Unique in vivo applications of spin traps

Hypoxia-induced reactive oxygen species formation in skeletal muscle

The existence of hypoxia-induced reactive oxygen species (ROS) production remains controversial. However, numerous observations with a variety of methods and in many cells and tissue types are supportive of this idea. Skeletal muscle appears to behave much like heart in that in the early stages of hypoxia there is a transient elevation in ROS, whereas in chronic exposure to very severe hypoxia there is evidence of ongoing oxidative stress. Important remaining questions that are addressed in this review include the following. Are there levels of PO2 in skeletal muscle, typical of physiological or mildly pathophysiological conditions, that are low enough to induce significant ROS production? Does the ROS associated with muscle contractile activity reflect imbalances in oxygen uptake and demand that drive the cell to a more reduced state? What are the possible molecular mechanisms by which ROS may be elevated in hypoxic skeletal muscle? Is the production of ROS in hypoxia of physiological significance, both with respect to cell signaling pathways promoting cell function and with respect to damaging effects of long-term exposure? Discussion of these and other topics leads to general conclusions that hypoxia-induced ROS may be a normal physiological response to imbalance in oxygen supply and demand or environmental stress and may play a yet undefined role in normal response mechanisms to these stimuli. However, in chronic and extreme hypoxic exposure, muscles may fail to maintain a normal redox homeostasis, resulting in cell injury or dysfunction.

Hydroxyl radical generation and oxidative stress in Carassius auratus liver as affected by 2,4,6-trichlorophenol

With Carassius auratus, one of the main economic fish species in Eastern China as test material, this paper studied the hydroxyl radical generation and oxidative stress in its liver under the effect of 2,4,6-trichlorophenol (2,4,6-TCP). Different doses of 2,4,6-TCP were injected intraperitoneally into the fishes, and the Electron paramagnetic resonance (EPR) spectra of hepatic free radicals, activities of superoxide dismutase (SOD), catalase (CAT) and glutathione-s-transferase (GST), levels of reduced glutathione (GSH) and oxidized glutathione (GSSG), and malondialdehyde (MDA) contents were determined 24h after injection. The results showed that under the effects of 2,4,6-TCP, the generation of free radical that was considered to be hydroxyl radical increased significantly, the activities of antioxidant enzymes decreased, with CAT most strongly affected and followed by SOD and GST, the GSH level decreased significantly while GSSG level had little difference, resulting in a decreased GSH/GSSG ratio, and the MDA content increased significantly. All the test parameters showed that C. auratus was subjected to oxidative stress and damage when exposed to 2,4,6-TCP.

Free radicals in exhaled breath condensate in cystic fibrosis and healthy subjects

Many markers of airway inflammation and oxidative stress can be measured non-invasively in exhaled breath condensate (EBC). However, no attempt has been made to directly detect free radicals using electron paramagnetic resonance (EPR) spectroscopy. Condensate was collected in 14 children with cystic fibrosis (CF) and seven healthy subjects. Free radicals were trapped by 5,5-dimethyl-1-pyrroline-N-oxide. EPR spectra were recorded using a Bruker EMX spectrometer. Secondly, to study the source of oxygen centered radical formation, catalase or hydrogen peroxide was added to the condensate. Radicals were detected in 18 out of 21 condensate samples. Analysis of spectra indicated that both oxygen and carbon centered radicals were trapped. Within-subject reproducibility was good in all but one subject. Quantitatively, there was a trend towards higher maximal peak heights of both oxygen and carbon centered radicals in the children with CF. Catalase completely suppressed the signals in condensate. Addition of hydrogen peroxide resulted in increased radical signal intensity. Detection of free radicals in EBC of children with CF and healthy subjects is feasible using EPR spectroscopy.

Hydrazyl-nitrones, novel hybrid molecules in free radical research

This work describes the synthesis and characterisation of some novel hybrid molecules which contains in the same molecule a free radical moiety of hydrazyl type and a spin-trap moiety of nitrone type. The new compounds synthesized have multiple and easy to follow spectroscopic properties, making them useful as sensors or probes in radical chemistry. The new class of hydrazyl-nitrone molecules can act, in a single step process, as both generator and spin-trap of short-lived radicals. The hybrid molecules can be also involved in acid-base or redox processes, and the chemical processes can be easily monitored by visible or electron paramagnetic resonance spectroscopy. The excellent generator and trap properties recommend them as valuable sensors and probes in radical chemistry.

Oxidative stress and electron spin resonance

The body constantly reacts with oxygen as part of the energy producing processes of cells. Oxidative stress is a dysbalance between the production of free radicals as products of these reactions and antioxidant properties of cells. The factors influencing the production of free radicals are physical agents, chemical agents and biological agents. Free radicals are paramagnetic molecules with short time-period for their detection by electron spin resonance (ESR) spectroscopy. The free radical stabilization can be gained by freezing a solution of an organic radical or bonding to spin trapping agents. The spin trapping agents are diamagnetic compounds which rapidly scavenge transient radicals to form stable paramagnetic spin adducts radicals. Because this secondary radical retains an unpaired electron, it can often be detected by electron spin resonance. From ESR spectra can be obtained structural information and kinetic information, information about the formation and decay of the radicals. To study the process of free radical generation is an important step towards reducing the deteriorating effects of oxidative stress.

Novel sparse component analysis approach to free radical EPR spectra decomposition

Free radicals play important roles in many physiological and pathological pathways in biological systems. These free radicals can be detected and quantified by their EPR spectra. The measured EPR spectra are often mixtures of pure spectra of several different free radicals and other chemicals. Blind source separation can be applied to estimate the pure spectra of interested free radicals. However, since the pure EPR spectra are often not independent of each other, the approach based on independent component analysis (ICA) cannot accurately extract the required spectra. In this paper, a novel sparse component analysis method for blind source separation, which exploits the sparsity of the EPR spectra, is presented to reliably extract the pure source spectra from their mixtures with high accuracy. This method has been applied to the analysis of EPR spectra of superoxide, hydroxyl, and nitric oxide free radicals, for both simulated data and real world ex vivo experiment. Compared to the traditional self-modeling method and our previous ICA-based blind source separation method, the proposed sparse component analysis approach gives much better results and can give perfect separation for mixtures of superoxide spectrum and hydroxyl spectrum in the ideal noise-free case. This method can also be used in other similar applications of quantitative spectroscopy analysis.

Non-invasive biomarkers of oxidative stress: reproducibility and methodological issues

Oxidative stress is the hallmark of various chronic inflammatory lung diseases. Increased concentrations of reactive oxygen species (ROS) in the lungs of such patients are reflected by elevated concentrations of oxidative stress markers in the breath, airways, lung tissue and blood. Traditionally, the measurement of these biomarkers has involved invasive procedures to procure the samples or to examine the affected compartments, to the patient's discomfort. As a consequence, there is a need for less or non-invasive approaches to measure oxidative stress. The collection of exhaled breath condensate (EBC) has recently emerged as a non-invasive sampling method for real-time analysis and evaluation of oxidative stress biomarkers in the lower respiratory tract airways. The biomarkers of oxidative stress such as H2O2, F2-isoprostanes, malondialdehyde, 4-hydroxy-2-nonenal, antioxidants, glutathione and nitrosative stress such as nitrate/nitrite and nitrosated species have been successfully measured in EBC. The reproducibility, sensitivity and specificity of the methodologies used in the measurements of EBC oxidative stress biomarkers are discussed. Oxidative stress biomarkers also have been measured for various antioxidants in disease prognosis. EBC is currently used as a research and diagnostic tool in free radical research, yielding information on redox disturbance and the degree and type of inflammation in the lung. It is expected that EBC can be exploited to detect specific levels of biomarkers and monitor disease severity in response to appropriate prescribed therapy/treatment.

“Distant spin trapping”: a method for expanding the availability of spin trapping measurements

The technique of spin trapping is used to study a wide range of free radicals in various systems, including those generated in vitro and in vivo. But unfortunately, EPR spectrometers are not always immediately accessible at the site of experimentation, and therefore it is important to find a method that can preserve a radical adduct over longer periods of time. We describe here an alternative method in which the samples can be frozen and transported for EPR measurements at another site. Various spin adducts of DEPMPO were frozen and measured at 0 degrees C at various intervals after freezing to determine their stability in the frozen state. The radical adducts were generated by established methods and stored at two different temperatures; -196 degrees C (liquid nitrogen) and -80 degrees C (dry ice). The experiments were carried out in an aqueous solution with and without a model of reducing environment (2 mM ascorbate). The results indicate that it is feasible to store and transport spin adducts for subsequent analysis. We conclude that this approach, which we term "distant spin trapping", makes it feasible to transport samples to another site for EPR measurements. This should significantly expand the ability to use spin trapping in biology and medicine.

In vitro and in vivo measurement of pH and thiols by EPR-based techniques

In vitro and in vivo measurements of pH and thiols provide critical information on physiology and pathophysiology of living organisms, particularly related to oxidative stress. Stable nitroxides of imidazoline and imidazolidine types provide the unique possibility of measuring local values of pH and glutathione content in various biological systems, including in vivo studies. The basis for these applications is the observation of specific chemical reactions of these nitroxides with protons or thiols, followed by significant changes in the electron paramagnetic resonance (EPR) spectra of these probes, measured by low-frequency EPR techniques. The applications of some newly developed pH and SH probes in model systems of pharmacological interest, biological fluids, tissues, and cells as well as in vivo studies in isolated hearts and in the gut of living animals are discussed.

Regulation of free radical outflow from an isolated muscle bed in exercising humans

Incremental knee extensor (KE) exercise performed at 25, 70, and 100% of single-leg maximal work rate (WR(MAX)) was combined with ex vivo electron paramagnetic resonance (EPR) spectroscopic detection of alpha-phenyl-tert-butylnitrone (PBN) adducts, lipid hydroperoxides (LH), and associated parameters in five males. Blood samples were taken from the femoral arterial and venous circulation that, when combined with measured changes in femoral venous blood flow, permitted a direct examination of oxidant exchange across a functionally isolated contracting muscle bed. KE exercise progressively increased the net outflow of LH and PBN adducts (100% > 70% > 25% WR(MAX), P < 0.05) consistent with the generation of secondary, lipid-derived oxygen (O(2))-centered alkoxyl and carbon-centered alkyl radicals. Radical outflow appeared to be more intimately associated with predicted decreases in intracellular Po(2) (iPo(2)) as opposed to measured increases in leg O(2) uptake, with greater outflow recorded between 25 and 70% WR(MAX) (P < 0.05 vs. 70-100% WR(MAX)). This bias was confirmed when radical venoarterial concentration differences were expressed relative to changes in the convective components of O(2) extraction and flow (25-70% WR(MAX) P < 0.05 vs. 70-100% WR(MAX), P > 0.05). Exercise also resulted in a net outflow of other potentially related redox-reactive parameters, including hydrogen ions, norepinephrine, myoglobin, lactate dehydrogenase, and uric acid, whereas exchange of lipid/lipoproteins, ascorbic acid, and selected lipid-soluble anti-oxidants was unremarkable. These findings provide direct evidence for an exercise intensity-dependent increase in free radical outflow across an active muscle bed that was associated with an increase in sarcolemmal membrane permeability. In addition to increased mitochondrial electron flux subsequent to an increase in O(2) extraction and flow, exercise-induced free radical generation may also be regulated by changes in iPo(2), hydrogen ion generation, norepinephrine autoxidation, peroxidation of damaged tissue, and xanthine oxidase activation.

EPR spin trapping of protein radicals

Electron paramagnetic resonance (EPR) spin trapping was originally developed to aid the detection of low-molecular-mass radicals formed in chemical systems. It has subsequently found widespread use in biology and medicine for the direct detection of radical species formed during oxidative stress and via enzymatic reactions. Over the last 15 years this technique has also found increasing use in detecting and identifying radicals formed on biological macromolecules as a result of either radical reactions or enzymatic processes. Though the EPR signals that result from the trapping of large, slowly tumbling radicals are often broad and relatively poor in distinctive features, a number of techniques have been developed that allow a wealth of information to be obtained about the nature, site, and reactions of such radicals. This article summarizes recent developments in this area and reviews selected examples of radical formation on proteins.

Measuring reactive species and oxidative damage in vivo and in cell culture: how should you do it and what do the results mean?

Free radicals and other reactive species (RS) are thought to play an important role in many human diseases. Establishing their precise role requires the ability to measure them and the oxidative damage that they cause. This article first reviews what is meant by the terms free radical, RS, antioxidant, oxidative damage and oxidative stress. It then critically examines methods used to trap RS, including spin trapping and aromatic hydroxylation, with a particular emphasis on those methods applicable to human studies. Methods used to measure oxidative damage to DNA, lipids and proteins and methods used to detect RS in cell culture, especially the various fluorescent "probes" of RS, are also critically reviewed. The emphasis throughout is on the caution that is needed in applying these methods in view of possible errors and artifacts in interpreting the results.

Biomarkers in breath condensate: a promising new non-invasive technique in free radical research

Oxidative stress is associated with a range of inflammatory lung diseases including asthma, adult respiratory distress syndrome, idiopathic pulmonary fibrosis, pneumonia, lung transplantation, chronic obstructive pulmonary disease, cystic fibrosis, bronchiectasis and lung cancer. Increased concentrations of reactive oxygen species (ROS) in the airways of such patients are reflected by elevated concentrations of oxidative stress markers in the breath, airways, lung tissue and blood. Traditionally, the measurement of these biomarkers has involved invasive procedures to procure the samples, or examine the compartments. As a consequence, there is a need for less invasive approaches to measure oxidative stress. Analysis of breath hydrocarbons has partly fulfilled this need, however only gas phase volatile constituents can be assessed by this approach. The collection of exhaled breath condensate (EBC) is a simple, non-invasive approach, which comprehensively samples the lower respiratory tract. It is currently used as a research and diagnostic tool in the free radical field, yielding information on redox disturbance and the degree and type of inflammation in the lung. With further technical developments, such an approach may ultimately have a role in the clinic, in helping to diagnose specific lung diseases. EBC can be exploited to assess a spectrum of potential biomarkers, thus generating a "finger print" characteristic of the disease. By assessing the nature of oxidative stress in this manner, the most appropriate therapy can be selected and the response to treatment monitored.

Free radical EPR spectroscopy analysis using blind source separation

In this paper, we propose a novel approach for electron paramagnetic resonance (EPR) mixture spectra analysis based on blind source separation (BSS) technique. EPR spectrum of a free radical is often superimposed by overlapping spectra of other species. It is important and challenging to accurately identify and quantify the 'pure' spectra from such mixtures. In this study, an automated BSS method implementing independent component analysis is used to extract the components from mixed EPR spectra that contain overlapping components of different paramagnetic centers. To apply this method, there is no requirement to know the component spectra or the number of components in advance. The method is applied to analyze free radical EPR spectra which are collected from standard chemical system, cultured cell suspense, and ex vivo rat kidneys by spin trapping EPR technique. Results show that the BSS method proposed here is capable of identifying the component EPR spectra from mixtures with unknown compositions. The BSS technique can offer powerful aids in resolving spectral overlapping problems in general EPR spectroscopy analysis.

Liquid chromatographic enantioseparation of spin-labelled β-amino acids

Direct and indirect high-performance liquid chromatographic (HPLC) methods were developed for the enantioseparation of spin-labelled, cyclic, chiral beta-amino acids containing nitroxide free radicals, trans-3-amino- 1-oxyl-2,2,5,5-tetramethylpyrrolidine-4-carboxylic acid (trans-POAC), cis-4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-3-carboxylic acid (cis-beta-TOAC) and their N-Fmoc-protected analogues, synthesized in racemic and enantiomerically pure forms. The direct method involved the use of a Chiralcel OD-RH column, while indirect separation was carried out by application of either 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate or (S)-N-(4-nitrophenoxycarbonyl)-phenylalanine methoxyethyl ester as chiral derivatizing agent. Use of 1-fluoro-2,4-dinitrophenyl-5-L-alanine amide (Marfey's reagent) as chiral derivatizing agent failed because of the low of yield of the derivatization reaction. Selection and variation of the mobile phase was restricted by the sensitivity of the spin-labelled amino acids to acidic conditions. Conditions affording the best resolution were found and the differences in separation capability of the methods were noted. The sequence of elution of the enantiomers was determined by different methods and, in the case of the beta-TOAC analogues, the absolute configurations of the enantiomers corresponding to each peak were identified.

Identification and quantification of free radicals during myocardial ischemia and reperfusion using electron paramagnetic resonance spectroscopy

There is general agreement that free radicals are involved in reperfusion injury. Electron paramagnetic resonance (EPR) spectroscopy can be considered as the more suitable technique to directly measure and characterize free radical generation during myocardial ischemia and reperfusion. There are essentially two approaches used in the detection of unstable reactive species: freezing technique and spin traps. The detection of secondary free radicals or ascorbyl free radicals during reperfusion might provide an index of oxidative stress. Spin trapping can also characterize nitric oxide. EPR spectroscopy can provide important data regarding redox state and free radical metabolism but ideally, the spin traps must not interfere with cell or organism function.

Detection of p53 mutations in precancerous gastric tissue

Intestinal-type gastric cancer is preceded by gastritis and intestinal metaplasia. There is uncertainty regarding the stage at which genetic alterations in the p53 gene occur. Reactive oxygen species (ROS) may participate in the production of mutations and the inactivation of p53 is due to infection by the bacterium Helicobacter pylori. We have investigated whether alterations of the p53 gene can be detected in gastritis and intestinal metaplasia using the restriction site mutation assay. We also assessed the potential contribution of ROS to p53 inactivation using electron spin resonance spectroscopy (ESR) and correlated with the presence of H. pylori. In all, 35% of the gastritis samples and 45% of the intestinal metaplasia samples were found to contain mutations in exons 5–8 of the p53 gene. Electron spin resonance spectroscopy analysis showed a significant increase in free radical levels in gastritis samples compared with normal, intestinal metaplasia and cancer samples, suggesting that free radicals present in gastritis may contribute to p53 mutations. There was no significant difference in free radical levels between the H. pylori-positive and -negative groups. However, a small subpopulation of the H. pylori-negative patients had much higher levels of free radicals. This suggests a more prominent role for other factors in ROS production.

Spin traps: in vitro toxicity and stability of radical adducts

We have evaluated the effects of DMPO, CMPO, EMPO, BMPO, and DEPMPO on functioning CHO cells and the stability of the radical adducts in the presence of cells. The potential toxic effects of the spin traps were measured by two estimates of cell viability (trypan blue exclusion and colony formation) and one of cell function (rate of oxygen consumption). We also studied the effects of the spin traps on colony formation in a second cell line, 9L tumor cells. Toxicity varied with the type of cell line and the parameter that was measured. In aqueous solutions the order of stability for all spin adducts was SO(3) > OH > CH(3), while in cell suspensions it was SO(3) > OH approximately CH(3). The radical adducts of the new spin traps have significantly increased stability as compared to DMPO. These results indicate that the new spin traps potentially offer increased stability of spin adducts in functioning cells. It also is clear that it is necessary to carry out appropriate studies of the stability and toxicity in the system that is to be studied for any particular use of these spin traps. It then should be feasible to select the spin trap(s) best suited for the proposed study.

EPR spectroscopic detection of free radical outflow from an isolated muscle bed in exercising humans

There is no direct evidence to support the contention that contracting skeletal muscle and/or associated vasculature generates free radicals in exercising humans. The unique combination of isolated quadriceps exercise and the measurement of femoral arterial and venous free radical concentrations with the use of electron paramagnetic resonance (EPR) spectroscopy enabled this assumption to be tested in seven healthy men. Application of ex vivo spin trapping using alpha-phenyl-tert-butylnitrone (PBN) resulted in the detection of oxygen- or carbon-centered free radicals (a(N) = 1.38 +/- 0.01 mT and a(beta)(H) = 0.17 +/-0.01 mT, where a(N) and a(beta)(H) are the nitrogen and beta-hydrogen coupling constants, respectively) with consistently higher EPR signal intensities of the PBN spin adduct observed in the venous compared with the arterial circulation (P < 0.05). Incremental exercise further increased the venoarterial intensity difference [85 +/- 58 arbitrary units (AU) at 24 +/- 6% maximal work rate (WR(max)) vs. 387 +/- 214 AU at 69 +/- 7% WR(max); P < 0.05]. When combined with measured changes in femoral venous blood flow (Q), this resulted in a net adduct outflow of 130 +/- 118 and 1,146 +/- 582 AU/min (P < 0.05), which was positively associated with leg oxygen uptake (r(2) = 0.47, P < 0.05) and Q (r(2) = 0.47, P < 0.05). These results provide the first evidence for oxygen- or carbon-centered free radical outflow from an active muscle bed in humans.

In vivo production of hydroxyl radical by Enterococcus faecalis colonizing the intestinal tract using aromatic hydroxylation

Enterococcus faecalis is an intestinal commensal that produces extracellular superoxide (O(2)(*-)) through autoxidation of membrane-associated demethylmenaquinone. To assess free radical production by E. faecalis in vivo, intestinal tracts of rats were colonized using wild-type E. faecalis or a mutant strain with attenuated O(2)(*-) production. Ex vivo electron paramagnetic resonance spin trapping study of colonic contents (mean +/- SD) showed 1.4 +/- 1.5 and 0.094 +/- 0.24 microM 5,5-dimethyl-1-pyrroline-N-oxide-hydroxyl radical adduct/gm stool for rats colonized with wild-type and mutant strains, respectively (p = .002). In vivo hydroxyl radical production was further assayed by aromatic hydroxylation using phenyl N-tert-butylnitrone (PBN) and D-phenylalanine. Hydroxylated PBN and D-phenylalanine products were recovered from stool (microM/gm colonic contents/10(9) colony forming units) and urine (microM/h/ml), respectively, and quantified using electrochemical detection. Hydroxylated (OH) PBNs and isomeric tyrosines (hydroxylated phenylalanine) were significantly increased (mean +/- SD) for rats colonized with wild-type E. faecalis (2-OH PBN, 63 +/- 58; 3-OH PBN, 63 +/- 84; ortho-tyrosine, 31 +/- 27; meta-tyrosine, 17 +/- 14) compared to the mutant strain (2-OH PBN, 2.5 +/- 7.3 (p < .001); 3-OH PBN, 3.9 +/- 12.3 (p = .01); ortho-tyrosine, 1.9 +/- 6.0 (p < .001); meta-tyrosine, 1.5 +/- 3.4 (p = .03)). Similar differences were observed following in vitro incubations of these bacteria with aromatic targets. These results confirm in vivo production of hydroxyl radical by E. faecalis colonizing the intestine, and indicate this bacterium may be a potent source of oxidative stress on the intestinal epithelium.

Endotoxin-induced liver hypoxia: defective oxygen delivery versus oxygen consumption

In vivo EPR was used to investigate liver oxygenation in a hemodynamic model of septic shock in mice. Oxygen-sensitive material was introduced either (i) as a slurry of fine particles which localized at the liver sinusoids (pO2 = 44.39 +/- 5.13 mmHg) or (ii) as larger particles implanted directly into liver tissue to measure average pO2 across the lobule (pO2 = 4.56 +/- 1.28 mmHg). Endotoxin caused decreases in pO2 at both sites early (5-15 min) and at late time points (6 h after endotoxin; sinusoid = 11.22 +/- 2.48 mmHg; lobule = 1.16 +/- 0.42 mmHg). The overall pO2 changes observed were similar (74.56% versus 74.72%, respectively). Blood pressures decreased transiently between 5 and 15 min (12.88 +/- 8% decrease) and severely at 6 h (59 +/- 9% decrease) following endotoxin, despite volume replacement with saline. Liver and circulatory nitric oxide was elevated at these times. Liver oxygen extraction decreased from 44% in controls to only 15% following endotoxin, despite severe liver hypoxia. Arterial oxygen saturation, blood flow (hepatic artery), and cardiac output were unaffected. Pretreatment with l-NMMA failed to improve endotoxin-induced oxygen defects at either site, whereas interleukin-13 preserved oxygenation. These site-specific measurements of pO2 provide in vivo evidence that the principal cause of liver hypoxia during hypodynamic sepsis is reduced oxygen supply to the sinusoid and can be alleviated by maintaining sinusoidal perfusion.

NMR spin trapping: detection of free radical reactions with a new fluorinated DMPO analog

Electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin trapping were used for detection of free radical reactions utilizing a new fluorinated analog of DMPO, 4-hydroxy-5,5-dimethyl-2-trifluoromethylpyrroline-1-oxide (FDMPO). The parent FDMPO spin trap exhibits a single 19F-NMR resonance at -66.0 ppm. The signal to noise ratio improved 10.4-fold compared to 31P-NMR sensitivity of the phosphorus-containing spin trap, DEPMPO. The spin adducts of FDMPO with .OH, .CH3, and .CH2OH were characterized. Competitive spin trapping of FDMPO with DMPO showed that both have similar rates of addition of .OH and C-centered radicals. The corresponding paramagnetic spin adducts of FDMPO were extremely stable to degradation. In the presence of ascorbate, reaction products from C-centered radicals resulted in the appearance of two additional 19F-NMR signals at -78.6 and -80 ppm for FDMPO/ .CH(3) and at -74.6 and -76.75 ppm for FDMPO/ .CH(2)OH. In each case, these peaks were assigned to the two stereoisomers of their respective, reduced hydroxylamines. The identification of the hydroxylamines for FDMPO/ .CH3 was confirmed by EPR and 19F-NMR spectra of independently synthesized samples. In summary, spin adducts of FDMPO were highly stable for ESR. For NMR spin trapping, FDMPO showed improved signal to noise and similar spin trapping efficiency compared to DEPMPO.