurements were taken on differently prepared samples of the quasi two-dimensional organic conductor κ-(BEDT—TTF) 2Cu[N(CN) 2]Cl to study the interesting pressure dependent phase transition of this charge-transfer compound. The MMMA results undoubtedly confirm that the substance becomes a superconductor below 12.5 K, if a moderate pressure, e.g. just by embedding the substance in silicon grease during cooling, is applied. At an onset temperature of 25 K an additional microwave absorption is observed, which is about a factor of 100 weaker than the superconducting MMMA signal. We relate this microwave absorption to an antiferromagnetic ordering resulting in weak ferromagnetism by spin-canting occurring at this temperature. The result corresponds to the EPR measurements, where considerable changes in the main EPR parameters, i.e. linewidth, signal amplitude and susceptibility, are observed, giving evidence for an antiferromagnetic ordering below 50 K and a change to weak ferromagnetism below 25 K.

Aging and Caloric Restriction Affect Mitochondrial Respiration and Lipid Membrane Status: An Electron Paramagnetic Resonance Investigation

Previous studies have indicated that reactive oxygen species (ROS) are likely involved in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). ROS, generated by succinate-stimulated mitochondria, have been reported to be spin trapped and detected by electron paramagnetic resonance (EPR). Our aim in the current study was to study the impact of aging on the effect of increased metabolic stimuli on mitochondrial respiration in terms of oxy-radical generation and possible lipid peroxidative changes in brain neocortical membranes. A mixed population of brain synaptosomes and mitochondria from brown norway male rats of differing ages being fed either ad lib (AL) or a caloric-restricted diet (DR) was prepared and labeled with 5-nitroxyl stearate (5-NS), a membrane lipid-specific spin label. The changes in anisotropic motion of the intercalated 5-NS spin probe also allows one to evaluate the status of the membrane fluidity in the lipid microenvironment via the order parameter. Upon succinate stimulation of mitochondria, the ROS generated resulted in a decrease in the EPR signal amplitude of the 5-NS reporter molecule indicative of the flux of oxy-radicals produced and possible peroxidation-induced changes in the synaptosomal lipid membrane. The line width remained constant, indicating that the overall intensity was reduced. The results showed a significant overall age effect in the ability to generate oxygen-derived radicals following metabolic stimulation ( p < .0001). Stimulation of state 4 mitochondrial respiration with 20 mM succinate resulted in greater oxy-radical production in 25-month-old animals as compared to younger animals, suggesting increased mitochondrial leakage with age. Free radical stress induced by metabolic stimulation also causes a concomitant increase in membrane fluidity ( p < .0001). There was also a significant age effect ( p < .0007) on the order parameter of the mixed population of membranes. Although caloric restriction attenuated the membrane rigidization caused by aging, it was not found to play a role in limiting the oxy-radical production following metabolic stimulation of mitochondria. The overall effect of age on membrane spin-label intensities EPR signal upon succinate stimulation suggests that progressive mitochondrial dysfunction may be a key factor in the aging process and in development of age-associated diseases.

EPR Signal Enhancement of Alanine Irradiated with Thermal Neutrons

A method to enhance the electron paramagnetic resonance signal amplitude response of alanine to thermal neutrons is presented. The method is based on the production of additional free radicals caused by alpha-particle and 7Li interactions on the alanine. These particles emerge from the neutron capture 10(5)B (n, alpha) 7(3)Li reaction. For this purpose, alanine and boric acid are intimately mixed. Mixture intimacy is critical for optimum enhancement. A way to achieve good mixture intimacy is suggested. A signal enhancement up to 2200% is obtained.

Use of Alanine-Silicone Pellets for Electron Paramagnetic Resonance γ Dosimetry

Silicone is proposed as an alternative binding substance in the production of D-L alanine pellets used in electron paramagnetic resonance (EPR) dosimetry of gamma rays. The dosimeters are manufactured at room temperature, making the production simple. Examination by EPR silicone-alanine pellets irradiated with 60Co gamma rays in the dose range 10 to 10(6) Gy shows that the proposed silicone binder does not affect typical alanine dose-response curves. Thermal stability of the pellets below 40 degrees C is good, but their pre-dose EPR signal amplitude is slightly higher than for nonirradiated alanine.

Supramolecular arrangement of Rhodospirillum rubrum B880 holochrome as studied by radiation inactivation and electron paramagnetic resonance.

Oxidation of the B880 antenna holochrome gives rise to a 3.8-G linewidth electron paramagnetic resonance (EPR) signal that is considerably narrower than the 13-G signal of monomeric bacteriochlorophyll (Bchl) cation. Radiation inactivation was used to verify a model according to which this linewidth narrowing is due to delocalization over several Bchl molecules. Chromatophores of the photoreaction centerless mutant F24 of Rhodospirillum rubrum were subjected to different doses of gamma-radiation. This induced not only a decay of the EPR signal amplitude but also its linewidth broadening. According to target theory, the induced amplitude decay of the EPR signal had a target size of 10.5 kDa. This is attributed to an elementary structure (alpha1beta1Bchl2), whose number in the membrane would limit the rate of encounter with ferricyanide and thus the formation of unpaired spins. We applied Bernoulli statistics to predict, for a given survival probability of the signal, the number of surviving elementary structures in aggregates of (alpha1beta1Bchl2)n where n was varied from 4 to 7. Using an equation that predicted the Bchl special pair in the photo-reaction center, we were able to simulate the observed relationship between the EPR linewidth and the dose of radiation. The best fit was obtained with a hexameric structure alpha1beta1Bchl2)6.

Paramagnetic probes of the domain structure of histidine-rich glycoprotein.

The interaction of Cu2+ and Fe3+-mesoporphyrin with histidine-rich glycoprotein (HRG) from rabbit serum was examined spectroscopically. The first equivalent of Cu2+ binds to HRG producing a type II electron paramagnetic resonance (EPR) spectrum with g[[ = 2.25, gm = 2.05, A[[ = 0.019 cm-1 (180 G), and superhyperfine along gm. These spectral parameters suggest moderately covalent coordination of Cu2+ to the protein by nitrogens. With increasing Cu2+ the superhyperfine disappears; however, the g and A values change only marginally. The increase in EPR signal amplitude throughout the addition of 1-15 equiv of Cu2+ is linear and thereafter maximizes, suggesting 18-22 equiv are bound. In contrast, changes in the circular dichroism spectrum at 280 nm appear sigmoidal and can be interpreted as the binding of Cu2+ to two structurally distinct regions of the protein. Evidence for two structurally distinct binding domains is found by comparing EPR spectra of Cu2+ complexes of HRG with spectra from complexes of two of its major proteolysis products (peptides). After binding 1 equiv of Cu2+, both the 30-kDa histidine-rich peptide and the native protein exhibit identical spectra including the pronounced superhyperfine. In contrast, the spectrum of the histidine-normal 45-kDa peptide with 1 equiv of Cu2+ bound lacks superhyperfine and parallels closely that of the native protein with 20 equiv bound. Finally, Fe3+-mesoporphyrin binds to HRG exhibiting both high-spin (g = 6.05) and low-spin (gz = 2.94, gy = 2.25, gx = 1.50) EPR resonances, and the latter imply bis(histidine) coordination.(ABSTRACT TRUNCATED AT 250 WORDS)