Cyclic Nitroxides Research Articles

Novel ferrocenyl nitroxides: Synthesis, structures, electrochemistry and antioxidative activity

Novel ferrocenyl nitroxides 3− 6 were prepared as models for studying the linkage structure–activity relationship. Single-crystal X-ray structures of compounds 4 and 5 were determined and comparatively studied. The in vitro antioxidative activities (e.g. scavenging superoxide anion and hydroxide radical) of compounds 3− 6 and 4-ferrocenamido-2,2,6,6-tetramethyl-piperidin-oxy ( 2) were evaluated. Compounds 3 and 5 exhibit high scavenging activities in low concentration. Results from electrochemistry and UV/Vis spectroscopy show that the redox property and antioxidative activity are closely related to the structure of the linkage bridging ferrocene and cyclic nitroxide moieties. The results further indicate that the ferrocene moiety plays a principal role in antioxidation. The modification of linkage was found to be able to decrease the ferrocene/ferrocenium potential and improve the antioxidative activity effectively.

1-tert-Butyl-3,3,5,5-tetraalkyl-2-piperazinon-4-oxyls: Highly Efficient Nitroxides for Controlled Radical Polymerization

The paper describes nitroxide-mediated radical polymerization of styrene and n-butyl acrylate by using four sterically highly hindered 6-membered cyclic nitroxides. The syntheses of the corresponding alkoxyamine initiators are described, and also rate constants for C−O bond homolysis of these systems are discussed. It is shown that rate constants of alkoxyamines can readily be determined by 1H NMR experiments. Polymerization results obtained are compared with data previously achieved with highly efficient nitroxides. Since this class of nitroxides is readily accessible, the 1-tert-butyl-3,3,5,5-tetraalkyl-2-piperazinon-4-oxyl radicals bearing alkyl groups that are ethyl or larger n-alkyl groups are probably the most efficient cyclic 6-membered nitroxides known to date for mediating NMP.

Electric, electrochemical and magnetic properties of novel ionic liquid nitroxides, and their use as an EPR spin probe

Novel metal-free paramagnetic ionic liquid imidazolium compounds, which contain a chiral cyclic nitroxide unit as the redox and spin sources in the cation, have been prepared, and their first use as an EPR spin probe in diamagnetic ionic liquids as well as their electric, electrochemical and magnetic properties are described.

Kinetics of the Reaction between Nitroxide and Thiyl Radicals: Nitroxides as Antioxidants in the Presence of Thiols

Cyclic nitroxides effectively protect cells, tissues, isolated organs, and laboratory animals from radical-induced damage. The present study focuses on the kinetics and mechanisms of the reactions of piperidine and pyrrolidine nitroxides with thiyl radicals, which are involved in free radical "repair" equilibria, but being strong oxidants can also produce cell damage. Thiyl radicals derived from glutathione, cysteine, and penicillamine were generated in water by pulse radiolysis, and the rate constants of their reactions with 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO), 4-OH-TPO, and 3-carbamoyl-proxyl were determined to be (5-7) x 10 (8) M (-1) s (-1) at pH 5-7, independent of the structure of the nitroxide and the thiyl radical. It is suggested that the reaction of nitroxide (>NO (*)) with thiyl radical (RS (*)) yields an unstable adduct (>NOSR). The deprotonated form of this adduct decomposes via heterolysis of the N-O bond, yielding the respective amine (>NH) and sulfinic acid (RS(O)OH). The protonated form of the adduct decomposes via homolysis of the N-O bond, forming the aminium radical (>NH (*+)) and sulfinyl radical (RSO (*)), which by subsequent reactions involving thiol and nitroxide produce the respective amine and sulfonic acid (RS(O) 2OH). Nitroxides that are oxidized to the respective oxoammonium cations (>N (+)O) are recovered in the presence of NADH but not in the presence of thiols. This suggests that the reaction of >N (+)O with thiols yields the respective amine. Two alternative mechanisms are suggested, where >N (+)O reacts with thiolate (RS (-)) directly generating the adduct >NOSR or indirectly forming >NO (*) and RS (*), which subsequently together yield the adduct >NOSR. Under physiological conditions the adduct is mainly deprotonated, and therefore nitroxides can detoxify thiyl radicals. The proposed mechanism can account for the protective effect of nitroxides against reactive oxygen- and nitrogen-derived species in the presence of thiols.

Experimental and Theoretical Studies of the Redox Potentials of Cyclic Nitroxides

The redox potentials of 25 cyclic nitroxides from four different structural classes (pyrrolidine, piperidine, isoindoline, and azaphenalene) were determined experimentally by cyclic voltammetry in acetonitrile, and also via high-level ab initio molecular orbital calculations. It is shown that the potentials are influenced by the type of ring system, ring substituents and/or groups surrounding the radical moiety. For the pyrrolidine, piperidine, and isoindolines there is excellent agreement (mean absolute deviation of 0.05 V) between the calculated and experimental oxidation potentials; for the azaphenalenes, however, there is an extraordinary discrepancy (mean absolute deviation of 0.60 V), implying that their one-electron oxidation might involve additional processes not considered in the theoretical calculations. This recently developed azaphenalene class of nitroxide represents a new variant of a nitroxide ring fused to an aromatic system and details of the synthesis of five derivatives involving differing aryl substitution are also presented.

Cyclic nitroxides inhibit the toxicity of nitric oxide-derived oxidants: mechanisms and implications

The substantial therapeutic potential of tempol (4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy) and related cyclic nitroxides as antioxidants has stimulated innumerous studies of their reactions with reactive oxygen species. In comparison, reactions of nitroxides with nitric oxide-derived oxidants have been less frequently investigated. Nevertheless, this is relevant because tempol has also been shown to protect animals from injuries associated with inflammatory conditions, which are characterized by the increased production of nitric oxide and its derived oxidants. Here, we review recent studies addressing the mechanisms by which cyclic nitroxides attenuate the toxicity of nitric oxide derived oxidants. As an example, we present data showing that tempol protects mice from acetaminophen-induced hepatotoxicity and discuss the possible protection mechanism. In view of the summarized studies, it is proposed that nitroxides attenuate tissue injury under inflammatory conditions mainly because of their ability to react rapidly with nitrogen dioxide and carbonate radical. In the process the nitroxides are oxidized to the corresponding oxammonium cation, which, in turn, can be recycled back to the nitroxides by reacting with upstream species, such as peroxynitrite and hydrogen peroxide, or with cellular reductants. An auxiliary protection mechanism may be down-regulation of inducible nitric oxide synthase expression. The possible therapeutic implications of these mechanisms are addressed.

Self-Assembly of β-Cyclodextrin in Water. 2. Electron Spin Resonance

The interaction of amphipilic spin labels with beta-cyclodextrin was investigated using conventional electron spin resonance (ESR) spectroscopy to explore the aggregation of cyclodextrins in water. Methyl 5-doxylstearate (5-DMS) and stearic acid spin probes (n-DSA), which contain a cyclic nitroxide moiety with unpaired electrons covalently linked to the aliphatic chain carbon in positions 5, 7, 12, and 16, show that different dynamic domains coexist in beta-CD water solutions above 3 mM. The results are consistent with the formation of beta-CD self-assembled structures in water above a critical aggregation concentration and confirm the previous findings that were reported in the part 1 article of this series.

The vibrational group frequency of the N–O [rad] stretching band of nitroxide stable free radicals

The group frequency of the N–O radical stretching vibration has received scant attention in the literature. The few existing treatments of the vibrational spectroscopy of nitroxides are incomplete at best and potentially misleading to workers in the field. To close this gap in the available knowledge, the existing literature on the vibrational spectra of nitroxide stable free radicals is critically reviewed with particular reference to the wavenumber position of the N–O stretching vibration, ν(N–O ). Poor evidentiary bases for the assignment ν(N–O ) were found in many instances. Ab initio Density Field Theory calculations using a model chemistry of UB3LYP at the 6-311++G(d,p) level were performed to obtain a theoretical band position of ν(N–O ) for comparison with the published data. Large discrepancies between the theoretical and experimental values were found for the radical 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxyl, which currently sets the lower limit of the accepted wavenumber range of ν(N–O ), as well as for the nitronyl and iminyl nitroxides. The wavenumber position of ν(N–O ) was found to occur in the range 1450–1420 cm −1 for 5-membered cyclic nitroxides and 1395–1340 cm −1 for 6-membered cyclic and acyclic nitroxides. In nitronyl nitroxides, the symmetric stretching vibration occurs in the region 1470 cm −1, but coupling to other modes makes specific band assignments problematic for the nitronyl nitroxide group.

Kinetics and Mechanism of Peroxyl Radical Reactions with Nitroxides

Cyclic nitroxides (>NO*) are stable radicals of diverse size, charge, lipophilicility, and cell permeability, which provide protection against oxidative stress via various mechanisms including SOD-mimic activity, oxidation of reduced transition metals and detoxification of oxygen- and nitrogen-centered radicals. However, there is no agreement regarding the reaction of nitroxides with peroxyl radicals, and many controversies in the literature exist. The question of whether nitroxides can protect by scavenging peroxyl radicals is important because peroxyl radicals are formed in biological systems. To further elucidate the mechanism(s) underlying the antioxidative effects of nitroxides, we studied by pulse radiolysis the reaction kinetics of piperidine, pyrrolidine, and oxazolidine nitroxides with several alkyl peroxyl radicals. It is demonstrated that nitroxides mainly reduce alkyl peroxyl radicals forming the respective oxoammonium cations (>N+=O). The most efficient scavenger of peroxyl radicals is 2,2,6,6-tetramethylpiperidine-N-oxyl (TPO), which has the lowest oxidation potential among the nitroxides tested in the present study. The rate constants of peroxyl reduction are in the order CH2(OH)OO*>CH3OO*>t-BuOO*, which correlate with the oxidation potential of these peroxyl radicals. The rate constants for TPO vary between 2.8x10(7) and 1.0x10(8) M-1 s-1 and for 3-carbamoylproxyl (3-CP) between 8.1x10(5) and 9.0x10(6) M-1 s-1. The efficacy of protection of nitroxides against inactivation of glucose oxidase caused by peroxyl radicals was studied. The results demonstrate a clear correlation between the kinetic features of the nitroxides and their ability to inhibit biological damage inflicted by peroxyl radicals.

Living radical polymerization of styrene mediated by a piperidinyl- N-oxyl radical having very bulky substituents

A new cyclic nitroxide 1 and the corresponding alkoxyamines 9 and 10 were synthesized and the polymerization of styrene (St) initiated with 10 was investigated. The NO–C bond of 9 is very weak, cleaving at room temperature. On the other hand, alkoxyamine 10 is stable at room temperature and the A act and E act for the NO–C bond homolysis were determined to be 1.4 × 10 15 s −1 and 124.5 kJ mol −1, respectively. When the polymerization of St was carried out at 70 °C, the resultant poly(St) showed narrow polydispersities below 1.25. In the polymerization at 90 °C, the resulting poly(St) showed narrow polydispersity until 60% conversion, but M w/ M n was rapidly increased above 60% conversion. On the other hand, the polymerization at 120 °C gave poly(St) with broad polydispersities. The unusual polymerization behavior was discussed on the basis of the SEC and ESR results.

A New Sterically Highly Hindered 7‐Membered Cyclic Nitroxide for the Controlled Living Radical Polymerization

AbstractThe synthesis of a new sterically highly hindered 7‐membered alkoxyamine, 2,2,7,7‐tetraethyl‐1‐(1‐phenylethoxy)‐1,4‐diazepan‐5‐one (4), starting from known 2,2,6,6‐tetraethyl‐1‐(1‐phenylethoxy)piperidin‐4‐one (3) via a Beckmann‐type rearrangement is presented. It is shown that ring‐enlargement by insertion of an NH moiety in going from 3 to 4 leads to a more efficient regulator for nitroxide‐mediated controlled living radical styrene (= ethenylbenzene) and butyl acrylate (= butyl prop‐2‐enoate) polymerization. In addition to the polymerization experiments, kinetic data on the reversible CO bond homolysis of alkoxyamines 3 and 4 are presented.

Thermodynamic Analysis of the Chemical Exchange of β−Phosphorylated Cyclic Nitroxides by Using Two-dimensional (Temperature versus Magnetic Field) Simulation of ESR Spectra: The Impact of Labile Solvent−Solute Interactions on Molecular Dynamics

A complete thermodynamic characterization of the chair-to-chair interconversion in beta-diphosphorylated piperidine-N-oxyl radicals was achieved by means of the analysis of temperature-dependent ESR spectra. A new two-dimensional simulation method was developed with the coordinates temperature and magnetic field, in which the entire set of spectra was simulated simultaneously by adjusting the coefficients in the power expansion, giving the temperature-dependent ESR parameters and the thermodynamic and kinetic parameters determining the site populations and exchange rates, respectively. The new method promotes elimination of the ambiguities inherent in the spectroscopic determination of thermodynamic parameters. Labile solvent-solute interactions can strongly influence the chemical exchange, producing a complex network of symmetric and asymmetric interconversions. The solvent dependence of magnetic relaxation was also analyzed.

Structure−Activity Relationship of Cyclic Nitroxides as SOD Mimics and Scavengers of Nitrogen Dioxide and Carbonate Radicals

Synthetic nitroxide antioxidants attenuate oxidative damage in various experimental models. Their protective effect reportedly depends on ring size and ring substituents and is greater for nitroxides having lower oxidation potential. The present study focuses on the kinetics and mechanisms of the reactions of piperidine, pyrrolidine and oxazolidine nitroxides with HO2*/O2*-, *NO2 and CO3*- radicals, which are key intermediates in many inflammatory and degenerative diseases. It is demonstrated that nitroxides are the most efficient scavengers of *NO2 at physiological pH (k = (3-9) x 10(8) M(-1) s(-1)) and among the most effective metal-independent scavengers of CO3*- radicals (k = (2 - 6) x 10(8) M(-1) s(-1)). Their reactivity toward HO2*, though not toward *NO2 and CO3*-, depends on the nature of the ring side-chain and particularly on the ring-size. All nitroxide derivatives react slowly with O2*- and are relatively inefficient SOD mimics at physiological pH. Even piperidine nitroxides, having the highest SOD-like activity, demonstrate a catalytic activity of about 1000-fold lower than that of native SOD at pH 7.4. The present results do not indicate any correlation between the kinetics of HO2*/O2*-, *NO2 and CO3*- removal by nitroxides and their protective activity against biological oxidative stress and emphasize the importance of target-oriented nitroxides, i.e., interaction between the biological target and specific nitroxides.

Characterization of the Crystalline Nature of the Racemates of Novel Chiral Five-Membered Cyclic Nitroxides

ABSTRACT Racemic samples of chiral five-membered cyclic nitroxides, 2,2,5,5-tetrasubstituted pyrrolidine-1-oxyls bearing one or two 4-hydroxyphenyl groups on the stereogenic centers [C(2) or C(2) & C(5)] adjacent to the NO radical moiety, exist as a racemic conglomerate in the crystalline state, whereas those having one or two 3-hydroxyphenyl groups at the same positions belong to a racemic compound. The key intermolecular interactions in controlling whether these racemates crystallize in a homochiral way or a heterochiral manner have been investigated by X-ray crystallographic analysis and magnetic susceptibility measurements.

Spontaneous Racemization and Epimerization Behavior in Solution of Chiral Nitroxides

[reaction: see text] Enantiomerically enriched samples of chiral cyclic nitroxides with a 4-hydroxyphenyl group on the stereogenic center bearing the NO radical group undergo unprecedented spontaneous racemization and/or epimerization in aprotic solvents, which can be well accounted for by the multistep equilibrations involving planar quinoid intermediates.

Kinetics and mechanism of the comproportionation reaction between oxoammonium cation and hydroxylamine derived from cyclic nitroxides

Cyclic nitroxides demonstrate antioxidative activity in numerous in vitro and in vivo models, which frequently involves the participation of the reduced and oxidized forms of the nitroxide, namely, the hydroxylamine and oxoammonium cation. Generally, cellular reducing equivalents facilitate rapid enzymatic as well as nonenzymatic reduction of nitroxides in the tissue. On the other hand, the reaction of nitroxides with various radicals yields the highly oxidizing oxoammonium cation, which mediates the catalytic effect of nitroxides in selective oxidation of alcohols. Hence, nitroxides might act as both anti- and pro-oxidants. Therefore, the comproportionation reaction between the oxoammonium cation and the hydroxylamine might play a role in lowering the pro-oxidative activity of nitroxides. Although the comproportionation reaction has previously been studied, there is no agreement regarding its kinetic features. We investigated the reaction of the reduced forms of 2,2,6,6-tetramethylpiperidinoxyl (TPO) and 4-OH-2,2,6,6-tetramethylpiperidinoxyl (4-OH-TPO) with the oxoammonium cation derived from TPO at various pHs using rapid-mixing stopped-flow and EPR spectrometry. From the pH dependence of the reaction rate constants we determined the p K 1 of the respective hydroxylamines to be 7.5 and 6.9, respectively. The reduction potentials of the hydroxylamines were determined by cyclic voltammetry, and from their dependence on pH, we obtained the same p K 1 values. The rate constant of the comproportionation reaction does not exceed 20 M −1 s −1 in the physiological pH range and, therefore, cannot greatly contribute toward recycling of the nitroxides in the tissue.

Magnetic properties of all-organic liquid crystals containing a chiral five-membered cyclic nitroxide unit within the rigid core.

Magnetic properties of all-organic liquid crystals containing a chiral five-membered cyclic nitroxide unit within the rigid core.

Magnetic Properties of All‐Organic Liquid Crystals Containing a Chiral Five‐Membered Cyclic Nitroxide Unit within the Rigid Core

Magnetic Properties of All‐Organic Liquid Crystals Containing a Chiral Five‐Membered Cyclic Nitroxide Unit within the Rigid Core

Reactions of nitric oxide, peroxynitrite, and carbonate radicals with nitroxides and their corresponding oxoammonium cations.

Cyclic nitroxides effectively protect biological systems against radical-induced damage. However, the mechanism of the reactions of nitroxides with nitrogen-derived reactive species and carbonate radicals is far from being elucidated. In the present study, the reactions of several representative piperidine- and pyrrolidine-based nitroxides with *NO, peroxynitrite, and CO3*- were investigated, and the results are as follows: (i) There is no evidence for any direct reaction between the nitroxides and the *NO. In the presence of oxygen, the nitroxides are readily oxidized by *NO2, which is formed as an intermediate during autoxidation of *NO. (ii) *NO reacts with the oxoammonium cations to form nitrite and the corresponding nitroxides with k1 = (9.8 +/- 0.2) x 10(3) and (3.7 +/- 0.1) x 10(5) M(-1) s(-1) for the oxoammonium cations derived from 2,2,6,6-tetramethylpiperidine-1-oxyl (TPO) and 3-carbamoyl-proxyl (3-CP), respectively. (iii) CO3*- oxidizes all nitroxides tested to their oxoammonium cations with similar rate constants of (4.0 +/- 0.5) x 10(8) M(-1) s(-1), which are about 3-4 times higher than those determined for H-abstraction from the corresponding hydroxylamines TPO-H and 4-OH-TPO-H. (iv) Peroxynitrite ion does not react directly with the nitroxides but rather with their oxoammonium cations with k(10) = (6.0 +/- 0.9) x 10(6) and (2.7 +/- 0.9) x 10(6) M(-1) s(-1) for TPO+ and 3-CP+, respectively. These results provide a better insight into the complex mechanism of the reaction of peroxynitrite with nitroxides, which has been a controversial subject. The small effect of relatively low concentrations of nitroxides on the decomposition rate of peroxynitrite is attributed to their ability to scavenge efficiently *NO2 radicals, which are formed during the decomposition of peroxynitrite in the absence and in the presence of CO2. The oxoammonium cations, thus formed, are readily reduced back to the nitroxides by ONOO-, while forming *NO and O2. Hence, nitroxides act as true catalysts in diverting peroxynitrite decomposition from forming nitrating species to producing nitrosating ones.

Do stable nitroxide radicals catalyze or inhibit the degradation of hyaluronic acid?

Reactive oxygen-derived species and particularly OH radicals can degrade hyaluronic acid (HA), resulting in a loss of viscosity and a subsequent decrease in its effectiveness as a joint-lubricating agent. The production of OH in the vicinity of HA can be catalyzed by bound redox-active metals, which participate in the Haber-Weiss reaction. Damage to HA can also occur as a result of hypochlorite formed by myeloperoxidase (MPO). The protective reagents commonly used to inhibit oxidative stress-induced degradation of HA include antioxidative enzymes, such as SOD and catalase, chelators that coordinate metal ions rendering them redox-inactive, and scavengers of radicals, such as OH, as well as nonradical reactive species. In recent years, stable cyclic nitroxides have also been widely used as effective antioxidants. In many cases, nitroxide antioxidants operate catalytically and mediate their protective effect through an exchange between their oxidized and reduced forms. It was anticipated, therefore, that nitroxides would protect HA from oxidative degradation as well. On the other hand, nitroxides serve as catalysts in many oxidation reactions of alcohols, sugars and polysaccharides, including hyalouronan. Such opposite effects of nitroxides on oxidative degradation are particularly intriguing and the aim of the present study was to examine their effect on HA when subjected to diverse forms of oxidative stress. The results indicate that nitroxides protect HA from OH radicals generated enzymatically or radiolytically. The protective effect is attributable neither to the scavenging of OH nor to the oxidation of reduced metal, but to the reaction of nitroxides with secondary carbohydrate radicals-most likely peroxyl radicals.