Stable Nitroxide Free Radicals Research Articles

Kabachnik–Fields reactions with stable nitroxide free radicals

Kabachnik–Fields reactions with stable nitroxide free radicals

Synthesis and Application of Stable Nitroxide Free Radicals Fused with Carbocycles and Heterocycles

Stable nitroxide free radicals have traditionally been associated with 2,2,6,6- tetramethylpiperidine-1-oxyl (TEMPO) or its 4-substituted derivatives as relatively inexpensive and readily accessible compounds with limited possibilities for further chemical modification. Over the past two decades, there has been a resurgence of interest in stable free radicals with proper functionalization tuned for various applications. The objective of this review is to present recent results with synthetic methodologies to achieve stable nitroxide free radicals fused with aromatic carbocycles and heterocycles. There are two main approaches for accessing stable nitroxide free radicals fused with arenes, e.g., isoindoline- like nitroxides: further functionalization and oxidation of phthalimide or inventive functionalization of pyrroline nitroxide key compounds. The latter also offers the constructions of versatile heterocyclic scaffolds (furan, pyrrole, thiophene, 1,2-thiazole, selenophene, pyrazole, pyrimidine, pyridine, pyridazine, 1,5-benzothiazepine) that are fused with pyrroline or tetrahydropyridine nitroxide rings. The possible applications of these new stable nitroxide free radicals, such as covalent spin labels and noncovalent spin probes of proteins and nucleic acids, profluorescent probes, building blocks for construction of dual active drugs and electroactive materials, and substances for controlled free radical polymerization, are discussed.

Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study.

Two stable nitronyl nitroxide free radicals {R1 = 4′-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and R2 = 2-(2′-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac)2 (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal–organic complexes Co(hfac)2(NNPhOMe)2, 1; Co(hfac)2(NNT)2·(H2O), 2; Mn(hfac)2(NNPhOMe)·x(C7H16), 3; and Mn(hfac)2(NNT)2, 4. The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds 1 and 4 crystallize in the triclinic, P1̅, space group, whereas complex 3 crystallizes in the monoclinic structure with the C2/c space group and forms chain-like structure along the c direction. The complex 2 crystallizes in the monoclinic symmetry with the P21/c space group in which the N–O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (J/KB) are derived using isotropic spin Hamiltonian H = −2J∑(SmetalSradical) for the model fitting to the magnetic susceptibility data for 1, 2, 3, and 4. The exchange interaction strengths are found to be −328, −1.25, −248, and −256 K, for the 1, 2, 3, and 4 metal–organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal–radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.

Covalently grafted TEMPO on graphene oxide: A composite material for selective oxidations of alcohols

4-Amino-TEMPO, a stable nitroxide free radical, was covalently grafted onto graphene oxides through an amide bond and the new composite solid materials thus obtained were characterized using scanning electron microscopy, thermal and elemental analysis, infrared, Raman and electron spin resonance spectroscopy. It was found that these materials can be successfully used as easily recoverable solid catalysts for selective oxidation of alcohols, using NOx as co-catalyst and oxygen as final oxidant, under very mild conditions.

Melanin and humic acid-like polymer complex from olive mill waste waters. Part I. Isolation and characterization

A water soluble humic acid and melanin-like polymer complex (OMWW–ASP) was isolated from olive mill waste waters (OMWW) by ammonium sulfate fractionation to be used as natural additive in food preparations. The dark polymer complex was further characterized by a variety of biochemical, physicochemical and spectroscopic techniques. OMWW–ASP is composed mainly of proteins associated with polyphenols and carbohydrates and the distribution of its relative molecular size was determined between about 5 and 190kDa. SDS–PAGE shows the presence of a well separated protein band of 21.3kDa and a low molecular weight peptide. The OMWW–ASP complex exhibits a monotonically increasing UV–Vis absorption spectrum and it contains stable radicals. Antioxidant activity measurements reveal the ability of the OMWW protein fraction to scavenge both the cationic 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS+) radical, as well as the stable nitroxide free radical 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPOL).

An experimental NEXAFS and computational TDDFT and ΔDFT study of the gas-phase core excitation spectra of nitroxide free radical TEMPO and its analogues

Core-hole spectroscopy adds to the fundamental understanding of the electronic structure of stable nitroxide free radicals thus paving way for a sensible design of new analogues with desired functionalities. We study the gas-phase C 1s, N 1s and O 1s excitation spectra of three nitroxide free radicals - TEMPO and two of its amide-substituted analogues - using the experimental NEXAFS technique and the theoretical TDDFT and ΔDFT methods in the unrestricted setting. The short-range corrected SRC1-BLYP and SRC2-BLYP exchange-correlation functionals are used with TDDFT, and the standard B3LYP functional with ΔDFT. The TDDFT-based detailed spectral assignment includes the valence, mixed valence-Rydberg and Rydberg portions of the spectra from the onset of absorptions to the vicinity of the core-ionization thresholds. The relative overlaps between the experimental and TDDFT-modelled spectra are reasonably good, in the range of 0.7-0.8, 0.6-0.8, and 0.7-0.8 for the C 1s, N 1s, and O 1s spectra, respectively. The extent of spin contamination within the unrestricted framework and its effect on the accuracy of the calculated excitation energies and dipole intensities are discussed in detail. It is concluded that, despite the sizeable spin contamination, the presently used methods are capable of predicting the core-excitation spectra of comparatively large free radical species fairly reliably over a wide spectral range.

Reliability of Density Functional and Perturbation Theories for Calculating Core-Ionization Spectra of Free Radicals

The C 1s, N 1s, and O 1s ionization energies were calculated for the three stable nitroxide free radicals, viz., tempo and its two analogues, and compared to their most recent high-resolution core photoelectron spectra. We compare the performance of unrestricted and restricted open shell based ΔHF, ΔMP2, and ΔB3LYP methods. A mixed basis set is employed in all cases, which consists of the core-valence correlation-consistent triple-ζ basis set (cc-pCVTZ or cc-pwCVTZ) on the atom whose core-electron binding energy is calculated, model core potentials on the remaining first row atoms, and the cc-pVDZ basis set on the hydrogen atoms. The best overall performance for the three free radicals is offered by the restricted open shell based ΔB3LYP method. Surprisingly, both the restricted open and unrestricted second-order perturbation theories perform relatively poorly and typically do not warrant additional computational effort over the reference ΔHF methods. This is particularly true of the ΔZAPT method, which exhibits a number of grave failures that render it unsuitable for calculating the core-ionization spectra.

Overhauser-Enhanced MRI of Elastase Activity from In Vitro Human Neutrophil Degranulation

BackgroundMagnetic resonance imaging can reveal exquisite anatomical details. However several diseases would benefit from an imaging technique able to specifically detect biochemical alterations. In this context protease activity imaging is one of the most promising areas of research.Methodology/Principal FindingsWe designed an elastase substrate by grafting stable nitroxide free radicals on soluble elastin. This substrate generates a high Overhauser magnetic resonance imaging (OMRI) contrast upon digestion by the target proteases through the modulation of its rotational correlation time. The sensitivity is sufficient to generate contrasted images of the degranulation of neutrophils induced by a calcium ionophore from 2×104 cells per milliliter, well under the physiological neutrophils concentrations.Conclusions/SignificanceThese ex-vivo experiments give evidence that OMRI is suitable for imaging elastase activity from neutrophil degranulation. Provided that a fast protease-substrate is used these results open the door to better diagnoses of a number of important pathologies (cystic fibrosis, inflammation, pancreatitis) by OMRI or Electron Paramagnetic Resonance Imaging in vivo. It also provides a long-expected method to monitor anti-protease treatments efficiency and help pharmaceutical research.

Nitroxide radical TEMPO reduces ozone-induced chemokine IL-8 production in lung epithelial cells

Exposure to high levels of ozone (O 3) damages respiratory tract epithelial cells. This research evaluated the ability of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), a stable nitroxide free radical, to decrease O 3-mediated injury to a respiratory tract-derived cell line (A549 cells) by monitoring in this cell system the interleukine-8 (IL-8) production. TEMPO reduced O 3-induced IL-8 production in A549 cells, as evidenced by PCR analysis, Western blot and ELISA assays. This behaviour is explainable on the basis of the reactivity between TEMPO with O 3 and/or O 3-derived free radicals in biological systems. The study provides evidence that TEMPO reacts with O 3 and/or its cytotoxic products and may provide protections against O 3-induced biotoxicities.

Antioxidant Properties of Fruits and Vegetables Shots and Juices: An Electron Paramagnetic Resonance Study

The antioxidant activity of some commercially available fruit and vegetable juices was evaluated with regard to their radical scavenging activity against the stable free radical 4-hydroxy-2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPOL) monitored by electron paramagnetic resonance (EPR) spectroscopy. TEMPOL is a stable nitroxide free radical characterized by a well-defined EPR spectrum consisting of three peaks. The integral intensity of the EPR spectra of TEMPOL was decreased upon juice addition, and the decrease was dose dependent. EPR spectroscopy using stable free radicals provides a simple, rapid, and sensitive method for the determination of antioxidant activity of fruit and vegetable juices. The method was standardized by using the standard antioxidant compound Trolox, and the antioxidant activity of the juices was expressed as Trolox equivalents. When concentrated juices of fruits and vegetables (shots) were considered, the evaluated antioxidant activity was almost twofold higher than that of the conventional, non-concentrated ones. Fruits and vegetables shots also showed very good stability during storage. This finding indicates that natural antioxidant compounds contained in commercially available concentrated juices are not eliminated or inactivated when the juices are kept refrigerated according to the instructions of the manufacturer.

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.

Spin‐Labeled Fulleropyrrolidines

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Spin-labeled fulleropyrrolidines

Abstract We describe the synthesis and properties of a series of fulleropyrrolidines bearing stable nitroxide free radical moieties covalently linked to the [60]fullerene cage. The nitroxide electron spin functions as a weakly interacting probe for the investigation of radical anions and paramagnetic excited states of functionalized fullerenes. Electron transfer processes, spin coupling in polyradical adducts, and interactions between radical and fullerene excited triplet unpaired electrons have been studied through electron paramagnetic resonance. Photogenerated quartet and quintet high spin species, obtained from the spin coupling of the fullerene triplet with the nitroxide spins, have been observed for the first time at room temperature. To cite this article: C. Corvaja et al., C. R. Chimie 9 (2006).

Rate enhancement for nitroxide stable free radical polymerization of n‐butyl acrylate by dodecylbenzenesulfonic acid

AbstractDodecylbenzenesulfonic acid, DDBSA, was chosen as a new rate‐accelerating additive for 2,2,6,6,‐teramethyl‐1‐piperidinyloxy (TEMPO)‐mediated stable free radical polymerization of n‐butyl acrylate (n‐BA) monomers with 2,2′‐azobisisobutyronitrile (AIBN). It was found that the number‐average molecular weight of polymers could reach about ten thousand with a narrow polydispersity index (PDI) of 1.4 in a few minutes, which was faster than other systems reported previously. But, at higher conversion, the molecular weight distribution of polymers became broad, and a bimodal distribution occurred. The macro‐initiators isolated from the former polymers with narrow PDI could be extended by polymerization with monomers by the addition of DDBSA. Furthermore, a proposed kinetic model demonstrated that the decay of the concentration of DDBSA would reduce the living polymer concentration and retard the growth of the polymers, which could be further propagated by the supplement of DDBSA. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 42–49, 2005

417 The ATM-Chk2 kinase pathway: molecular interaction maps and therapeutic rationale

The antiproliferative effect of Tempol, a stable nitroxide free radical, was investigated on the p53-negative human leukemia cell line HL60. A concentration- and time-dependent inhibition of cell growth was observed that appears to be due to induction of apoptosis. Involvement of oxidative stress is indicated by a concentration-dependent increase in intracellular peroxides and a parallel decrease in total cellular glutathione; in addition, increased survival rates were observed in cells simultaneously treated with Tempol and the antioxidant N-acetylcysteine. Tempol did not affect the relative levels of Bax and Bcl2, whereas p21WAF1/CIP1 was enhanced in a concentration- and time-dependent fashion; this effect was partially inhibited by N-acetylcysteine, was maintained for up to 8 h after Tempol removal, and seemed to depend on continuing protein synthesis. The increase in p21WAF1/CIP1 was accompanied by a parallel accumulation of cells in the G1 phase of the cycle and by a decrease in the 110 kDa form of pRb. Our results suggest that p53-independent induction of p21WAF1/CIP1 mediates the antiproliferative effect of Tempol; on the basis of this observation, the nitroxide could be proposed as an useful adjunct to the treatment of p53-deficient tumors, which are often refractory to standard chemotherapy.

Temperature and Solvent Effects on the Reactions of o-Positronium with Nitroso Spin Traps in Methanol−Water Mixed Solvents

Positron annihilation lifetime spectroscopy (PALS) is used to study the reactions of the ortho-positronium (o-Ps) atom with the following nitroso spin traps: nitrosobenzene (NOB), N,N-dimethyl-4-nitroso-aniline (DMNA), and tert-nitroso-butane (t-NOBu). The reaction rate constants (k) are close to the range that corresponds to that of diffusion-controlled reactions and exhibit a similar trend to those reported in the literature for the analogous nitro compounds. The differences between the aromatic and aliphatic nitroso and nitro compounds and the observed substituent effect are in good correlation with the calculated LUMO energies of the molecules. The nucleophilic o-Ps attacks the molecules at their electron deficient aromatic ring. Solvent effects on the reaction rates in methanol−water solvent mixtures exhibit a complex and solute-dependent nature. The reactions are strongly affected by solvent viscosity; however, the expected 1/η versus k variation is valid only for limited ranges of solvent compositions. The dielectric permittivity effect on the reaction rate constants does not follow the expected trend; k values in water are significantly greater than in the less polar methanol. The average activation enthalpies (ΔH⧧ and entropies (ΔS⧧) evaluated for the reaction of o-Ps with DMNA exhibit maxima at a solvent composition which corresponds to the most rigid solvent structure, where the activation energies of the transport processes such as viscosity and self-diffusion have extremes too. The validity of the isokinetic relation between ΔH⧧ and ΔS⧧ shows that the mechanism of the reaction of o-Ps does not depend on the solvent composition. ΔH⧧ exhibits an “Onsager reaction field” type (ε − 1)/(ε + 2) dependence on solvent permittivity. The results are compared to literature data for the reactions of o-Ps with stable nitroxide free radicals and reaction of solvated electron with nitrobenzene (NO2B) in methanol−water mixtures. The observed similarities and differences are discussed.

Synthesis, Equilibrium and Kinetic Properties of Gd3+ Complexes of Three DTPA‐Bis(Amide) Derivatives Containing Stable Nitroxide Free Radical Substituents

AbstractTwo new DTPA‐bis(amide) derivatives containing stable nitroxide free radicals, H3L2 and H3L3, were synthesized. The complexation properties of L2 and L3 with Gd3+ were compared with those of H3L1, synthesized earlier. The sums of the protonation constants ΣlogKiH of the DTPA‐bis(amide) derivative ligands with stable nitroxide free radicals are somewhat higher than that for DTPA‐bis(methylamide) (H3L4). The ligands L1, L2, and L3 form complexes with a metal‐to‐ligand ratio of 1:1 with Gd3+ (GdL), while the species formed with Zn2+ are ZnL, ZnHL, ZnH2L, and Zn2L. The formation of the dinuclear complex Zn2L results in a lower selectivity of the ligands for Gd3+ over Zn2+ than was assumed for L4. The relaxivities of the complexes GdL1, GdL2, and GdL3 are close to the sums of the relaxivities of the complex GdL4 and of the free ligands L1, L2, and L3. The relaxivities of GdL1, GdL2, and GdL3 are practically independent of temperature in the 7−30 °C range, which is probably a result of the relatively long lifetime of the water molecules in the inner spheres of the [GdL(H2O)] complexes. The kinetic stabilities of the Gd3+ complexes were characterized by the rates of the exchange reactions taking place between the complexes GdL and Lu3+ or Zn2+. The reactions occur through the proton‐assisted dissociation of the complexes, with the formation of a protonated intermediate. Another pathway is the direct reaction between the Gd3+ complex and Lu3+ or Zn2+, which occurs with the formation of a dinuclear intermediate, GdLM. The exchange reactions proceed about five times more rapidly with Zn2+ than with Lu3+, indicating the importance of the formation of an intermediate containing an N−Zn2+ bond, which is stronger than the N−Lu3+ bond in an intermediate with similar structure. The exchange reactions of the complex GdL3 formed with the disubstituted ligand L3 are significantly slower than those of the complexes of the monosubstituted ligands L1 and L2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Study of in vitro and in vivo effects of the piperidine nitroxide Tempol—a potential new therapeutic agent for gliomas

The identification of novel therapeutic agents for the management of malignant gliomas represents an area of active research. Here, we show that Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl; TPL), a stable nitroxide free radical, inhibits the growth of C6 glioma cells both in vitro and in vivo. Morphological features of apoptosis were apparent in C6 cells following in vitro treatment with TPL. Cell death was preceded by dose-dependent increase in p21waf1/cip1 expression, without apparent stabilisation of the TP53 gene product. When C6 cells were grown as xenografts in nude mice, treatment with TPL induced a significant dose-dependent decrease in tumour growth, without signs of general or organ toxicity. Tumours from treated mice showed an increase in the number of apoptotic cells and a decrease in the rate of neo-vascularisation compared with tumours from control mice. Our findings suggest a potential use for TPL as a novel antiproliferative agent for the treatment of malignant gliomas.

Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization

A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120°C in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88wt%. All ‘synthesized’ composites gave stable suspensions in toluene and tetrahydrofuran.

Cesium Adsorption in Hydrated Iron Oxide Particles Suspensions: An NMR Study

137Cs is an important component of nuclear waste which may pollute water. Its migration in natural environments is slowed down by adsorption on minerals. Cesium adsorption on akaganeite (β-FeOOH) particles, dextran-coated ferrihydrite (5 Fe 2O 3–9H 2O) particles, and ferritin in aqueous solutions is studied with 133Cs nuclear magnetic resonance measurements. The longitudinal relaxation time ( T 1) of 133Cs in the presence of such magnetic particles depends on whether the ions bind to the particle or not. T 1 of 133Cs ions in aqueous solutions containing the same amount of magnetized particles will not depend on cesium concentration if relaxation is governed by diffusion (when cesium is not able to bind), but it will depend on cesium concentration if exchange governs relaxation (when cesium is able to bind). The method is successfully tested using TEMPO, a nitroxide stable free radical whose relaxation is due to diffusion. 133Cs relaxation in solutions of ferritin, akaganeite, and dextran-coated ferrihydrite particles is found to result from a cationic exchange of cesium ions between particles surface and bulk ions, owing to adsorption. The effect of pH on 133Cs relaxation in solutions of the particles is consistent with the adsorption properties of cations on hydrated iron oxides.