Electron Paramagnetic Resonance Evidence Research Articles

2-chlorophenol induced hydroxyl radical production in mitochondria in Carassius auratus and oxidative stress – An electron paramagnetic resonance study

In our previous study, electron paramagnetic resonance (EPR) evidence of reactive oxygen species (ROS) production in Carassius auratus following 2-chlorophenol (2-CP) administration was provided. To further investigate the potential pathway of ROS production, liver mitochondria of C. auratus was isolated and incubated with 2-CP for 30min. An EPR analysis indicated ROS was produced, and intensities of ROS increased with increasing concentrations of 2-CP. The ROS was then assigned •OH by comparing with Fenton reaction. Either catalase or superoxide dismutase, extinguished •OH completely in the mitochondria mixture. These facts suggested that O2·- and H2O2 contributed to the formation of •OH in mitochondria in C. auratus stressed by 2-CP. Combining previous references and our own data, it is reasonable to suggest that 2-CP is first oxidized by H2O2 present in vivo to form phenoxyl radical under the catalytic action of cellular peroxidase (1); phenoxyl radical oxidizes mitochondria NADH to NAD• in the presence of NADH (2); NAD• reacts with oxygen in vivo to produce O2·- (3); O2·- is spontaneously dismutated by SOD to form H2O2 and O2, which creates a renewable supply of H2O2 as the initiators of the chain reactions until NADH is consumed (4); simultaneously with reaction (4), O2·- reacts with H2O2 to form •OH radical via the Haber–Weiss reaction (5). A strong negative correlation (r=−0.9278, p<0.01) between glutathione (GSH) pool and •OH production was observed after fish were i.p. injected with 2-CP (250mgkg−1), indicating the depletion of GSH caused by •OH.

Biochemical and spectroscopic studies of human melanotransferrin (MTf): Electron-paramagnetic resonance evidence for a difference between the iron-binding site of MTf and other transferrins

Melanotransferrin (MTf) is a member of the transferrin (Tf) family of iron (Fe)-binding proteins that was first identified as a cell-surface marker of melanoma. Although MTf has a high-affinity Fe-binding site that is practically identical to that of serum Tf, the protein does not play an essential role in Fe homeostasis and its precise molecular function remains unclear. A Zn(II)-binding motif, distinct from the Fe-binding site, has been proposed in human MTf based on computer modelling studies. However, little is known concerning the interaction of its proposed binding site(s) with metals and the consequences in terms of MTf conformation. For the first time, biochemical and spectroscopic techniques have been used in this study to characterise metal ion-binding to recombinant MTf. Initially, the binding of Fe to MTf was examined using 6 M urea gel electrophoresis. Although four different iron-loaded forms were observed with serum Tf, only two forms were found with MTf, the apo-form and the N-monoferric holo-protein, suggesting a single high-affinity site. The presence of a single Fe(III)-binding site was also supported by EPR results which indicated that the Fe(III)-binding characteristics of MTf were unique, but somewhat comparable to the N-lobes of human serum Tf and chicken ovo-Tf. Circular dichroism (CD) analysis indicated that, as for Tf, no changes in secondary structure could be observed upon Fe(III)-binding. The ability of MTf to bind Zn(II) was also investigated using CD which demonstrated that the single high-affinity Fe-binding site was distinct from a potential Zn(II)-binding site.

EPR and DRS evidence for NO2 sensing in Al-doped ZnO

Abstract Zinc oxide (ZnO) is a well-known semiconducting multifunctional material wherein properties right from the morphology to gas sensitivity can be tailor-made by doping or surface modification. Aluminum (Al)-incorporated porous zinc oxide (Al:ZnO) exhibits good response towards NO 2 at low-operating temperature. The NO 2 gas concentration as low as 20 ppm exhibits S = 17% for 5 wt.% Al-incorporated ZnO. The NO 2 response increases with operating temperature and concentration and reaches to its maximum at 300 °C without any interference from other gases such as SO 3 , HCl, LPG and alcohol. Physico-chemical characterization likes differential thermogravimetric analysis (TG-DTA) electron paramagnetic resonance (EPR) and diffused reflectance spectroscopy (DRS) have been used to understand the sensing behavior for pure and Al-incorporated ZnO. The TG-DTA depicts formation of ZnO phase at 287 °C. The EPR study reveals distinct variation for O − ( g = 2.003) and Zn interstitial ( g = 1.98) defect sites in pure and Al:ZnO. The DRS studies elucidate signature of adsorbed NO x species in aluminium-incorporated zinc oxide indicating its tendency to adsorb these species even at low temperatures. This paper is an attempt to correlate the gas sensing behavior with the physico-chemical studies such as EPR and DRS.

Edaravone inhibits lipid peroxidation in neonatal hypoxic-ischemic rats: An in vivo microdialysis study

The occurrence of hypoxia-ischemia (HI) during early fetal or neonatal stages of an individual leads to the damaging of immature neurons resulting in behavioral and psychological dysfunctions. Free radical-mediated lipid peroxidation is the main cause of neurotoxicity including neonatal brain damage. Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) is a novel anti-oxidant agent and the drug of choice in the treatment of acute ischemic brain disorders in adult patient. The purpose of this study is to determine the direct effects of edaravone in inhibiting the lipid peroxidation production in the neonatal rat brains during hypoxic-ischemic insult by electron paramagnetic resonance (EPR) spectoroscopy and in vivo brain microdialysis. Seven-day-old Wistar rats were subjected to left common carotid artery ligation and a probe was inserted in the rat hippocampus. Edaravone (5, 50, or 100μM) or saline was perfused with a spin trap agent (α-(4-pyridyl-N-oxide)-N-tert-butylnitrone; POBN) before, during and after hypoxia (1h of 8% O2 exposure) and then analyzed by EPR. Edaravone (100μM) did not show any EPR evidence of POBN adduct formation during and after hypoxic-ischemic insult. However, the EPR signal increased, but not significantly during the hypoxic period in the hypoxic and edaravone 50μM-treated groups compared to control. Edaravone at 5μM significantly increased the EPR signals compared to control. This study shows that edaravone directly and dose-dependently inhibited the formation of lipid free radicals produced during hypoxic-ischemic insult in the neonatal rat brain. These results suggest that edaravone is able to attenuate neuronal damage in the rat neonatal brain by inhibiting the formation of lipid radicals.

Electron Paramagnetic Resonance Study of the Mononuclear Al Species Formed in the Reaction of Al Atoms and Cyclopropylamine at 77 K in an Adamantane Matrix

Ground-state Al atoms have been reacted with cyclopropylamine (CpNH2) in an adamantane matrix at 77 K. The four paramagnetic species generated in the reaction have been studied by electron paramagnetic resonance (EPR) spectroscopy and their magnetic parameters extracted from the EPR spectra. The isotropic Al hyperfine interactions (hfis) for these radicals vary from 801 to 962 MHz and are consistent with values previously reported for structurally similar Al-centered radicals. The major species formed in the reaction has a highly anisotropic a tensor indicative of an Al-centered radical with a substantial amount of the unpaired spin density residing in the Al 3p orbital. In addition, there is EPR evidence that the insertion products, CpNH(AlH) and CpAlNH2, are formed in the reaction, suggesting that Al atoms can activate the N-H and C-N bonds of amines. It has been shown that a molecule of CpNH2 coordinates to CpNH(AlH), causing the Al, N and H hfi of the N-H insertion product to decrease. The values of the nuclear hfis calculated for the insertion products, using a density functional theory (DFT) method, are comparable to the experimental values.

Electron paramagnetic resonance evidence of hydroxyl radical generation and oxidative damage induced by tetrabromobisphenol A in Carassius auratus

Tetrabromobisphenol A (TBBPA) is one of the most widely used brominated flame retardants (BFRs). To confirm its putative oxidative stress-inducing activity, freshwater fish Carassius auratus were injected intraperitoneally with TBBPA. One experiment lasted 3 h to 28 days after a single injection of 100 mg/kg TBBPA, and the other lasted 24 h after a single injection of 0–300 mg/kg TBBPA. Reactive oxygen species (ROS) were trapped by phenyl- tert-butyl nitrone (PBN) and detected by electron paramagnetic resonance (EPR). Protein carbonyl (PCO) and lipid peroxidation product (LPO) content were also determined. A six-line EPR spectrum was detected in the sample prepared in air, and a multiple one was obtained in nitrogen. The observed spectrum in nitrogen fits the simulation one with PBN/ OCH 3 and PBN/ CH 3 quite well. As compared to the control group, TBBPA significantly induced ROS production marked by the intensity of the prominent spectra in liver and bile. TBBPA (100 mg/kg) also significantly increased PCO content in liver starting 24 h and LPO content 3 days after injection. Either PCO or LPO content showed significant relation with ROS production. Based on the hyperfine constants and shape of the spectrum, ROS induced by TBBPA was determined as OH. The results clearly indicated that TBBPA could induce OH generation and result in oxidative damage in liver of C. auratus.

Properties of the Cysteine Residues and Iron−Sulfur Cluster of the Assimilatory 5‘-Adenylyl Sulfate Reductase from Pseudomonas aeruginosa

APS reductase from Pseudomonas aeruginosa has been shown to contain a [4Fe-4S] cluster. Thiol determinations and site-directed mutagenesis studies indicate that the single [4Fe-4S] cluster contains only three cysteine ligands, instead of the more typical arrangement in which clusters are bound to the protein by four cysteines. Resonance Raman studies in the Fe-S stretching region are also consistent with the presence of a redox-inert [4Fe-4S](2+) cluster with three cysteinate ligands and indicate that the fourth ligand is likely to be an oxygen-containing species. This conclusion is supported by resonance Raman and electron paramagnetic resonance (EPR) evidence for near stoichiometric conversion of the cluster to a [3Fe-4S](+) form by treatment with a 3-fold excess of ferricyanide. Site-directed mutagenesis experiments have identified Cys139, Cys228, and Cys231 as ligands to the cluster. The remaining two cysteines present in the enzyme, Cys140 and Cys256, form a redox-active disulfide/dithiol couple (E(m) = -300 mV at pH 7.0) that appears to play a role in the catalytic mechanism of the enzyme.

EPR evidence of hydroxyl radical generation as an initiator of lipid peroxidation in amyloid β-protein-stimulated PC12 cells

Recent data from several groups suggest that the primary mechanism of amyloid β-protein (Aβ) neurotoxicity may be mediated by free radicals. To evaluate this hypothesis, our aim is to make the mechanism of Aβ neurotoxicity clear, especially in the formation of free radicals. In this study, rat pheochromocytoma (PC12) cells were exposed to Aβ25–35 and confirmed free radical generations using two kinds of spin trap agents, 5,5-dimethyl-1-pyrroline- N-oxide; DMPO and α-(4-pyridyl-1-oxide)- N- tert-butylnitrone; POBN. DMPO spin adduct revealed that hydroxyl radical ( OH), while POBN spin adduct identified a lipid radical ( L) as electron paramagnetic resonance (EPR) evidence of lipid peroxidation in the process of cell damage by Aβ25–35 exposure. An Aβ cytotoxicity assay also was performed by using WST-8 reduction system and histochemical analysis. These analyses showed cell damage induced by Aβ. This study provides EPR evidence that Aβ neurotoxicity is derived from hydrogen abstraction from polyunsaturated lipid acid by hydroxyl radical as a cause of lipid peroxidation.

Fluorescence resonance energy transfer from tryptophan to a chromium(III) complex accompanied by non-specific cleavage of albumin: a step forward towards the development of a novel photoprotease

A chromium(III) complex, transdiaqua [ N , N ′ -propylenebis(salicylideneimino)chromium(III)]perchlorate ([Cr(salprn)(H 2 O) 2 ]ClO 4 ) in the presence of sodium azide and upon photoexcitation was found to bring about non-selective cleavage of bovine serum albumin (BSA). Electron paramagnetic resonance (EPR) evidence has been obtained for the formation of a Cr(V) species upon photolysis of a solution containing the chromium(III) complex and sodium azide. This Cr(V) species non-selectively cleaves BSA. The fluorescence excitation spectrum of BSA-[Cr(salprn)(H 2 O) 2 ] + adduct showed a band at λ ex max =370 nm due to charge transfer transition of the chromium(III) complex as well as a prominent band at 290 nm attributable to tryptophan absorption. This indicated an efficient Forster type fluorescence energy transfer (FRET) from the tryptophan residues to the chromium(III) complex indicating that the Cr(III) complex binds in the vicinity of the tryptophan residue.

Fluorescence resonance energy transfer from tryptophan to a chromium(III) complex accompanied by non-specific cleavage of albumin: a step forward towards the development of a novel photoprotease

A chromium(III) complex, transdiaqua [ N, N ′-propylenebis(salicylideneimino)chromium(III)]perchlorate ([Cr(salprn)(H 2O) 2]ClO 4) in the presence of sodium azide and upon photoexcitation was found to bring about non-selective cleavage of bovine serum albumin (BSA). Electron paramagnetic resonance (EPR) evidence has been obtained for the formation of a Cr(V) species upon photolysis of a solution containing the chromium(III) complex and sodium azide. This Cr(V) species non-selectively cleaves BSA. The fluorescence excitation spectrum of BSA-[Cr(salprn)(H 2O) 2] + adduct showed a band at λ ex max=370 nm due to charge transfer transition of the chromium(III) complex as well as a prominent band at 290 nm attributable to tryptophan absorption. This indicated an efficient Forster type fluorescence energy transfer (FRET) from the tryptophan residues to the chromium(III) complex indicating that the Cr(III) complex binds in the vicinity of the tryptophan residue.

Characterization of expressed pigmented core light harvesting complex (LH 1) in a reaction center deficient mutant of Blastochloris viridis.

The utility of photosynthetically defective mutants in the purple photosynthetic bacterium Blastochloris viridis (formerly Rhodopseudomonas viridis)was demonstrated with construction of a reaction-center deficient mutant, LH 1-H. This LH 1-H mutant has a photosynthetic apparatus in which most of the puf operon genes were deleted, resulting in an organism containing only the genes for the light harvesting polypeptides and the H subunit of the reaction center. This B. viridisstrain containing a truncation of the puf operon was characterized by gel electrophoresis, lipid-to-protein ratio analysis, optical spectroscopy, electron paramagnetic resonance and transmission electron microscopy. Optical and electron paramagnetic resonance spectroscopies revealed no photoactivity in this LH 1-H mutant consistent with the absence of intact reaction centers. Electron paramagnetic resonance evidence for assembled LH 1 complexes suggested that the interactions between light harvesting polypeptide complexes in membranes were largely unchanged despite the absence of their companion reaction center cores. The observed increase in the lipid-to-protein ratio was consistent with modified interactions between LH 1s, a view supported by transmission electron microscopy analysis of membrane fragments. The results show that B. viridis can serve as a practical system for investigating structure-function relationships in membranes and photosynthesis through the construction of photosynthetically defective mutants.

SO 4−–SO 3− radical pair formation in Ce doped and Ce, U co-doped K 3Na(SO 4) 2: EPR evidence and its role in TSL

Thermally stimulated luminescence (TSL) and electron paramagnetic resonance (EPR) studies were carried out on cerium doped and cerium–uranium co-doped K 3Na(SO 4) 2 samples after γ-irradiation. Three glow peaks around 352, 415 and 475 K were observed and their spectral characteristics have shown that Ce 3+ and UO 2 2+ act as the emission centres in K 3Na(SO 4) 2:Ce and K 3Na(SO 4) 2:Ce, U, respectively. In Ce–U co-doped sample, energy transfer from cerium to uranium takes place. The commonly occurring radiation-induced centres in sulphates, viz SO 3 − and SO 4 − were observed by EPR and SO 4 − radical ion was found to take part in the TSL emission at 415 K . The hitherto unknown information, however, is the formation of SO 4 −–SO 3 − radical pair creating deep traps in these lattices, apparently assisted by the dopants. This is the first observation of such radical pair formation leading to the identification of deep traps in this lattice. The radical pair, (SO 3 −–SO 4 −) which is stable up to 970 K , decreases the intensity of the peak at 415 K due to the depletion of SO 4 − centres.

Electron Paramagnetic Resonance Evidence for Binding of Cu 2+ to the C-terminal Domain of the Murine Prion Protein

Transmissible spongiform encephalopathies in mammals are believed to be caused by scrapie form of prion protein (PrP Sc), an abnormal, oligomeric isoform of the monomeric cellular prion protein (PrP C). One of the proposed functions of PrP C in vivo is a Cu(II) binding activity. Previous studies revealed that Cu 2+ binds to the unstructured N-terminal PrP C segment (residues 23–120) through conserved histidine residues. Here we analyzed the Cu(II) binding properties of full-length murine PrP C (mPrP), of its isolated C-terminal domain mPrP(121–231) and of the N-terminal fragment mPrP(58–91) in the range of pH 3–8 with electron paramagnetic resonance spectroscopy. We find that the C-terminal domain, both in its isolated form and in the context of the full-length protein, is capable of interacting with Cu 2+. Three Cu(II) coordination types are observed for the C-terminal domain. The N-terminal segment mPrP(58–91) binds Cu 2+ only at pH values above 5.0, whereas both mPrP(121–231) and mPrP(23–231) already show identical Cu(II) coordination in the pH range 3–5. As the Cu 2+-binding N-terminal segment 58–91 is not required for prion propagation, our results open the possibility that Cu 2+ ions bound to the C-terminal domain are involved in the replication of prions, and provide the basis for further analytical studies on the specificity of Cu(II) binding by PrP.

Electron paramagnetic resonance evidence for a C3' sugar radical in crystalline d(CTCTCGAGAG) X-irradiated at 4 K.

Debije, M. G. and Bernhard, W. A. Electron Paramagnetic Resonance Evidence for a C3' Sugar Radical in Crystalline d(CTCTCGAGAG) X-Irradiated at 4 K. Radiat. Res. 155, 687-692 (2001). A neutral sugar radical formed by the net loss of hydrogen from C3' has been identified in crystalline DNA X-irradiated at 4 K. Crystals of duplex d(CTCTCGAGAG), known to be of B conformation, were studied using electron paramagnetic resonance (EPR) spectroscopy. The C3' radical was identified by using information from dose saturation, power saturation, thermal annealing, and spectrum simulation. The yield of the C3' radical, G(C3'), is 0.03 +/- 0.01 micromol/J, and its concentration does not appear to saturate up to at least 100 kGy. In the region in which total radical concentration increases linearly with dose, the C3' radical makes up about 4.5% of the total radical population trapped in the oligodeoxynucleotide crystal at 4 K. Based on free base release measured in other oligodeoxynucleotides, we suggest that in d(CTCTCGAGAG) the C3' radical is responsible for about one-third of the strand breakage events.

EPR spectroscopic study of the reaction of ground-state aluminium atoms with acyclic alcohols in a rotating cryostat

Ground-state aluminium atoms have been reacted with methanol, ethanol and 2-methylpropan-2-ol, in an adamantane matrix, in a rotating cryostat at 77 K. We found no electron paramagnetic resonance (EPR) evidence under these experimental conditions for aluminium atom insertion into the C–C, C–H or O–H bonds of these alcohols. There are, however, several mononuclear aluminium species formed with large aluminium hyperfine interactions (hfi) (1206–1537 MHz). A comparison of the magnetic parameters with those determined for similar systems suggests that in the aluminium–methanol reaction, Al(OCH3)2 is the major product. In addition, our results suggest that HOAlOCH3 and Al(OH)2 are present in the reaction mixture. We have calculated the aluminium hfi for these species using density functional theory (DFT) methods and the results are in close agreement with the experimental values. The aluminium–ethanol reaction gives Al(OCH2CH3)2 and HOAlOCH2CH3 while only HOAlOC(CH3)3 is observed from reaction of aluminium atoms with 2-methylpropan-2-ol.

Chelation of hypocrellin B with zinc ions with electron paramagnetic resonance (EPR) evidence of the photodynamic activity of the resulting chelate

Hypocrellin B (HB), a perylenequinone derivative, is an efficient phototherapeutic agent. The chelation of HB with Zinc ions (Zn2+) results in a metal chelate (Zn-HB) which exhibits considerable absorption (λmax = 612nm) in the phototherapeutic window. The structure of this chelate has been characterized by UV-Vis, IR and mass spectra. The redox potentials of the Zn-HB chelate were Eox = +1.1V (vs. SCE) and Ere = -0.7V (vs. SCE) as measured using the circle volt curve. The quantum yield of singlet oxygen generated by the Zn-HB chelate was 0.86, which both the electron spin trap (EPR) method and the chemical trap method show to be about 0.1 higher than that of its parent compound HB. In irradiated oxygen-saturated solutions of Zn-HB chelate, superoxide radical anions and hydroxyl radicals were detected by EPR spectroscopy using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as the spin-trapping agent.

Electron Paramagnetic Resonance Evidence for a Novel Interconversion of [3Fe-4S]+ and [4Fe-4S]+Clusters with Endogenous Iron and Sulfide in Anaerobic Ribonucleotide Reductase Activase in Vitro

We report an EPR study of the iron-sulfur enzyme, anaerobic ribonucleotide reductase activase from Lactococcus lactis. The activase (nrdG gene) together with S-adenosyl-L-methionine (AdoMet) give rise to a glycyl radical in the NrdD component. A semi-reduced [4Fe-4S](+) cluster with an axially symmetric EPR signal was produced upon photochemical reduction of the activase. Air exposure of the reduced enzyme gave a [3Fe-4S](+) cluster. The Fe(3)S(4) cluster was convertible to the EPR-active [4Fe-4S](+) cluster by renewed treatment with reducing agents, demonstrating a reversible [3Fe-4S](+)- to-[4Fe-4S](+) cluster conversion without exogenous addition of iron or sulfide. Anaerobic reduction of the activase by a moderate concentration of dithionite also resulted in a semi-reduced [4Fe-4S](+) cluster. Prolonged reduction gave an EPR-silent fully reduced state, which was enzymatically inactive. Both reduced states gave the [3Fe-4S](+) EPR signal after air exposure. The iron-sulfur cluster interconversion was also studied in the presence of AdoMet. The EPR signal of semi-reduced activase-AdoMet had rhombic symmetry and was independent of which reductant was applied, whereas the EPR signal of the [3Fe-4S](+) cluster after air exposure was unchanged. The results indicate that an AdoMet-mediated [4Fe-4S](+) center is the native active species that induces the formation of a glycyl radical in the NrdD component.

Study of dichlophenac polymeric derivative with 12-DOXYL spin probe

Continuous wave (CW) electron paramagnetic resonance (EPR) studies of the 12-doxylmethyl stearic acid methyl ester (12-DOXYL) spin probe were made in the solutions of a triblock copolymer of acrylamide, vinylchloroethyl ether and an ester of the vinylchloroethyl ether with 2-[(2,6-dichlorophenyl)amino] benzene acetic acid, “dichlophenac” (DPH). Light scattering experiments show that the hydrophobic DPH group influences the clustering of the polymer in solution. The EPR evidence suggests that when the polymer is dissolved in buffer solutions of 12-DOXYL, the spin probe undergoes a partitioning between the solution, the polymer clusters and a minor third environment. The rate of the 12-DOXYL partitioning from the solution into the polymer cluster is fast compared with the time scale of the changes observed in light scattering and exhibits a dependence on the concentration of the polymer in solution and the pH. The 12-DOXYL environment in the polymer cluster is characterized as restricting 12-DOXYL to slow molecular motion with rotational correlation time τ=1.3×10 −8 s/rad and permitting relatively infrequent collisions between the 12-DOXYL molecules. The solution spectrum of the 12-DOXYL is dominated by the spin exchange narrowing expected for concentrated assemblies of the 12-DOXYL molecules, such as a micelle or bilayer. Partitioning of a hydrophobic spin probe in an aqueous solution is shown to be a sensitive measure of the presence of stabilizing hydrophobic regions in a polymer aggregate.

In vivo EPR evidence for free radical adducts of nifedipine.

Nifedipine [3,5-pyridinedicarboxylic acid, 1,4-dihydro-2, 6-dimethyl-4-(2-nitrophenyl)-dimethyl ester] is a calcium channel blocker that has been widely used as a prescription drug for patients with hypertension. After illumination by ordinary light for 24 hr, nifedipine is converted completely to its nitroso analog without further photochemical degradation. Evidence for stable, nitroxyl-like free radical generation in mice was observed 15 min after intramuscular (i.m.) or intraperitoneal (i.p.) injection of illuminated nifedipine as monitored by in vivo L-band electron paramagnetic resonance (EPR) spectrometry. This was confirmed in more detail by ex vivo measurements on excised muscle and liver tissue. The nature of these radicals was surmised by comparing the reaction of illuminated nitroso-nifedipine with polyunsaturated fatty acids. Surprisingly, identical radical spectra were detected from excised liver doped with nonilluminated nifedipine, suggesting that this drug can be enzymatically converted in vivo to its nitroso analog without the requirement for illumination. This is one of the first reports of in vivo EPR evidence for a class of unsaturated fatty acid radical conjugates resulting from the normal metabolism of a common drug. Magn Reson Med 42:691-694, 1999.

Electron paramagnetic resonance evidence for reversible transformation of thermal donor into shallow donor-type center in hydrogen-implanted silicon

Electron paramagnetic resonance spectrum of the proton-related thermal donor (TD) assigned as NL8 paramagnetic center has been detected at 110 K after heat treatment of the hydrogen-implanted Czochralski-Si at 773 K. The effect of temperature on reversible transformations of the anisotropic spectrum of NL8 center into the isotropic singlet line was studied in the temperature region from 110 to 240 K. The analysis of the singlet provides an evidence that this signal originates from the proton-related shallow donor type at g=1.9987. The changes in the linewidth have been used to evaluate the parameters [1/τ=0.66×1012 exp(−ΔE/kT); ΔE=169 meV] for thermally activated electron emission to the conduction band from the second donor state of the NL8 center. These results represent direct experimental evidence of reversible transformation of the TD+ charged center into the shallow donor-type center.