Spin-trapping Experiments Research Articles

Reactivity of Pd3(dppm)3(CO) n+ and Pd3(dppm)3(CO)(RCCR) n+ (n = 0, +1, +2) Towards F−. Evidence of Reactive Intermediates and X-Ray Structure of [Pd3(dppm)3(MeO2CC≡CCO2Me)(F)]PF6

The reactivity of the trinuclear palladium cluster [Pd3(dppm)3(CO)] n+ (dppm = bis(diphenylphosphinomethane); n = 2, 1) towards F− was investigated by electrochemical and spectroscopic methods. The reaction depends on the charge of the cluster. The chemical reduction of the cluster dication is observed in the presence of F− generating the paramagnetic monocationic cluster. Spin-trapping experiments with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) provided evidence for the radical F• as an intermediate. In a similar manner to the dication, the monocationic cluster [Pd3(dppm)3(CO)]+ is also reduced, but in a slower process, by the F− ion to produce [Pd3(dppm)3(CO)]0. Additionally, the alkyne cluster adducts [Pd3(dppm)3(CO)(RCCR)] n+ (n = 2, 1; R = CO2Me) are also reactive towards F−. Particularly, the dication adduct leads to a metastable fluoride adduct [Pd3(dppm)3(CO)(RCCR)(F)]+. The electroreductive behavior of this adduct involves electron-transfer steps and F− exchange equilibriums, for which digital simulation enables the extraction of the thermodynamic parameters (standard potentials and equilibrium constants). Concurrently, the monocation adduct [Pd3(dppm)3(CO)(RCCR)]+ with F−, leads to a disproponation generating 0.5 equiv. of [Pd3(dppm)3(CO)(RCCR)(F)]+ and 0.5 equiv. of [Pd3(dppm)3(CO)(RCCR)]0. The former slowly evolves to [Pd3(dppm)3(RCCR)(F)]+, which was described by X-ray diffraction method. [Pd3(ddpm)3(CO)] n+ (n = 2, 1) are reduced by F− to form [Pd3(dppm)3(CO)](n+)−1. Concurrently, [Pd3(dppm)3(CO)(RCCR)] n+ (n = 2, 1) with F− generate the corresponding adducts [Pd3(dppm)3(CO)(RCCR)(F)](n+)−1, but, in the case of n = 1, disproponation is observed into [Pd3(dppm)3(CO)(RCCR)(F)]+ and [Pd3(dppm)3(CO)(RCCR)]0.

On the Favorable Interaction of Metal Centered Radicals with Hydroperoxides for an Enhancement of the Photopolymerization Efficiency Under Air

Four metal-based photoinitiating systems (benzene−tricarbonylchromium(0), ArCr(CO)3, cyclopentadienylmolybdenum tricarbonyl dimer, Cp2Mo2(CO)6, diiron nonacarbonyl, Fe2(CO)9, and bis(cyclopentadienylruthenium dicarbonyl), Cp2Ru2(CO)4) are investigated by laser flash photolysis (LFP) and ESR−spin trapping (ESR−ST) experiments and checked as photoinitiators for both free radical promoted cationic photopolymerization (FRPCP) and free radical photopolymerization (FRP). New combinations with silanes instead of alkyl halides improve both the polymerization rates and the final conversions upon irradiation with UV/visible or visible light and laser diodes (405, 457, 473, and 532 nm) under air. The enhancement of the photopolymerization reaction under air in the presence of the selected compounds is ascribed to the reuse of the inherently present hydroperoxides through a set of reactions leading to additional initiating species. These systems are particularly attractive for FRPCP with excellent polymerization rate...

Is the Oxygen Radical Absorbance Capacity (ORAC) Method a Peroxyl-radical Scavenging Assay?

Abstract The oxygen radical absorbance capacity (ORAC) method employs a water soluble azo-radical initiator, AAPH (2,2’-azobis(2-amidinopropane) dihydrochloride) as a free radical generator, by which the fluorescent probe fluorescein is damaged to result in the loss of fluorescence. Antioxidants can protect the probe from the damage and the degree of protection is quantified. Because AAPH has been used as a lipid-peroxidation reagent, “oxygen radical” in ORAC is generally accepted as peroxyl radicals; however, in the present spin trapping experiments using a newly developed spin trap, CYPMPO, there was no indication of peroxyl-radical formation in AAPH decomposition in aqueous media. These spin trapping studies demonstrated that alkoxyl (RO·) radical adduct was the sole product of AAPH decomposition. In contrast, alkyl-peroxyl (ROO·) radical was spin-trapped during the decomposition of a lipid soluble azo-radical initiator AIBN (azobis(isobutyronitrile)) in non-aqueous media. We speculate that alkyl-peroxyl radicals are short-lived in water, rapidly converted into alkoxyl radicals. Although the possibility that ORAC method monitors peroxyl-radical scavenging activity cannot be completely eliminated, spin trapping evidence strongly indicates that ORAC method is a scavenging capacity assay for alkoxyl radicals.

Carbon nanotubes induce inflammation but decrease the production of reactive oxygen species in lung

With the rapid spread of carbon nanotubes (CNTs) applications, the respiratory toxicity of these compounds has attracted the attention of many scientists. Several studies have reported that after lung administration, CNTs could induce granuloma, fibrosis, or inflammation. By comparison with the mechanisms involved with other toxic particles such as asbestos, this effect could be attributed to an increase of oxidative stress. The aim of the present work was to test this hypothesis in vivo. Mice were intranasally instilled with 1.5 mg/kg of double walled carbon nanotubes (DWCNTs). Six, 24, or 48 h after administration, inflammation and localisation of DWCNTs in lungs were microscopically observed. Local oxidative perturbations were investigated using ESR spin trapping experiments, and systemic inflammation was assessed by measuring the plasma concentration of cytokines TNF-α, IL-1α, IL-1β, IL-6, IGF-1, Leptin, G-CSF, and VEGF. Examination of lungs and the elevation of proinflammatory cytokines in the plasma (Leptin and IL-6 at 6 h) confirmed the induction of an inflammatory reaction. This inflammatory reaction was accompanied by a decrease in the local oxidative stress. This effect could be attributed to the scavenger capability of pure CNTs.

Hydroxyl radical release from dental resins: Electron paramagnetic resonance evidence

It is well known that polymeric free radicals remain trapped inside dental resins for a long time after photopolymerization. Moreover, although these high molecular mass compounds have very limited mobility, there is evidence to suggest that they disappear progressively over time. The purpose of this study was to provide new experimental data to help understand this phenomenon. To determine whether low molecular mass free radicals are released by dental composites stored in hydrophilic media, we used electron paramagnetic resonance spectroscopy to perform spin-trapping experiments on experimental and commercial samples stored in ethanol. Under these conditions, ethoxy radicals were produced. Further experiments demonstrated that (1) hydroxyl radicals were released from the methacrylated resin and (2) they reacted with ethanol molecules to produce "secondary" ethoxy free radicals. In addition to the well-known monomer toxicity of methacrylated resins, we may have identified a new source of concern for these biomaterials.

Mechanism of light-induced oxidation of nitrosylmyoglobin

Exposure of nitrosylmyoglobin (MbFe IINO), to light in the presence of oxygen increases rate of oxidation still with metmyoglobin (MbFe III) as main product, as shown by electron paramagnetic resonance and visible absorption spectroscopy. Despite previous suggestions of interfering radical species, capable of initiating lipid oxidation in cured meat, no radical adducts were detected during photolysis in spin trapping experiments. Flash photolysis showed separation of myoglobin (MbFe II) and nitric oxide (NO ) after excitation, followed by fast rebinding under anaerobic conditions. In the presence of oxygen, nitric oxide rebinding was inhibited suggesting formation of a nitrosyldioxyl-radical (ONOO ). As NO failed to diffuse into bulk solution, ONOO formation is proposed to occur in the heme cavity in competition with NO rebinding and, to a lesser degree, O 2 binding to yield MbFe IIO 2. Subsequent oxidation of MbFe II to MbFe III by ONOO was expected to result in the highly reactive peroxynitrite intermediate (ONOO −), of which occurrence would be characterized by typical chemical footprints, such as nitrated tyrosine or tryptophan residues. However, results from direct infusion mass spectrometry suggested neat conversion from nitrosylmyoglobin to metmyoglobin and questioned the existence of ONOO − as reaction product. Catalytic activity of the resulting ferric centre in MbFe III, however, induces fast isomerisation of ONOO − to the stable nitrate, which was identified directly as ion m/ z 62 or indirectly by adduct formation with MbFe III.

Enhanced acyl radical formation in the Au nanoparticle-catalysed aldehydeoxidation

EPR spectroscopy and spin-trapping experiments showed that polymer-encapsulated Au nanoparticles promote aldehyde oxidation via a radical pathway by initiating formation of acyl radicals.

Bis(germyl)ketones: Toward a New Class of Type I Photoinitiating Systems Sensitive Above 500 nm?

The ability of a bis(germyl)ketone Ph(3) GeCOGePh(3) to act as a photoinitiator of free radical polymerization under visible light is investigated. The results suggest that this compound could be the starting point of a new high performance class of photoinitiators. The excited state processes, as well as the generation and the reactivity of the germyl radicals, are studied by laser flash photolysis, ESR spin trapping experiments, and molecular orbital calculations; they are compared to the results obtained on a mono- (germyl)ketone CH(3) COGePh(3) . Time dependent density functional theory calculations allow discussion of the strongly red-shifted ground state absorption of Ph(3) GeCOGePh(3) .

Long Wavelength Cationic Photopolymerization in Aerated Media: A Remarkable Titanocene/Tris(trimethylsilyl)silane/Onium Salt Photoinitiating System.

A new system for visible light induced polymerization of cationic resins working through a free radical promoted process under air is presented. This system is based on a fluorinated titanocene free radical initiator bis(cyclopentadienyl)bis[2,6-difluoro-3-(1-pyrryl)phenyl]titanium Ti, a silane (tris(trimethylsilyl)silane TTMSS) together with an onium salt (diphenyliodonium hexafluorophosphate). The polymerization process is highly efficient, and better under aerated than under laminated conditions; it appears as a suitable system for long wavelength cationic photopolymerization under air. This system could also be efficient for the polymerization under sunlight irradiation. The involved mechanisms investigated by laser flash photolysis and ESR spin trapping experiments are detailed.

Chemiluminescence and energy transfer mechanism of lanthanide ions in different media based on peroxomonosulfate system

The chemiluminescence (CL) phenomena of lanthanide (Ln) ions and their coordinate complexes in peroxomonosulfate system and the energy transfer mechanism during the process were investigated in this work. A strong and sharp CL signal was yielded when the Eu(III) or Tb(III) solution was added to the peroxymonosulfate solution. The CL intensity was greatly enhanced by 2,6-pyridinedicarboxylic acid (DPA) ligand [maximum enhancement reached when Ln(III):DPA was 1:1] and hexadecyltrimethylammonium chloride micelles. The degree of enhancement of DPA and micelles on Ln(III) CL was related to the fluorescence lifetimes of Ln(III) in different media. According to the ESR spin-trapping experiments of 2,2,6,6-tetramethyl-4-piperidone and the specific quenching experiments of 1,4-diazabicyclo[2.2.2]octane and sodium azide, singlet oxygen was generated though the Ln(III) ion-catalyzed decomposition of peroxymonosulfate. From the comparisons of the fluorescence and CL spectra, lanthanide ions were the luminescence emitter and the ligand DPA absorbed the energy from singlet oxygen and transferred it to Ln(III) ions in the coordinate complexes. Micelles can enhance the CL intensity by improving intermolecular energy transfer efficiencies, removing the quenching effect of water and prolonging the lifetime of singlet oxygen.

Toxic effects of carvacrol, caryophyllene oxide, and ascaridole from essential oil of Chenopodium ambrosioides on mitochondria

Chenopodium ambrosioides have been used for centuries in the Americas as a popular remedy for parasitic diseases. The essential oil of this plant possesses anthelmintic activity and is still used in some regions to treat parasitosis and leishmaniasis. However, the Chenopodium oil caused also some fatalities, leading to its commercial disuse. In this work, we studied the mechanism of toxicity of the essential oil and its major pure ingredients (carvacrol, caryophyllene oxide, and ascaridole, which was synthesized from α-terpinene) with respect to mammalian cells and mitochondria. We observed that all products, but especially caryophyllene oxide, inhibited the mitochondrial electron transport chain. This effect for carvacrol and caryophyllene oxide was mediated via direct complex I inhibition. Without Fe 2+, ascaridole was less toxic to mammalian mitochondria than other major ingredients. However, evidence on the formation of carbon-centered radicals in the presence of Fe 2+ was obtained by ESR spin-trapping. Furthermore, it was shown that Fe 2+ potentiated the toxicity of ascaridole on oxidative phosphorylation of rat liver mitochondria. The increase of the α-tocopherol quinone/α-tocopherol ratio under these conditions indicated the initiation of lipid peroxidation by Fe 2+-mediated ascaridole cleavage. Further ESR spin-trapping experiments demonstrated that in addition to Fe 2+, reduced hemin, but not mitochondrial cytochrome c can activate ascaridole, explaining why ascaridole in peritoneal macrophages from BALB/c mice exhibited a higher toxicity than in isolated mitochondria.

The role of multiwalled carbon nanotubes in enhancing the catalytic activity of cobalt tetraaminophthalocyanine for oxidation of conjugated dyes

Multiwalled carbon nanotubes (MWCNTs) used to support a metallophthalocyanine catalyst (CoTAPc–MWCNTs) were prepared using covalent immobilization of cobalt tetraaminophthalocyanine (CoTAPc) on them, and characterized by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectra and thermogravimetric analysis. The oxidative decoloration of rhodamine 6G (Rh6G) in the presence of CoTAPc–MWCNTs and H 2O 2 was investigated by examination of UV–Vis absorption spectra. The results showed that Rh6G was oxidized efficiently in the CoTAPc–MWCNTs/H 2O 2 system. The introduction of MWCNTs resulted in a marked enhanced catalytic activity that CoTAPc does not have. Electron paramagnetic resonance spin-trap experiments indicated that CoTAPc–MWCNTs have a novel non-radical pathway, which is different from common CoTAPc catalytic systems. Furthermore, the result of online electrochemical measurement in the CoTAPc–MWCNTs/H 2O 2 system suggested that MWCNTs might participate directly in the electron transfer process in the catalytic oxidation of this conjugated dye. In this catalytic system, MWCNTs provide strong adsorption to conjugated Rh6G due to their special sp 2-hybridized surface, and are able to rapidly oxidize the conjugated dye by a special electron transfer pathway.

Inclusion complexes of some antipyrine derivatives with cyclodextrins: influence of guest configuration

Formation of inclusion complexes between α- and β-cyclodextrins and three antipyrine type compounds (antipyrine, 4-amino-antipyrine, 4-nitroso-antipyrine), in solution and in solid state, has been investigated. UV–Vis measurements indicate that in solution antipyrine derivatives do not form inclusion complexes and oxidative process of amino group is not influenced by the presence of cyclodextrins. The oxidation of 4-amino-antipyrine to 4-nitroso-antipyrine using DPPH type radicals was investigated using spin trapping experiments in organic solvents and in water. These results were explained by betainic configuration of antipyrine derivatives in aqueous solutions. Formation of inclusion complexes in solid states was evidenced by DSC, as peaks associated with various thermodynamic events (dehydration, melting, thermal decomposition) are shifted towards higher temperature or disappear in case of inclusion complexes.

Glycosaminoglycans are fragmented by hydroxyl, carbonate, and nitrogen dioxide radicals in a site-selective manner: implications for peroxynitrite-mediated damage at sites of inflammation

Glycosaminoglycans (long-chain polysaccharides) are major components of the extracellular matrix, glycocalyx, and synovial fluid. These materials provide strength and elasticity to tissues and play a key role in regulating cell behavior. Modifications to these materials have been linked to multiple human pathologies. Although modification may occur via both enzymatic and nonenzymatic mechanisms, there is considerable evidence for oxidant-mediated matrix damage. Peroxynitrite (ONOO −/ONOOH) is a potential mediator of such damage, as elevated levels of this oxidant are likely to be present at sites of inflammation. In this study we demonstrate that hyaluronan and chondroitin sulfate are extensively depolymerized by HO ⋅ and CO 3 ⋅−, but not NO 2 ⋅, which may be formed from peroxynitrite. Polymer fragmentation is shown to be dependent on the radical flux, to be O 2-independent, and to occur in a site-selective manner as indicated by the detection of disaccharide fragments. EPR spin trapping experiments with polymers, oligomers, and component monosaccharides, including 13C-labeled materials, have provided evidence for the formation of specific carbon-centered sugar-derived radicals. The time course of formation of these radicals is consistent with these species being involved in polymer fragmentation.

Evidence of free radical liberation by dental resins: Spin-trapping experiments

Evidence of free radical liberation by dental resins: Spin-trapping experiments

Superoxide Dismutase 1-mediated Production of Ethanol- and DNA-derived Radicals in Yeasts Challenged with Hydrogen Peroxide: MOLECULAR INSIGHTS INTO THE GENOME INSTABILITY OF PEROXIREDOXIN-NULL STRAINS

Peroxiredoxins are receiving increasing attention as defenders against oxidative damage and sensors of hydrogen peroxide-mediated signaling events. In the yeast Saccharomyces cerevisiae, deletion of one or more isoforms of the peroxiredoxins is not lethal but compromises genome stability by mechanisms that remain under scrutiny. Here, we show that cytosolic peroxiredoxin-null cells (tsa1Deltatsa2Delta) are more resistant to hydrogen peroxide than wild-type (WT) cells and consume it faster under fermentative conditions. Also, tsa1Deltatsa2Delta cells produced higher yields of the 1-hydroxyethyl radical from oxidation of the glucose metabolite ethanol, as proved by spin-trapping experiments. A major role for Fenton chemistry in radical formation was excluded by comparing WT and tsa1Deltatsa2Delta cells with respect to their levels of total and chelatable metal ions and of radical produced in the presence of chelators. The main route for 1-hydroxyethyl radical formation was ascribed to the peroxidase activity of Cu,Zn-superoxide dismutase (Sod1), whose expression and activity increased approximately 5- and 2-fold, respectively, in tsa1Deltatsa2Delta compared with WT cells. Accordingly, overexpression of human Sod1 in WT yeasts led to increased 1-hydroxyethyl radical production. Relevantly, tsa1Deltatsa2Delta cells challenged with hydrogen peroxide contained higher levels of DNA-derived radicals and adducts as monitored by immuno-spin trapping and incorporation of (14)C from glucose into DNA, respectively. The results indicate that part of hydrogen peroxide consumption by tsa1Deltatsa2Delta cells is mediated by induced Sod1, which oxidizes ethanol to the 1-hydroxyethyl radical, which, in turn, leads to increased DNA damage. Overall, our studies provide a pathway to account for the hypermutability of peroxiredoxin-null strains.

Acylgermanes: Excited state processes and reactivity

The excited state processes of an acetyltriphenylgermane as well as the generation and the reactivity of the germyl radical are studied by laser flash photolysis, ESR spin trapping experiments and MO calculations. The transient spectra, the dissociation quantum yield, the singlet excited state S 1 lifetime, the S 1 quenching by an iodonium salt, the triplet state energy level E T, the Ge–C bond dissociation energy BDE, the interaction of the germyl radical with oxygen, double bonds and an iodonium salt, and the ESR spectra are determined. A triplet cleavage process is clearly evidenced; a singlet cleavage should likely contribute. The overall reactivity is discussed and compared to that of parent compounds. The acetyltriphenylgermane behaves as a high performance photoinitiator in the free radical polymerization and free radical promoted cationic polymerizations.

Vitamin B12 model complex catalyzed methyl transfer reaction to alkylthiol under electrochemical conditions with sacrificial electrode

Catalytic methyl transfer reactions from methyl tosylate to 1-octanethiol catalyzed by heptamethyl cobyrinate perchlorate, [Cob(II)7C(1)ester]ClO(4), were investigated under electrochemical conditions. As a model study for the cobalamin-dependent methyl transfer reaction from methyltetrahydrofolate to homocysteine, controlled-potential electrolyses were carried out at -1.0 V vs. Ag/AgCl using a zinc plate as the sacrificial anode at 50 degrees C in the dark. A turnover behaviour for the methyl transfer reaction was observed for the first time under non-enzymatic reaction conditions. Co(I) species, which is generated from the continuous electrolysis of [Cob(II)7C(1)ester]ClO(4), and its methylated CH(3)-Co complex were found to be important intermediates. The mechanism for such a methyl transfer reaction was investigated by product analysis, electronic spectroscopy and ESR spin-trapping experiments. A simple vitamin B(12) model complex was also utilized as the catalyst for the methyl transfer reaction.

Alpha-lipoic acid potently inhibits peroxynitrite-mediated DNA strand breakage and hydroxyl radical formation: implications for the neuroprotective effects of alpha-lipoic acid

Alpha-lipoic acid (LA) has recently been reported to afford protection against neurodegenerative disorders in humans and experimental animals. However, the mechanisms underlying LA-mediated neuroprotection remain an enigma. Because peroxynitrite has been extensively implicated in the pathogenesis of various forms of neurodegenerative disorders, this study was undertaken to investigate the effects of LA in peroxynitrite-induced DNA strand breaks, a critical event leading to peroxynitrite-elicited cytotoxicity. Incubation of phi X-174 plasmid DNA with the 3-morpholinosydnonimine (SIN-1), a peroxynitrite generator, led to the formation of both single- and double-stranded DNA breaks in a concentration- and time-dependent fashion. The presence of LA at 100-1,600 microM was found to significantly inhibit SIN-1-induced DNA strand breaks in a concentration-dependent manner. The consumption of oxygen induced by 250 microM SIN-1 was found to be decreased in the presence of high concentrations of LA (400-1,600 microM), indicating that LA at these concentrations may affect the generation of peroxynitrite from auto-oxidation of SIN-1. It is observed that incubation of the plasmid DNA with authentic peroxynitrite resulted in a significant formation of DNA strand breaks, which could also be dramatically inhibited by the presence of LA (100-1,600 microM). EPR spectroscopy in combination with spin-trapping experiments, using 5,5-dimethylpyrroline-N-oxide (DMPO) as spin trap, resulted in the formation of DMPO-hydroxyl radical adduct (DMPO-OH) from authentic peroxynitrite and LA at 50-1,600 microM inhibited the adduct signal. Taken together, these studies demonstrate for the first time that LA can potently inhibit peroxynitrite-mediated DNA strand breakage and hydroxyl radical formation. In view of the critical involvement of peroxynitrite in the pathogenesis of various neurodegenerative diseases, the inhibition of peroxynitrite-mediated DNA damage by LA may be responsible, at least partially, for its neuroprotective activities.

Free Radical Scavenging and Antioxidant Activity of Ascorbigen Versus Ascorbic Acid: Studies in Vitro and in Cultured Human Keratinocytes

Ascorbigen (ABG) is formed by indole-3-carbinole and ascorbic acid in brassica vegetables. In the present study, ABG has been systematically analyzed for its free radical scavenging and antioxidant capacity. To directly measure the free scavenging activity of ABG and ascorbic acid (used as a positive control), electron spin resonance and spin trapping experiments were performed. Ascorbic acid exhibited a potent free radical scavenging activity, whereas ABG did not scavenge superoxide anion free radicals and showed only little scavenging activity toward 2,2-diphenyl-1-picrylhydrazyl free radicals. Similar data were obtained for the ferric reducing ability of plasma and trolox equivalent antioxidant capacity assays. In cultured human keratinocytes, ABG counteracted tert-buthylhydroperoxide-induced cytotoxicity, whereas ascorbic acid did not exhibit any protective activity. Furthermore, in ABG-treated human keratinocytes, a decrease in tert-buthylhydroperoxide-induced lipid peroxidation was detected, whereas an ascorbic acid pretreatment did not result in the prevention of lipid peroxidation. These data indicate that ascorbic acid seems to be a more potent free radical scavenger than ABG in vitro, while ABG prevented tert-buthylhydroperoxide cytotoxicity more effectively as compared to ascorbic acid in cultured cells.