Macroradical reactivity studied by electron spin resonance

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

This chapter summarizes the state of the art in electron paramagnetic resonance (EPR) spectroscopy. It should give the reader an overview of the tremendous methodological developments and applications, which have a huge impact on the fields of biophysics, biology, biochemistry, material sciences, solid state physics, and physical organic chemistry. The basic theory and principles of continuous wave (CW) EPR are introduced, as well as principles and common experiments of pulse EPR. We discuss, how the choice of microwave frequency or pulse sequence allow to separately address different interactions that contribute to the spin Hamiltonian and thus the EPR spectrum. An important family of advanced EPR experiments focusses on the determination of couplings between electron spins and nearby nuclear spins. EPR imaging (EPRI) is introduced as a method to add spatial resolution to EPR spectroscopy. How the use of spin labels or spin probes makes diamagnetic systems, which initially do not offer an EPR signal, accessible to EPR spectroscopy and EPR distance measurements is discussed in the last section of the chapter. Keywords: electron paramagnetic resonance (EPR) spectroscopy; electron spin resonance (ESR); electron Zeeman interaction; spin Hamiltonian; spin labels; spin probes; transition metal ions

Electron Paramagnetic Resonance Spectroscopy at Surfaces

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.

The fate of free radicals in a cellulose based hydrogel: Detection by electron paramagnetic resonance spectroscopy

Asphaltenes precipitation has become a major issue in oil production. It can clog oil wells and increase the cost of production. The precipitation can occur in the near-wellbore region, inside the wellbore, in subsea flowlines, and in the separator. Currently, there is no effective solution for monitoring asphaltenes concentration and precipitation in real time. In this work, we propose a novel method for monitoring asphaltenes concentration in crude oil, utilizing the physical principle of Electron Paramagnetic Resonance (EPR). The phenomenon of EPR is based on the interaction of electron spins with electromagnetic fields in the presence of an external DC magnetic field. As asphaltenes have paramagnetic centers, they generate EPR signals that can be measured with our proprietary EPR sensor. The sensor is implemented in a 0.13µm SiGe BiCMOS process technology. The sensor chip can operate in both continuous wave (CW) and pulse modes. The frequency is tunable from 770MHz to 970MHz, corresponding to Zeeman magnetic fields from 28mT to 35mT for a free electron. The chip consists of a voltage-controlled oscillator, a power amplifier, a low-noise amplifier, a down-conversion mixer, baseband amplifiers, and a pulse generation block. The EPR sensor uses a loop-gap resonator built on a PCB board to interact with the sample. The measurement results of the asphaltenes samples are reported. 1. Introduction Asphaltenes are among the most complex components in crude oil. They are commonly defined as hydrocarbon-based materials that are toluene soluble and n-heptane insoluble [1]. In oil production, for a long time, asphaltenes deposit removal has been remained a major challenge. Asphaltenes percipitation can clog oil pipelines, thereby imposing strong and negative impacts on production [2]. In order to effectively and timely mitigate the effects of asphaltenes precipitation, the asphaltenes concentration in the crude oil must be monitored in real time. Unfortunately, no practical solution exists to achive this goal. In this work, we propose a novel monitoring method that can detect the asphaltenes concentration in crude oil in real time, utilizing the physical principle of Electron Paramagnetic Resonance (EPR). Asphaltenes contain unpaired electrons, which produce EPR signal that can be measured using conventional EPR sensors [3]. However, existing EPR sensors comprise components that are expensive, heavy, and large, which prevent their use in real-time flow-assurance applications. In this work, for the first time, we have used advanced integrated circuit (IC) technology to implement an EPR transceiver on a chip smaller than 4mm2. Measurement results show that the sensors can effectively detect asphaltenes extracted from crude oil. 2. Electron Paramagnetic Resonance (EPR) EPR phenomenon is based on the interaction of electron spins with electromagnetic fields in the presence of an external DC magnetic field [4]. EPR data provides valuable information about electronic structures and spin interactions in paramagnetic materials. The EPR data of a sample can be measured using an EPR sensor, which typically consists of the following major components: transceiver circuits, a magnet with adjustable magnetic field, and a resonator.

For photovoltaic applications the electronic properties of the employed materials are of great importance. A powerful method to investigate electronic states in organic and inorganic materials is electron spin resonance (ESR) spectroscopy. This technique is sensitive for the detection of paramagnetic species, i.e., species carrying electron spins non-zero. A relevant example is polarons in organic semiconductors. This Chapter gives a short introduction to the working principle of ESR spectroscopy and provides an overview what type of information can be revealed with respect to polymer-based photovoltaics. An important task in this field concerns the investigation of the charge transfer process which is an important elementary step in donor/acceptor solar cells. To enable studying charge transfer as a process subsequent to the generation of electron-hole pairs by light absorption, the basic ESR spectroscopy technique can be modified to allow for illumination of the sample during the measurements. This gives rise to so-called light-induced ESR spectroscopy (L-ESR). Beyond the charge transfer process, the recombination of charge carriers in donor/acceptor blends can be studied by L-ESR spectroscopy, as well.

In this study, we deal with the design and implementation of microsystems for electron spin resonance (ESR) applications. Three different microsystems are designed with different approaches for microwave magnetic field generation, ESR detection and sample handling. To the best of our knowledge, these microsystems are the first ever realized fully integrated solutions providing X-band ESR spectroscopy functionality on a single silicon chip. The first microsystem implements absorption mode ESR detection by means of a differential approach. It consists of two pairs of planar excitation/detection coils, a voltage controlled oscillator (VCO), a low noise amplifier (LNA) and a mixer. The excitation/detection coil pairs are identical. The excitation coil has a diameter of 300 microns, and it also functions as the LC-tank inductor of the VCO. The detection coil, which is placed in the middle of the excitation coil, has a diameter of 100 microns. The ESR sample is placed over one of the detection coils. This coil functions as the sample coil, while the empty coil functions as the reference coil. The excitation coils generate the microwave magnetic field acting on the detection coils. In the presence of ESR, the induced voltage on the sample coil differs from the induced voltage on the reference coil. This difference is amplified by the LNA and down-converted by the mixer. Two such microsystems with operating frequencies at 6 and 8 GHz are prototyped on a single standard CMOS chip with an area of 8.6 mm2. The ESR spin sensitivities achieved with the 6 and 8 GHz microsystems are 6.7×1010 and 3.9×1010 spins/GHz1/2, respectively. The second microsystem is an improved version of the first microsystem, with a slightly different topology. Furthermore, LNAs are implemented with SiGe bipolar transistors for higher gain with lower noise figure. It is also prototyped on a single silicon chip with an area of 4 mm2. The ESR spin sensitivity achieved with this microsystem is 3×1010 spins/GHz1/2. The third microsystem implements dispersion mode ESR detection by means of a differential approach in frequency domain. It consists of two VCOs, a mixer, a high frequency buffer, and a frequency divider. The VCO LC-tank inductor has a diameter of 100 microns, and it functions both as the excitation and detection coil. The two coils are placed perpendicularly in order to prevent injection locking. The VCO outputs are multiplied by the mixer, and a difference frequency output is generated. This output is amplified by the high frequency buffer. Its frequency is then divided by the frequency divider, and a 1-bit digital frequency output is generated. In the presence of ESR, the center frequencies of one of the VCOs changes slightly. This change is observed at the output in the form of frequency. This microsystem is prototyped on a standard CMOS chip with an area of 1 mm2. The ESR spin sensitivity achieved with this microsystem is 2×1010 spins/GHz1/2. The realized microsystems already have performances comparable to commercial systems. Their performances can further be improved; and they may be used in different micro scale applications of ESR spectroscopy and imaging.

ROS: a step closer to elucidating their role in the etiology of light-induced skin disorders.

Properly Interpreting Lipid-Protein Specificities in Pulmonary Surfactant

Aldehyde oxidase from rabbit liver was studied by electron paramagnetic resonance (EPR) spectroscopy. Reduction by substrate elicited three types of signals characteristic of free radicals, molybdenum(V), and a non-heme iron complex, respectively. The properties of these signals are described, and the representation of each species in the corresponding signal is quantitatively assessed. Under the specific conditions which yielded a maximum for each of the signal types, 24% of the flavin was in the semiquinone form (g = 2.00), 25% of the iron was in the reduced form responsible for the signal of g = 1.94, and 15 to 20% of the molybdenum was in the Mo(V) state with a signal at g = 1.97. Anaerobic titration of the enzyme with dithionite indicated that reduction of only 1 iron out of each 4 (per flavin) results in the appearance of the g = 1.94 signal, in agreement with the quantitative recovery in the EPR signal. When titration of the enzyme with N-methylnicotinamide was followed by EPR spectroscopy, partial formation of flavin semiquinone and accumulation of reducing equivalents in the non-heme iron complex was observed during consumption of the first 5 to 6 electrons per flavin. Extensive reduction of molybdenum ensued after an additional 1 to 2 eq. Spectrophotometry suggests that the enzymic components which accept electrons from substrate and are not detectable by EPR spectroscopy are that portion of the flavin which is fully reduced and the remainder of the non-heme iron. Rapid titrations, insufficient to permit equilibration between enzyme and substrate, showed migration of reducing equivalents from molybdenum toward flavin and non-heme iron with increasing time. These experiments show that intramolecular electron transfer can be effectively stopped at a low temperature. Measurements of electron spin relaxation of reduced enzyme suggest intramolecular magnetic interactions among the various paramagnetic species generated during the titrations.

Chapter 13 - Brief review on applications of continuous-wave electron paramagnetic resonance spectroscopy in natural product free radical research

This chapter presents a theoretical background on continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy, focusing on electron Zeeman, nuclear Zeeman, and hyperfine interactions. It discusses electron spin angular momentum and considers the effect of placing the unpaired electron in an external magnetic field from a classical perspective. It also examines the influence of the hyperfine interaction and elaxation mechanism on the EPR spectrum. The chapter regards electrons as negatively charged particles that possess an intrinsic property called spin, which is characterized by the spin angular momentum S. It cites a sample in EPR spectroscopy that contains unpaired electrons, which is irradiated with electromagnetic radiation in the presence of an external field.

Copolymers of acrylonitrile : studies of the mechanisms of copolymerization and degradation

Chapter 1 - ESR Standard Methods for Detection of Irradiated Food