Complex Electron Paramagnetic Resonance Research Articles

Low-temperature trapping of N2 reduction reaction intermediates in nitrogenase MoFe protein-CdS quantum dot complexes.

The biological reduction of N2 to ammonia requires the ATP-dependent, sequential delivery of electrons from the Fe protein to the MoFe protein of nitrogenase. It has been demonstrated that CdS nanocrystals can replace the Fe protein to deliver photoexcited electrons to the MoFe protein. Herein, light-activated electron delivery within the CdS:MoFe protein complex was achieved in the frozen state, revealing that all the electron paramagnetic resonance (EPR) active E-state intermediates in the catalytic cycle can be trapped and characterized by EPR spectroscopy. Prior to illumination, the CdS:MoFe protein complex EPR spectrum was composed of a S = 3/2 rhombic signal (g = 4.33, 3.63, and 2.01) consistent with the FeMo-cofactor in the resting state, E0. Illumination for sequential 1-h periods at 233K under 1 atm of N2 led to a cumulative attenuation of E0 by 75%. This coincided with the appearance of S = 3/2 and S = 1/2 signals assigned to two-electron (E2) and four-electron (E4) reduced states of the FeMo-cofactor, together with additional S = 1/2 signals consistent with the formation of E6 and E8 states. Simulations of EPR spectra allowed quantification of the different E-state populations, along with mapping of these populations onto the Lowe-Thorneley kinetic scheme. The outcome of this work demonstrates that the photochemical delivery of electrons to the MoFe protein can be used to populate all of the EPR active E-state intermediates of the nitrogenase MoFe protein cycle.

Alteration in the molecular structure of the adenine base exposed to gamma irradiation: An ESR study

Abstract Adenine polycrystals were obtained from their powder form under effective crystallization conditions by adjustment of the concentration of chemical solutions. The adenine samples were irradiated with a 60Co gamma-ray source at room temperature for 12, 24, 48, and 72 h, and then these samples were investigated between 240 and 400 K using an electron spin resonance (ESR) spectrometer. No signals were observed in the non-irradiated samples irradiated for 12, 24, and 48 h; however, when irradiated for 72 h, the samples exhibited complex ESR spectra. The analysis indicated the presence of a radical structure, NH ̇ \dot{{NH}} . The hyperfine splitting and g-values were calculated from the spectra by simulation programs. Also, during the measurements, it was observed that the shapes of the ESR lines changed slightly with the temperature.

Simulations of complex electron paramagnetic resonance spectra for radiation-induced defect centres in advanced ceramic breeder pebbles

Advanced ceramic breeder (ACB) pebbles consisting of lithium orthosilicate (Li4SiO4) and the strengthening phase lithium metatitanate (Li2TiO3) are accepted as the European Union’s reference solid-state material for tritium breeding in thermonuclear fusion reactors. Previously, the influence of various radiation types on Li4SiO4-based ceramic breeder materials has already been investigated and described by several groups of researchers. Electron paramagnetic resonance (EPR) spectroscopy is one of the most frequently used analytical and non-destructive techniques that can be selectively applied for qualitative and quantitative research of radiation-induced defect centres with paramagnetic properties (electron spin S ≠ 0). In the present work, individual signals of paramagnetic radiation-induced defect centres in the ACB pebbles after irradiation with X-rays were separated by EPR spectra simulations for the first time. Multiple signals of spin S = ½ systems with distinctive symmetries and g-factor (g) values were identified and characterised. Individual signal decay at room temperature was monitored and isochronal annealing of the irradiated ACB pebbles was performed in order to evaluate the stability of accumulated radiation-induced defect centres. Based on the obtained results, it was determined that radiation-induced defect centres with EPR signals in the g > 2.00 region are similar to irradiated single-phase Li4SiO4 ceramic materials, while additions of Li2TiO3 as the second phase in the ACB pebbles also stimulates the formation of several titanium-related electron centres with signals in the g < 2.00 region and different thermal properties.

Identification of radiation processing of different plant foods of Pakistan origin using the rapid technique of Electron Spin Resonance (ESR) spectrometry

Electron Spin Resonance (ESR) spectrometry is a potential technique for detection of irradiated foods like plant foods, which contain cellulosic materials as well as low molecular weight sugars and like the meat and fish, which contain bones. This study is focused on identification of irradiated foods such as tree nuts (pine nuts and walnuts), beans and pulses (red grams, white beans, pinto beans and green peas), and food containing low molecular weight sugars (pan masala and dried dates). The food samples were obtained from Pakistan and exposed to different permissible doses of radiation using electron beam linear accelerator. The control and irradiated samples were kept at room temperature in the laboratory for the record of ESR spectra. For post-irradiation stabilities the food samples were analysed at different intervals of time (15, 90 and 300 days). The controlled samples in case of cellulosic foods showed ESR signal at g0 = 2.004 and irradiated samples were shown with triplet signal indicative of the presence of radiation-induced cellulose radicals. Similarly, in case of Pan Masala and dried dates, a straight signal at zero level was observed for un-irradiated samples and a complex ESR signal was appeared for irradiated samples. The observed radiation-induced signals were dose-dependent and showed an increasing trend with an increase in radiation dose. The comparison of intensities of ESR signals was also carried out along with the study of post-irradiation stabilities of the ESR signals and method proved to be valid for differentiation of controlled and irradiated samples of food.

Novel Deconvolution Approach for the Analysis of Complex Electron Paramagnetic Resonance Spectra of Irradiated Sweet Potatoes

Electron paramagnetic resonance (EPR) spectroscopy is an unambiguous technique to analyze free radicals. Dehydrated sweet potatoes (SPs) can provide crystalline cellulose environments to stabilize irradiation-derived free radicals and allow their analysis using EPR spectroscopy. This study demonstrated a peak enhancement method to deconvolute complex EPR spectra for the quantitative assessment of irradiation. The SPs were prepared with two moisture contents (48.3 and 9.7%) and irradiated at 0, 5, 10, 20, 30, and 50 kGy. The slow-tumbling EPR spectra of nonirradiated SPs showed a singlet peak located at 334.5 mT, while irradiated SPs were characterized by cellulose satellite peaks located 3 mT apart from the central peak, and a glucose shoulder 0.9 mT away from the center. The free radical formation was quantified by using total peak areas, cellulose satellite peak areas (SPAs), and glucose peak areas with and without artificial intelligence-assisted peak enhancement, which allowed analysis of glucose peaks and improved dosage sensitivity and reduced variability of SPA analysis (coefficient of variation less than 25%), especially for high-moisture samples. The high-field SPA was more sensitive to the irradiation dose as compared to the low-field signal. Overall, this study demonstrates the applicability of the peak enhancement procedure to improve the EPR analysis of irradiated fruits and vegetables.

An EPR study of gamma irradiated mixed materials based on sucrose for dosimetric purposes

Since the recent past it was known that sucrose showed some good results as dosimeter using Electron Paramagnetic Resonance (EPR) technique. However there is still considerable interest in improving its accuracy and precision as well as in developing new systems with improved characteristics. In view of this fact, materials consisting of mixture at two different ratios (70:30 and 50:50%) between sucrose and ascorbic acid, sucrose and cysteine and sucrose and diphenylamine were studied by EPR spectroscopy. The obtained new materials were irradiated by γ-rays having doses varying from 0.5 up to 20 kGy. Before irradiation no EPR spectra were recorded. After irradiation complex EPR spectra with dominants of the sucrose spectrum in them were observed. The influence of the instrumental settings on the EPR signal intensity was reported. It was found out that the dose response of sucrose has linear dependence up to 20 kGy. The dose response of the new materials is compared with those in case of sucrose. The results showed that the addition of ascorbic acid, cysteine and diphenylamine to sucrose were leading to decrease in its sensitivity, but the EPR dose response remains unchanged. It was supposed that the decrease in the sensitivity was probably due to the antiradical activity of the added substances. To check this assumption the 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical scavenging activity (FRSA) of the used substances was studied. It was concluded that these mixed materials can be used for dosimetric purposes, however they have lower sensitivity compared to sucrose.

Quantum dynamics of Mn2+ in dimethylammonium magnesium formate.

Dimethylammonium magnesium formate, [(CH3)2NH2][Mg(HCOO)3] or DMAMgF, is a model used to study high temperature hybrid perovskite-like dielectrics. This compound displays an order-disorder phase transition at about 260 K. Using multifrequency electron spin resonance in continuous wave and pulsed modes, we herein present the quantum dynamics of the Mn2+ ion probe in DMAMgF. In the high temperature paraelectric phase, we observe a large distribution of the zero field splitting that is attributed to the high local disorder and further supported by density functional theory computations. In the low temperature ferroelastic phase, a single structure phase is detected and shown to contain two magnetic structures. The complex electron paramagnetic resonance signals were identified by means of the Rabi oscillation method combined with the crystal field kernel density estimation.

An analysis of cellulose- and dextrose-based radicals in sweet potatoes as irradiation markers.

Dried sweet potatoes (SPs) are often irradiated for improved safety and shelf life. Formation of irradiation-derived radicals was analyzed using electron paramagnetic resonance (EPR) spectroscopy. These irradiation-specific radicals can be used to characterize the irradiation history of dry plant-based foods containing cellulose and sugars. The signal characteristics (intensity and peak shape) were evaluated at different sample locations (skin and flesh), as a function of sample preparation method (grinding, sieving, and pelletizing). The signal intensity was quantified using a double integration method of the peaks based on the area under the curve. The sieving caused ca. 50% decrease in total signal intensity as compared to nonsieved samples due to loss of cellulose-based radicals. The flesh of irradiated SP showed complex EPR spectra with multiple satellite peaks of cellulose radicals (333.5 and 338.8 mT) and split peak of dextrose radicals (337.4 mT); while skin spectra were distinctive of cellulose radicals. In this study, we demonstrated the effects of sample composition and preparation method on formation and analysis of irradiation-specific radicals based on EPR. PRACTICAL APPLICATION: In the last decade or so, there have been health concerns related to the consumption of irradiated pet food products. Electron paramagnetic resonance spectroscopy can be used to analyze the irradiation history of dry products containing cellulose and sugar, such as the popular dog treat dried sweet potatoes, to ensure the products were irradiated within safe limits. This work demonstrates that the formation of irradiation-specific radicals is affected by the sample location (skin and flesh) and moisture content.

An inter-laboratory comparison to evaluate the suitability of EN 1787 standard to detect irradiation in plant-origin foods with health benefits

This paper reports the results of a study carried out to verify the applicability of the EN 1787 method, which uses the Electron Spin Resonance (ESR) technique for the identification of irradiated plant-origin foods with health benefits. The method was tested on samples of herbal ingredients of Plant Food Supplements (PFSs), nuts and fresh blueberries. Untreated and irradiated samples of Camellia sinensis (leaves) Ginkgo biloba (leaves), Glycine max (seeds), Silybum marianum (fruits), Vaccinium myrtillus (fruits), almonds, hazelnuts, peanuts, pistachios, walnuts and fresh blueberries were analysed. The work includes an inter-laboratory blind test involving five Italian laboratories that perform routine analyses for the official control of irradiated food. A total of 180 untreated and irradiated samples of PFS ingredients, nuts and fresh blueberries were analysed. The analyses on the irradiated samples were replicated even a long time after irradiation (up to two years depending on the matrix) to test the reliability of the method throughout the shelf life of the products.The results were matrix-dependent: all the 5 kGy irradiated nuts and the 1 kGy-irradiated blueberries were correctly classified, whereas herbal ingredients showed complex ESR spectra with spurious signals which often prevented the correct classification of the sample.

Application of High-Frequency Electron Paramagnetic Resonance/Electron Spin Echo for the Identification of the Impurity Composition and Electronic Structure of Ceramics Based Garnets

Electron paramagnetic resonance (EPR) spectra of Ce3+, Yb3+, Cr3+, and Gd3+ impurity ions in yttrium aluminum garnet Y3Al5O12 (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd3+ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

Применение высокочастотного ЭПР/ЭСЭ для идентификации примесного состава и электронной структуры керамик на основе гранатов

Electron paramagnetic resonance (EPR) spectra of Ce3+, Yb3+, Cr3+, and Gd3+ impurity ions in yttrium aluminum garnet Y3Al5O12 (YAG) ceramics were detected and identified at frequency of 94 GHz. The advantage of measuring the EPR spectra in the high-frequency range compared to the standard EPR technique is shown, which makes it possible to separate the EPR spectra characterized by different anisotropic g-factors and also to isolate the EPR signals due to the splitting of the fine structure for centers with high-spin states. In ceramics with a high content of magnetic Gd3+ ions, EPR and electron spin echo (ESE) spectra of multi-ionic gadolinium complexes were observed, and EPR spectra of complexes with the maximum number of exchange-coupled gadolinium ions are seen at low temperatures. The temperature dependences of the EPR spectra indicates ferromagnetic ordering of exchange-coupled complexes of gadolinium.

Electron beam irradiation studies of polyethylene maleic anhydride on their Chemical and Thermal properties.

The electron beam (e-beam) irradiation effects on polyethylene maleic anhydride (MANPE) are investigated by spectroscopic and thermal methods. The MANPE irradiated with electron beam to a radiation dose of 30, 60 and 90 K Gy and the resultant changes in their chemical structure and thermal properties have been investigated by Electron spin resonance (ESR), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods. The irradiated MANPE has shown complex ESR line shapes under different conditions. The analysis of ESR spectra suggests the formation of macro-radicals of the type (–CH 2 -ĊH-CH 2 -)(I) and peroxy radicals of the ROO (II). The temperature dependent ESR studies suggest that the free radicals are trapped in the amorphous regions of the polymer. The g- values of free radicals at different temperatures are measured and reasons for variation in g- values is explained. Bloch analysis is used to calculate activation energy associated with the decay of free radicals. The FTIR analysis indicates the shifting of absorption bands and formation of new absorption bands corresponding to unsaturated groups and carbonyl groups centered around 1600 cm and 929 cm -1 . The e-beam irradiation of MANPE caused the decrease of transition temperature and increase of melting enthalpy and degree of crystallinity with increase of radiation dose as per differential scanning calorimeter studies. The results indicate that on irradiation MANPE preferably undergo degradation at low dose of irradiation and preferably undergo crosslinking at high doses.

Impact of disorder on formation of free radicals by gamma-irradiation: Multi-frequency EPR studies of trehalose polymorphs

Electron paramagnetic resonance (EPR) studies of the radiation-induced radicals in two anhydrous trehalose polymorphs, beta-crystalline (TREc) and glassy (TREg), were conducted with the aim to resolve whether different types of free radicals are induced in a differently disordered environment. A multifrequency approach (9.5 GHz, 94 GHz, and 244 GHz) was applied to improve the g-tensor resolution of the complex EPR spectra. In addition, the thermal stability of the EPR spectra and the respective decay kinetics were analyzed in a series of thermal annealing studies in the temperature interval from 333 K to 363 K. It was found that in the crystalline matrix the transformation process of the induced radicals is more complex than in the glassy host matrix. Qualitative decomposition of the experimental spectra, assuming four contributing species, reproduced characteristic EPR spectral features in both matrices. These were interpreted as carbon-centered radicals while the possibility of the formation of alkoxy radicals due to the abstraction of a hydrogen atom could be ruled out. Only in one case, the tentative assignment of the EPR spectral components revealed the formation of the same radical species in both TREc and TREg. Furthermore, by thermal annealing TREg lost one of the radical species, whereas in TREc all 4 radical species pertained irrespective of the treatment. The results presented here, therefore, provide experimental evidence that the extent of disorder present in the material strongly affects the type and stability of radicals induced by ionizing radiation.

Radiation induced paramagnetic radicals in synthetic octacalcium phosphate

Created at room temperature by X-ray irradiation at the dose of about 5 kGy radiation induced centres in stable octacalcium phosphate (OCP) obtained by double transformation of α-tricalcium phosphate (α-TCP) are studied by means of the conventional and pulsed electron paramagnetic resonance (EPR). No EPR signals within the sensitivity of the used equipment in the non-irradiated samples were detected. In the irradiated species complex EPR spectrum signals due to H0 and CO-like radicals appear. The obtained results could be used for the tracing of the mineralization processes from its initiation to the completion of the final product and for the in situ identification of the OCP phase.

Application of the ‘Spiking’ method to the measurement of low dose radiation (≤ 1 Gy) using alanine dosimeters

Alanine dosimeters are limited in radiotherapy by poor sensitivity at low doses (< 5Gy). A set of alanine dosimeters were ‘spiked’ with a large dose of radiation, (~30Gy, 6MV X-rays) and additional doses ranging between 0.5 and 10Gy. The radical yield was measured by Electron Paramagnetic Resonance (EPR) spectroscopy, and after subtraction of the contribution from the "spike" dose, a linear correlation between the radiation dose and the area of the central EPR signal was obtained for doses between 0.5 and 10Gy (regression value of 0.9890), and for the central peak's amplitude (regression value of 0.9895). Overall, this method is easy to perform, requires no complex EPR signal analysis, and offers much potential to extend the current usage of alanine dosimeters in radiotherapy.

Magnetic characterization of microcrystalline Na3Ln0.99–xEr0.01Crx(PO4)2 orthophosphates synthesized by Pechini method (Ln = La, Gd)

Abstract Na3Ln(PO4)2 orthophosphates (Ln = La, Gd) doped with Er3+ and co-doped with Cr3+ ions were synthesized by Pechini method and characterized by electron paramagnetic resonance (EPR) and magnetic susceptibility measurements. Low temperature EPR spectra were detected and analyzed in terms of temperature dependence and the structure of the obtained materials. They show that erbium and chromium ions substitute Ln3+ and also Na+ ions or Na+ channels forming complex EPR spectra. Both kinds of ions reveal ferromagnetic type of interaction which shows some anomaly at the temperature between 10 K and 15 K. Magnetic susceptibility reveals a weak antiferromagnetic kind of interaction dominating in the whole temperature range, from 3.5 to 300 K.

Investigation of radiosterilization feasibility of sulfamethoxazole by ESR spectroscopy

ABSTRACTIn the present study, the spectroscopic features of the radiolytic intermediates that were produced in gamma-irradiated (5, 10, 25 and 50 kGy) sulfamethoxazole (SMX) have been investigated by electron spin resonance (ESR) spectroscopy and the radiation sterilization feasibility of SMX by ionizing radiation was examined. Gamma-irradiated SMX exhibited a complex ESR spectrum consisting of 13 resonance lines where spectral parameters for the central resonance line were found to be g = 2.0062 and ΔHpp = 0.6 mT. The radiation yield of SMX was calculated to be relatively low (G = 0.1) by ESR spectroscopy and no meaningful difference was observed in the comparison of unirradiated and 50 kGy gamma irradiated SMX by the Fourier transform infrared (FT-IR) technique, confirming that SMX is a radioresistive material. Although SMX could not be accepted to be a good dosimetric material, the identification of irradiated SMX from the unirradiated sample was possible even for the low absorbed radiation doses and for a relatively long time (three months) after the irradiation process. Decay activation energy of the radical species, which is mostly responsible for the central intense resonance line, is calculated to be 45.15 kJ/mol by using the signal intensity decay data derived from annealing studies. Four radical species with different spectroscopic properties were accepted to be responsible for the ESR spectra of gamma-irradiated SMX, by simulation calculations. It is concluded that SMX and SMX-containing drugs can be sterilized by gamma radiation and ESR spectroscopy is an appropriate technique for the characterization of these induced radical intermediates during the gamma irradiation process of SMX. Toxicology tests should also be done for its safe usage.

Multiple States of Nitrile Hydratase from Rhodococcus equi TG328-2: Structural and Mechanistic Insights from Electron Paramagnetic Resonance and Density Functional Theory Studies.

Iron-type nitrile hydratases (NHases) contain an Fe(III) ion coordinated in a characteristic "claw setting" by an axial cysteine thiolate, two equatorial peptide nitrogens, the sulfur atoms of equatorial cysteine-sulfenic and cysteine-sulfinic acids, and an axial water/hydroxyl moiety. The cysteine-sulfenic acid is susceptible to oxidation, and the enzyme is traditionally prepared using butyric acid as an oxidative protectant. The as-prepared enzyme exhibits a complex electron paramagnetic resonance (EPR) spectrum due to multiple low-spin (S = 1/2) Fe(III) species. Four distinct signals can be assigned to the resting active state, the active state bound to butyric acid, an oxidized Fe(III)-bis(sulfinic acid) form, and an oxidized complex with butyric acid. A combination of comparison with earlier work, development of methods to elicit individual signals, and design and application of a novel density functional theory method for reproducing g tensors to unprecedentedly high precision was used to assign the signals. These species account for the previously reported EPR spectra from Fe-NHases, including spectra observed upon addition of substrates. Completely new EPR signals were observed upon addition of inhibitory boronic acids, and the distinctive g1 features of these signals were replicated in the steady state with the slow substrate acetonitrile. This latter signal constitutes the first EPR signal from a catalytic intermediate of NHase and is assigned to a key intermediate in the proposed catalytic cycle. Earlier, apparently contradictory, electron nuclear double resonance reports are reconsidered in the context of this work.

Preparation and Properties of Metal Organic Framework/Activated Carbon Composite Materials.

Metal organic frameworks (MOFs) have unique properties that make them excellent candidates for many high-tech applications. Nevertheless, their nonconducting character is an obstacle to their practical utilization in electronic and energy systems. Using the familiar HKUST-1 MOF as a model, we present a new method of imparting electrical conductivity to otherwise nonconducting MOFs by preparing MOF nanoparticles within the conducting matrix of mesoporous activated carbon (AC). This composite material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), gas adsorption measurements, and electron paramagnetic resonance (EPR) spectroscopy. We show that MOF nanoparticles grown within the carbon matrix maintain their crystalline characteristics and their surface area. Surprisingly, as a result of the composition process, EPR measurements revealed a copper signal that had not yet been achieved. For the first time, we could analyze the complex EPR response of HKUST-1. We demonstrate the high conductivity of the MOF composite and discuss various factors that are responsible for these results. Finally, we present an optional application for using the conductive MOF composite as a high-performance electrode for pseudocapacitors.

Nature of Paramagnetic Species in Nitrogen-Doped SnO2: A Combined Electron Paramagnetic Resonance and Density Functional Theory Study

A combination of electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) calculations is used to characterize the paramagnetic species in rutile N-doped SnO2 samples synthesized by wet chemistry methods. In particular, the nature of paramagnetic N species, substitutional or interstitial, and their effect on the electronic structure are discussed. Complex EPR spectra generated by the interaction of the unpaired electron with N and Sn nuclei have been accurately simulated to obtain the EPR properties (g and A tensors). The results suggest that the N dopants form a rather symmetric structure with three magnetically equivalent or nearly equivalent Sn atoms surrounding the N impurity. After a careful assessment of an all-electron basis set for Sn atoms, realistic models of substitutional and interstitial N-doped SnO2 structures have been designed, and the corresponding hyperfine coupling constants (hpcc) were computed. The comparison between computed and measured hpcc values leads to the assignment of the paramagnetic centers in N-doped SnO2 to substitutional N dopants that take the position of the O atoms in the lattice. The DFT calculations finally suggest the N impurities induce the formation of localized empty states (electron holes) in the intra band gap region.