Cyclic Nitroxides Research Articles

Reaction of cyclic nitroxides with nitrogen dioxide: the intermediacy of the oxoammonium cations.

Piperidine and pyrrolidine nitroxides, such as 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 3-carbamoylproxyl (3-CP), respectively, are cell-permeable stable radicals, which effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The kinetics and mechanism of their reactions with .OH, superoxide, and carbon-centered radicals have been extensively studied, but not with .NO2, although the latter is a key intermediate in cellular nitrosative stress. In this research, .NO2 was generated by pulse radiolysis, and its reactions with TPO, 4-OH-TPO, 4-oxo-TPO, and 3-CP were studied by fast kinetic spectroscopy, either directly or by using ferrocyanide or 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate), which effectively scavenge the product of this reaction, the oxoammonium cation. The rate constants for the reactions of .NO2 with these nitroxides were determined to be (7-8) x 10(8) M(-)(1) s(-)(1), independent of the pH over the range 3.9-10.2. These are among the highest rate constants measured for .NO2 and are close to that of the reaction of .NO2 with .NO, that is, 1.1 x 10(9) M(-1) s(-1). The hydroxylamines TPO-H and 4-OH-TPO-H are less reactive toward .NO2, and an upper limit for the rate constant for these reactions was estimated to be 1 x 10(5) M(-1) s(-1). The kinetics results demonstrate that the reaction of nitroxides with .NO2 proceeds via an inner-sphere electron-transfer mechanism to form the respective oxoammonium cation, which is reduced back to the nitroxide through the oxidation of nitrite to .NO2. Hence, the nitroxide slows down the decomposition of .NO2 into nitrite and nitrate and could serve as a reservoir of .NO2 unless the respective oxoammonium is rapidly scavenged by other reductant. This mechanism can contribute toward the protective effect of nitroxides against reactive nitrogen-derived species, although the oxoammonium cations themselves might oxidize essential cellular targets if they are not scavenged by common biological reductants, such as thiols.

Crystal structure and magnetic properties of novel chiral nitroxides existing as racemic conglomerates

Racemic samples of novel chiral cyclic nitroxides 2-(4-hydroxyphenyl)-2,5,5-trimethylpyrrolidine-1-oxy 1, trans-2-(4-hydroxyphenyl)-2,5-dimethyl-5-phenylpyrrolidine-1-oxy 2 and trans-2,5-bis(4-hydroxyphenyl)-2,5-dimethylpyrrolidine-1-oxy 3 having 4-hydroxyphenyl groups on the stereogenic centres adjacent to the NO radical moiety have been found to exist as racemic conglomerates in the crystalline state, and the origin of the conglomerate formation has been studied by X-ray crystallographic analysis and magnetic susceptibility measurements.

Site-activity relationship of nitroxide radical's antioxidative effect.

A relatively new strategy in preventing oxidative damage employs cyclic nitroxides. These stable radicals have been widely used as biophysical probes, spin labels, and are currently tested as contrast agents for nuclear magnetic resonance imaging. Nitroxides were found to protect cells, organs, and whole animals against diverse oxidative insults. The present study concentrated on comparing the antioxidative activity of nitroxides against oxidative damage, initiated either in the lipid or aqueous phase, to egg phosphatidylcholine acyl chains (13.4% polyunsaturated fatty acids) in small unilamellar vesicles. We determined the lipophilicity and liposome-membrane/aqueous-medium partition coefficient for several nitroxides and compared their specific protective effects. The aim was to study the relation between nitroxides' concentration, location in the lipid bilayer, and their protection against oxidative damage. Both 6-membered- and 5-membered-ring nitroxides were studied for: (i) partitioning between the lipid bilayer and the aqueous phase (nitroxides were quantified using EPR spectroscopy); (ii) the intrabilayer distribution, using three different fluorescent probes of known location of their fluorophors in the lipid bilayer; and (iii) the specific antioxidative effect (protection per concentration) against radicals formed in a liposomal dispersion. The radicals were generated using the thermolabile, radical-generating compounds 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) in the aqueous phase, and 2,2'-azobis (2,4-dimethyl-valeronitrile) (AMVN) in the lipid phase. The results show that nitroxides react, in a concentration-dependent manner, with deleterious species at their formation sites, both in the aqueous and the lipid phase, and that their specific protective effects for the lipophilic target, the lipid bilayer, are similar for both the lipophilic and the hydrophilic nitroxides.

The role of oxoammonium cation in the SOD-mimic activity of cyclic nitroxides.

Cyclic nitroxides (RNO(*)) mimic the activity of superoxide dismutase (SOD) and demonstrate antioxidant properties in numerous in vitro and in vivo models. Their broad antioxidant activity may involve the participation of their reduced and oxidized forms, that is, hydroxylamine (RNO-H) and oxoammonium cation (RNO(+)). To examine this possibility we studied the reactions of RNO(*) and RNO(+) with HO(2)(*)/O(2)(*)(-) and with several reductants by pulse radiolysis and rapid-mixing stopped-flow techniques. The oxoammonium cations were generated by electrochemical and radiolytic oxidation of 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidinoxyl (3-CP). The rate constant for the reaction of RNO(*) with HO(2)(*) to form RNO(+) was determined to be (1.2 +/- 0.1) x 10(8) for TPO and (1.3 +/- 0.1) x 10(6) M(-)(1) s(-)(1) for 3-CP. The kinetics results demonstrate that the reaction of RNO(*) with HO(2)(*) proceeds via an inner-sphere electron-transfer mechanism. The rate constant for the reaction of RNO(*) with O(2)(*)(-) is lower than 1 x 10(3) M(-)(1) s(-)(1). The rate constant for the reaction of RNO(+) with O(2)(*)(-) was determined to be (3.4 +/- 0.2) x 10(9) for TPO(+) and (5.0 +/- 0.2) x 10(9) M(-)(1) s(-)(1) for 3-CP(+). Hence, both nitroxides catalyze the dismutation of superoxide through the RNO(*)/RNO(+) redox couple, and the dependence of the catalytic rate constant, k(cat), on pH displayed a bell-shaped curve having a maximum around pH 4. The oxoammonium cation oxidized ferrocyanide and HO(2)(-) by a one-electron transfer, whereas the oxidation of methanol, formate, and NADH proceeded through a two-electron-transfer reaction. The redox potential of RNO(*)/RNO(+) couple was calculated to be 0.75 and 0.89 V for 3-CP and TPO, respectively. The elucidated mechanism provides a clearer insight into the biological antioxidant properties of cyclic nitroxides that should permit design of even more effective antioxidants.

Kinetics and mechanism of hydroxyl radical and OH-adduct radical reactions with nitroxides and with their hydroxylamines.

Stable nitroxide radicals are potent antioxidants and are among the most effective non-thiol radioprotectants, although they react with hydroxyl radicals more slowly than typical phenolic antioxidants or thiols. Surprisingly, the reduced forms of cyclic nitroxides, cyclic hydroxylamines, are better reductants yet have no radioprotective activity. To clarify the reason for this difference, we studied the kinetics and mechanisms of the reactions of nitroxides and their hydroxylamines with (*)OH radicals and with OH-adducts by using pulse radiolysis, fluorimetric determination of phenolic radiation products, and electron paramagnetic resonance spectrometric determination of nitroxide concentrations following radiolysis. Competition kinetics with phenylalanine as a reference compound in pulse radiolysis experiments yielded rate constants of (4.5 +/- 0.4) x 10(9) M(-1) s(-1) for the reaction of (*)OH radical with 2,2,6,6-tetramethylpiperidine-N-oxyl (TPO), 4-hydroxy-TPO (4-OH-TPO), and 4-oxo-TPO (4-O-TPO), (3.0 +/- 0.3) x 10(9) M(-1) s(-1) for deuterated 4-O-TPO, and (1.0 +/- 0.1) x 10(9) M(-1) s(-1) for the hydroxylamine 4-OH-TPO-H. The kinetic isotope effect suggests the occurrence of both (*)OH addition to the aminoxyl moiety of 4-O-TPO and H-atom abstraction from the 2- or 6-methyl groups or from the 3- and 5-methylene positions. This conclusion was further supported by final product analysis, which demonstrated that (*)OH partially oxidizes 4-O-TPO to the corresponding oxoammonium cation. The rate constants for the reactions of the nitroxides with the OH-adducts of phenylalanine and terephthalate have been determined to be near 4 x 10(6) M(-1) s(-1), whereas the hydroxylamine reacted at least 50 times slower, if at all. These findings indicate that the reactivity toward (*)OH does not explain the differences between the radioprotective activities of nitroxides and hydroxylamines. Instead, the radioprotective activity of nitroxides, but not of hydroxylamines, can be partially attributed to their ability to detoxify OH-derived secondary radicals.

EPR investigation of the free radicals generated during the photosensitization of TiO2 colloid by hypocrellin B

The cation radical of dye produced from the interfacial electron transfer from a surface chelated dye to the conduction band of the colloidal TiO2 was studied by laser flash photolysis and electron paramagnetic resonance (EPR) techniques. The study employed hypocrellin B (HB), a natural photodynamic pigment with strong absorption over the visible light region, as a sensitizer and titanium dioxide as a colloid semiconductor. HB formed a chelate with this colloid semiconductor and exhibited a red-shifted and strongly enhanced absorption in the visible spectrum. Laser photolysis indicated that the electron excitation in the visible absorption band of the chelate resulted in extremely rapid and efficient electron injection from the excited triplet state of the dye into the conduction band of the semiconductor. A transient absorption of cation radical of HB at 570 nm was observed. The appearance of cation radical of HB was characterized by EPR spectrometry: the photoinduced EPR signal was not quenched by oxygen and its intensity decreased in the presence of NaI, a typical hole scavenger. The generation of conduction band electrons in HB-sensitized TiO2 system was also verified by the spin elimination of a stable cyclic nitroxide, 2,2, 6,6-tetramethylpiperidine-1-oxyl (TEMPO), and by the reduction of methyl viologen (MV2+) to its radical MV+·.

Synthesis and Characterization of Poly(1,3-phenylene)-Based Polyradical Carrying Cyclic Nitroxides. Observation of Ferromagnetic Interaction

Synthesis and Characterization of Poly(1,3-phenylene)-Based Polyradical Carrying Cyclic Nitroxides. Observation of Ferromagnetic Interaction

Differential scanning calorimetry

This study was aimed to investigate the physicochemical changes induced in 200 nm extruded oligolamellar DPPC:DPPG (10:1) liposomes by freezing, followed by γ-irradiation, in the absence and presence of 5 mM stable cyclic nitroxide radicals, 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol). The characterization is based on the use of differential scanning calorimetry (DSC) and was aimed to differentiate the contribution of freezing and γ-irradiation in the presence and absence of nitroxides. Liposomal preparations of DPPC/DPPG which have sub-, pre- and main-phase transitions in the temperature range (0°C<Tm<50°C) were used.

Separation of cyclic nitroxide free radicals and their redox forms with dual microelectrochemical detection

A gradient elution method with conventional electrochemical detection was developed to separate four analogs of the nitroxide 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radical in less than 3 min. The corresponding hydroxylamines and oxoammonium ions of these free radicals were separated using the identical chromatographic method. A dual microelectrochemical detector was constructed that allowed for simultaneous anodic and cathodic detection of all three redox forms. This detector was superior to a conventional electrochemical detector because it could provide immediate information about the oxidation state of the nitroxide, it nearly eliminated post-column band broadening, and it was less susceptible to changes in oxygen concentration of the mobile phase during gradient elutions.

Fast-Scan Voltammetry of Cyclic Nitroxide Free Radicals

Fast-scan cyclic voltammetry at carbon fiber microelectrodes is used to detect the cyclic nitroxide 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical (TEMPO) and three analogs. The electrochemical behavior of the TEMPO analogs at unmodified carbon fiber electrodes is found to differ greatly from their behavior at glassy carbon electrodes. After the electrode is coated with the polymer Nafion, the electrodes exhibit increased sensitivity to TEMPO and 4-amino-TEMPO. Voltammograms of the nitroxides at Nafion-coated electrodes indicate that the oxidized form (oxoammonium ion) and the free radical form have greatly different mobilities through the polymeric coating. Response times to changes in nitroxide concentration vary from subsecond at bare electrodes (all four analogs) and 4-hydroxy-TEMPO at modified electrodes to 1-3 s for TEMPO and 4-amino-TEMPO at modified electrodes. The detection limit for 4-amino-TEMPO is 50 μM at an unmodified electrode and 5 μM at a Nafion-coated carbon fiber electrode. The sensitivity of the Nafion-modified electrode to TEMPO, 4-hydroxy-TEMPO, and 4-amino-TEMPO can be improved by choosing a resting potential at which the oxoammonium ion form of the nitroxide is preconcentrated into the Nafion film. Using fast-scan cyclic voltammetry and the modified carbon fiber electrodes, the reaction of two nitroxide free radicals with ascorbate can be monitored. This work shows that fast-scan voltammetry at microelectrodes is a sensitive method that can be used to follow reactions of cyclic nitroxide free radicals in solution.

Structural dependence of nitroxide spin labels and nitroxide spin adducts on their reducibility by ascorbate ion.

The reducibility of a series of nitroxides (aminoxyls) by ascorbate was tested by measuring the nitroxide decay rates with a stopped-flow electron paramagnetic resonance technique in aqueous phosphate buffer solution. The dependence of reactivity on the structures and pH of the medium was found for both cyclic nitroxides and nitroxide adducts of phenyl N-tert butyl nitrone (PBN). In cyclic nitroxides, the ring size is a dominant factor in determining reaction rates but substituents have additional effects on the rate depending on their electronegativity. For alkyl and hydroxyalkyl adducts of PBN, at fixed ascorbate concentration, half-lives increase with lengthening of the substituent, suggesting that a long chain in the substituent sterically protects the nitroxide group and thus prevents its reduction by ascorbate.

Reactions of conjugated dienes with nitrogen monoxide and dioxide

Addition of ˙NO to dilute (ca. 10–4 mol l–1) solutions of 2,3-dimethylbuta-1,3-diene gave at least two nitroxyl radicals. The major radical, with a half-life of ca. 30 min, had an EPR spectrum with 1H hyperfine splitting of ca. 10 G from four equivalent protons. Similarly, the major nitroxyl radical formed from 2,5-dimethylhexa-2,4-diene gave a major triplet showing no resolved 1H hyperfine coupling, but satellite features from 13C (a= 10 G) from two equivalent nuclei. A minor radical with slightly different 14N coupling and a 13C splitting of 6.5 G was also detected. These radicals were very stable. On dilution of the diene, the second species was favoured over the former, and became the major component at ⩽ 10–5 mol l–1. These reactions occurred in the complete absence of ˙NO2. No radicals were detected using pure ˙NO2, but mixtures gave strong signals from nitroxyl radicals.Addition of ˙NO to dilute solutions of phorone gave a long-lived nitroxyl radical with a(13C)= 6.5 G from four equivalent carbon nuclei. However, in this case, ˙NO2 gave a 30 G triplet assigned to an alkoxy-nitroxyl radical, and mixtures of ˙NO +˙NO2 gave both signals in about the same proportion.It is suggested that for the butadienes the ˙NO adds reversibly to give allylic radicals. These can cyclise, dimerise or add a second ˙NO. The resulting nitroso derivatives can then add another radical to give nitroxyl radicals. For 2,3-dimethylbuta-1,3-diene, the cyclic nitroxide has a very short lifetime and hence was not detected. However, for the hexadiene, although the species with a(13C)= 10 G cannot be the simple cyclic derivative, the second species with a(13C)= 6.5 G probably has a cyclic structure. The results for phorone can all be understood in terms of a cyclic 1:1 adduct, both for ˙NO and ˙NO2, and we favour this assignment. Hence, it is suggested that phorone, or derivatives thereof, may be useful for EPR detection of ˙NO radicals, and possibly of both ˙NO and ˙NO2 in mixtures.

Reaction of Phenyl-Substituted o-Quinodimethanes with Nitric Oxide. Are Benzocyclobutenes Suitable Precursors for Nitric Oxide Cheletropic Traps?

In order to elucidate the potential of substituted o-quinodimethanes as reagents for the trapping of nitric oxide (NO) in biological systems, the reaction of alkoxyl- and alkyl-substituted 7,8-diphenyl- and 7,7,8-triphenyl-o-quinodimethanes with nitric oxide in solution was investigated by ESR spectroscopic and UV/vis stopped-flow techniques. Photolytic decarbonylation of 1,3-diphenyl- and 1,1,3-triphenylindan-2-ones gave the corresponding phenyl-substituted benzocyclobutenes as the major products and low photostationary concentrations of o-quinodimethanes. During 266-nm laser flash photolysis (LFP) of 1,3-dimethoxy-1,3-diphenylindan-2-one and 1-methoxy-1,3,3-triphenylindan-2-one in acetonitrile, species absorbing in the 400-600 nm range were produced, which were attributed to configurational isomers of the corresponding 7,7,8,8-substituted o-quinodimethanes. The isomeric o-quinodimethanes decayed at significantly different rates, indicating a strong influence of the relative orientation of the terminal substituents on their stability. Reaction of the raw photolysates of the 2-indanones with NO produced strong ESR spectra of the corresponding cyclic nitroxide radicals, isoindolin-2-oxyls. The nitroxide radicals were generated in a two-phase process, the first, rapid phase being attributed to the reaction of NO with the photolytically formed o-quinodimethanes and the second, slow phase reflecting the reaction with small amounts of o-quinodimethanes, generated by thermal ring opening of the phenyl-substituted benzocyclobutenes and probably a direct reaction of NO with the benzocyclobutenes. The kinetics of both steps, as evaluated by stopped-flow UV/vis and ESR spectroscopy, revealed a strong dependence of the rate constants of the o-quinodimethane + NO reaction on the substitution pattern of the o-quinodimethanes, with rate constants spanning a range of 10-4000 M(-)(1) s(-)(1). The rate constants ((0.4-7.5) x 10(-)(4) s(-)(1)) for the reaction of NO with the 7,7,8,8-tetrasubstituted benzocyclobutenes are much less influenced by the substitution pattern. The utility of phenyl-substituted benzocyclobutenes as "reservoirs" for o-quinodimethane-type nitric oxide traps is discussed.

A stable nitroxide radical effectively decreases mucosal damage in experimental colitis.

TEMPOL, a cyclic nitroxide stable radical blocks biological damage by breaking chain reactions through termination reaction with free radicals, and by inhibiting the catalytic effect of transition metals. This study tested its protective effect on two models of experimental colitis as free radicals play an important part in their pathogenesis. TEMPOL was given intragastrically immediately after induction of colitis with acetic acid or trinitrobenzene sulphonic acid (TNB) and mucosal damage was assessed one, three, or seven days later. Cellular partition of TEMPOL was determined by electron paramagnetic resonance spectroscopy. In vitro experiments showed that TEMPOL immediately penetrates colonic mucosa and, following its intragastric administration, it persists in both gastric and colonic mucosa for several hours. Intragastric administration of TEMPOL, 0.5 g/kg/bw, immediately after intracaecal administration of 5% acetic acid significantly decreased mucosal lesion area, myeloperoxidase activity, and leukotriene B4 and C4 generation when assessed 24 hours after damage induction. Intragastric administration of TEMPOL, 0.5 g/kg/bw, immediately after intracolonic administration of 30 mg TNB in 0.25 ml 50% ethanol, and once daily thereafter, significantly decreased mucosal lesion area assessed after one, three, and seven days, having no effect on LTC4 generation and affecting colonic weight, myeloperoxidase activity, and LTB4 generation only sporadically. In conclusion, TNB and acetic acid induced colitis can be pharmacologically manipulated by TEMPOL. TEMPOL may be beneficial in the treatment or prevention of inflammatory bowel disease.

Ab initio study of possible intermediates in the no(4Π) catalyzed geometric isomerizations of olefins

The geometries of acyclic and three-membered ring (nitroxide) H4C2NO radicals in their ground 2Π electronic states have been optimized completely at ab initio UHF and ROHF theoretical levels with the STO-3G and the 6-31G** basis sets. The optimizations favour the cyclic nitroxide structure energetically. However ΔE(acyclic - cyclic) at the UHF and ROHF/6-31G** levels are only 3.2 and 1.9 kcal mol-1, respectively. Incomplete MP2/6-311G** optimizations support these results. The zero-point energy computed at the ROHF/6-31G** level for the nitroxide radical is 2.5 kcal mol-1 higher than that for the acyclic structure, thus reversing the relative energies by 0.6 kcal mol-1. The energies of the two radical structures, relative to the sum of those for ethylene and NO, are very close to literature values of the activation energies for the thermal, NO catalyzed geometrical isomerizations of olefins. Thus cyclic nitroxide intermediates may play a role not only in the Hg 6(3P1) photosensitized, but also in the thermal, NO-catalyzed geometric isomerizations of olefins.

Radikalreaktionen an N-Heterocyclen. XI. ESR-spektroskopischer Nachweis von Radikalen der 3-Methyl-pyrazolin-5-one

Free Radical Reactions of N-Heterocyclic Compounds. XI. ESR-Study of Radicals of 3-Methyl-pyrazolin-5-ones The oxidation of 1,4-substituted 3-methylpyrazolin-5-ones (1a – p) led to pyrazolinonyl radicals (2a – p), which were detected directly or after trapping with nitrosobenzene as phenyl nitroxides. In a few cases, the products of oxidation of pyrazolin-5-ones (1) with electron-withdrawing groups in 4-position by peroxy radicals were endo cyclic nitroxides (8), which were also detected by ESR spectroscopy.

β-Phosphorylated cyclic nitroxides. 2. Synthesis of pyrrolidin- and piperidin-2-yl phosphonates and the corresponding stable nitroxides.

Alkenyl α-aminophosphonates are cyclized by intramolecular aminomercuration to give pyrrolidin- and piperidin-2-yl phosphonates which lead, after oxidation with m-CPBA, to the corresponding stable β-phosphorylated nitroxides.

Superoxide Reaction with Nitroxides

Stable, free radical nitroxides are commonly used ESR spectroscopy tools. However, it has recently been found that ESR observable signal from 5-membered ring spin-adducts or stable label nitroxides is lost or diminished by reaction with superoxide. A similar radical-radical annihilation was not found for six membered ring nitroxide radicals. To discern why six-membered ring nitroxides are not reduced under superoxide flux generated by hypoxanthine/xanthine oxidase, spectrophoptmetric (Cyt CIII) and chemiluminescence (lucigenin) and ESR assays were used to follow the reactions. Spectrophotometry and chemiluminescence clearly demonstrated that the six-membered piperidine-1-oxyl compounds (TEMPO, TEMPOL, and TEMPAMIN) rapidly react with superoxide: rate constants at pH 7.8 ranging from 7 x 10(4) to 1.2 x 10(5) M-1 s-1. The absence of detectable ESR signal loss results from facile re-oxidation of the corresponding hydroxylamine by superoxide. To fully corroborate the efficiency of the 6-membered nitroxide superoxide dismutase activity, they were shown to protect fully mammalian cells from oxidative damage resulting from exposure to the superoxide and hydrogen peroxide generating system hypoxanthine/xanthine oxidase. Since six-membered cyclic nitroxides react with superoxide about 2 orders of magnitude faster than the corresponding 5-membered ring nitroxides, they may ultimately be more useful as superoxide oxide dismutase mimetic agents.

Radical mechanism in the photoreaction of organic N-oxides: oxygen transfer from photoexcited paradiazine di- N-oxides to amines

Photolysis of paradiazine di- N-oxides in solution leads to the production of N-hydroxy- N′-oxyl radicals via hydrogen abstraction. For example, a chloroform solution of 2-methyl-3-acetylquinoxaline 1,4-dioxide gives the radical 1-hydroxy-2-methyl-3-acetylquinoxalinyl-4-oxyl. In the presence of amines, photoinduced oxygen transfer has been observed and electron spin resonance studies have indicated that reaction leads to the formation of amine-derived nitroxide radicals in systems involving triethylamine and secondary amines. Thus, the two typical cyclic nitroxides pyrrolidinyl-1-oxyl and 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxyl are generated through photolysis of chloroform solutions containing 2-methyl-3-acetylquinoxaline 1,4-dioxide and the corresponding amines. The evidence presented suggests that the active oxygenating species are the lowest triplet states of the N-oxides generated by photolysis. Exciplexes formed through collision of the T 1 state N-oxides with the amines may be intermediates in the oxygen-transfer processes. Amine-derived radicals are generated from the exciplexes through further ON bond formation and α cleavage of the amine moiety.

Investigation of naturally abundant17O hyperfine satellites in the electron spin resonance spectrum of 5- and 6-membered cyclic nitroxide (aminoxyl) radicals

Hyperfine satellites of naturally abundant 17O-isotopes have been detected for twelve nitroxide spin labels; line width, saturation, and magnetic parameters are discussed in terms of hyperfine anisotropy and molecular geometry.