Showing papers on "Hydrogen peroxide published in 2005"

Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions

Abstract: Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.

Boronate-Based Fluorescent Probes for Imaging Cellular Hydrogen Peroxide

Abstract: The syntheses, properties, and biological applications of the Peroxysensor family, a new class of fluorescent probes for hydrogen peroxide, are presented. These reagents utilize a boronate deprotection mechanism to provide high selectivity and optical dynamic range for detecting H2O2 in aqueous solution over similar reactive oxygen species (ROS) including superoxide, nitric oxide, tert-butyl hydroperoxide, hypochlorite, singlet oxygen, ozone, and hydroxyl radical. Peroxyresorufin-1 (PR1), Peroxyfluor-1 (PF1), and Peroxyxanthone-1 (PX1) are first-generation probes that respond to H2O2 by an increase in red, green, and blue fluorescence, respectively. The boronate dyes are cell-permeable and can detect micromolar changes in H2O2 concentrations in living cells, including hippocampal neurons, using confocal microscopy and two-photon microscopy. The unique combination of ROS selectivity, membrane permeability, and a range of available excitation/emission colors establishes the potential value of PR1, PF1, PX1,...

Review: organic matter removal from soils using hydrogen peroxide, sodium hypochlorite, and disodium peroxodisulfate

Abstract: We compare the performance of three most accepted reagents for organic matter removal: hydrogen peroxide (H 2 O 2 ), sodium hypochlorite (NaOCI) and disodium peroxodisulfate (Na 2 S 2 O 8 ). Removal of organic matter from soil is mostly incomplete with the efficiency of removal depending on reaction conditions and sample properties. Generally, NaOCI and Na 2 S 2 O 8 are more effective in organic C removal than H 2 O 2 . Alkaline conditions and additives favoring dispersion and/or desorption of organic matter, such as sodium pyrophosphate, seem to be crucial for C removal. Pyrophosphate and additives for pH control (bicarbonate) may irreversibly adsorb to mineral surfaces. In soils with a large proportion of organic matter bound to the mineral matrix, for example subsoils, or rich in clay-sized minerals (Fe oxides, poorly crystalline Fe and Al phases, expandable phyllosilicates), C removal can be little irrespective of the reagents used. Residual organic C seems to seems to represent largely refractory organic matter, and comprises mainly pyrogenic materials and aliphatic compounds. If protected by close association with minerals, other organic constituents such as low-molecular weight carboxylic acids, lignin-derived and N-containing compounds may escape chemical destruction. For determination of mineral phase properties, treatment with H 2 O 2 should be avoided since it may promote organic-assisted dissolution of poorly crystalline minerals at low pH, disintegration of expandable clay minerals, and transformation of vermiculite into mica-like products due to NH + 4 fixation. Sodium hypochlorite and Na 2 S 2 O 8 are less harmful for minerals than H 2 O 2 . While the NaOCI procedure (pH 9.5) may dissolve Al hydroxides, alkaline conditions favor the precipitation of metals released upon destruction of organic matter. Prolonged heating to >40°C during any treatment may transform poorly crystalline minerals into more crystalline ones. Sodium hypochlorite can be used at 25°C, thus preventing heat-induced mineral alteration.

Peroxide detoxification by brain cells.

Abstract: Peroxides are generated continuously in cells that consume oxygen. Among the different peroxides, hydrogen peroxide is the molecule that is formed in highest quantities. In addition, organic hydroperoxides are synthesized as products of cellular metabolism. Generation and disposal of peroxides is a very important process in the human brain, because cells of this organ consume 20% of the oxygen used by the body. To prevent cellular accumulation of peroxides and damage generated by peroxide-derived radicals, brain cells contain efficient antioxidative defense mechanisms that dispose of peroxides and protect against oxidative damage. Cultured brain cells have been used frequently to investigate peroxide metabolism of neural cells. Efficient disposal of exogenous hydrogen peroxide was found for cultured astrocytes, oligodendrocytes, microglial cells, and neurons. Comparison of specific peroxide clearance rates revealed that cultured oligodendrocytes dispose of the peroxide quicker than the other neural cell cultures. Both catalase and the glutathione system contribute to the clearance of hydrogen peroxide by brain cells. For efficient glutathione-dependent reduction of peroxides, neural cells contain glutathione in high concentration and have substantial activity of glutathione peroxidase, glutathione reductase, and enzymes that supply the NADPH required for the glutathione reductase reaction. This article gives an overview on the mechanisms involved in peroxide detoxification in brain cells and on the capacity of the different types of neural cells to dispose of peroxides.

Asymmetric Organocatalytic Epoxidation of α,β-Unsaturated Aldehydes with Hydrogen Peroxide

Abstract: The first asymmetric organocatalytic epoxidation of α,β-unsaturated aldehydes is presented. A chiral bisaryl−silyl-protected pyrrolidine acts as a very selective epoxidation organocatalyst using simple oxidation agents, such as hydrogen peroxide and tert-butyl hydroperoxide. The asymmetric epoxidation reactions proceed under environmental friendly reaction condition in, for example, water mixtures of alcohols, and the scope of the reaction is demonstrated by the formation of optically active α,β-epoxy aldehydes in high yields and enantioselectivities >94% ee. Furthermore, the direct synthesis of the sex pheromone from an acaric mite by asymmetric epoxidation of citral is presented.

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide.

Abstract: Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L(-1), pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at -0.5 V vs. Ag from 5 mmol L(-1) AgNO3/0.1 mol L(-1) TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at -0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0 x 10(-6) mol L(-1) for hydrogen peroxide in phosphate buffer (0.05 mol L(-1), pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode.

Antioxidant activity of the flavonoid hesperidin in chemical and biological systems.

Abstract: The antioxidant hesperidin, a major flavonoid in sweet orange and lemon, was evaluated using chemical and biological systems. The chemical assay evaluates the hesperidin capacity to sequester 1,1-diphenyl-2-picrylhydrazyl (DPPH*). Biological studies were done using the eukaryotic cells of superoxide-dismutase proficient and deficient strains of Saccharomyces cerevisiae treated with hesperidin and the stressing agents hydrogen peroxide or paraquat (methylviologen; 1,1'-dimethyl-4,4'-bipyridinium dichloride). Hesperidin was able to reduce significantly the level of the free radical DPPH* with similar efficacy of trolox (positive control). When the yeast cells were exposed to the flavonoid hesperidin before the stressing agents, there was a significant increase in the survival of all strains. Paraquat induced higher catalase and superoxide dismutase than did hydrogen peroxide, which only increased catalase activity. Previous addition of hesperidin to these treatments was able to reduce significantly both enzymatic levels. These observations clearly demonstrate that hesperidin provides strong cellular antioxidant protection against the damaging effects induced by paraquat and peroxide hydrogen.

Interaction of adsorption and catalytic reactions in water decontamination processes: Part I. Oxidation of organic contaminants with hydrogen peroxide catalyzed by activated carbon

Abstract: This series of papers addresses the role of sorption in heterogeneous catalysis aimed at the removal of organic contaminants (OCs) from water. This first part is focused on the oxidative treatment of OCs by H2O2 catalyzed by activated carbon (AC). The relative reaction rates of compounds with different hydrophobicities and therefore different sorption tendencies on AC (methyl tert-butyl ether, trichloroethene, 2,4,5-trichlorophenol) in the AC/H2O2 system differed drastically from those observed in a classical homogeneous Fenton system. Quantitative considerations that take into account the ratio of the reaction rate constants of MTBE and TCE in the AC/H2O2 system and the homogeneous Fenton system as well as the ratio of their freely dissolved fractions lead to the conclusion that the predominant pathway for the degradation reaction in the AC/H2O2 system is the attack of OH radicals on the OC fraction that is freely dissolved in the pore volume of the AC. In contrast, the sorbed fraction is nearly unreactive, i.e. protected against radical attack. Quenching experiments with methanol, a strong competitor for reactions with OH radicals in the solution phase, further confirmed this hypothesis. Consequently, sorption on AC has an adverse effect on the oxidation of OCs via OH radicals, even though the radicals are formed directly on the AC surface, i.e. in close proximity to the sorbed OCs.

Response of the antioxidant defense system to tert-butyl hydroperoxide and hydrogen peroxide in a human hepatoma cell line (HepG2).

Abstract: The aim of this work was to investigate the response of the antioxidant defense system to two oxidative stressors, hydrogen peroxide and tert-butyl hydroperoxide, in HepG2 cells in culture. The parameters evaluated included enzyme activity and gene expression of superoxide dismutase, catalase, glutathione peroxidase, and activity of glutathione reductase. Besides, markers of the cell damage and oxidative stress evoked by the stressors such as cell viability, intracellular reactive oxygen species generation, malondialdehyde levels, and reduced glutathione concentration were evaluated. Both stressors, hydrogen peroxide and tert-butyl hydroperoxide, enhanced cell damage and reactive oxygen species generation at doses above 50 microM. The concentration of reduced glutathione decreased, and levels of malondialdehyde and activity of the antioxidant enzymes consistently increased only when HepG2 cells were treated with tert-butyl hydroperoxide but not when hydrogen peroxide was used. A slight increase in the gene expression of Cu/Zn superoxide dismutase and catalase with 500 microM tert-butyl hydroperoxide and of catalase with 200 microM hydrogen peroxide was observed. The response of the components of the antioxidant defense system evaluated in this study indicates that tert-butyl hydroperoxide evokes a consistent cellular stress in HepG2.

Hydrogen Peroxide Induced Oxidative Stress Damage and Antioxidant Enzyme Response in Caco-2 Human Colon Cells

Abstract: Studies were conducted to evaluate the cell damage caused by exposing human colon carcinoma cells, Caco-2, to hydrogen peroxide at concentrations varying from 0 to 250 μM for 30 min. Evaluation of cell viability, as measured by trypan blue dye exclusion test, showed that the loss of viability was 100 μM hydrogen peroxide compared to those of the control. Antioxidant mechanisms in Caco-2 cells were evaluated by measuring catalase, superoxide dismutase, and glutathione peroxidase activities. Catalase activities remained constant in cells treated with 50−250 μM hydrogen peroxide. Superoxide dismutase activity decreased, whereas glutathione peroxidase activity increased in cells treated with H2O2 concentrations of >50 μM. This study showed that with increasing hydrogen peroxide concentration, ...

NAD(P)H Oxidase–Dependent Self-Propagation of Hydrogen Peroxide and Vascular Disease

Abstract: Excessive production of reactive oxygen species in the vasculature contributes to cardiovascular pathogenesis. Among biologically relevant and abundant reactive oxygen species, superoxide (O2·−) an...

Decomposition of Hydrogen Peroxide at Water−Ceramic Oxide Interfaces

Abstract: The thermal decomposition of hydrogen peroxide, H2O2, was determined in aqueous suspensions of SiO2, Al2O3, TiO2, CeO2, and ZrO2 nanometer-sized particles. First-order kinetics were observed for the decomposition in all cases. Temperature dependence studies found that the activation energy was 42 ± 5 kJ/mol for the overall decomposition of H2O2 independent of the type of oxide. Oxide type had a strong effect on the preexponential rate term with increasing rate in the order of SiO2 < Al2O3 < TiO2 < CeO2 < ZrO2. The rate coefficient for H2O2 decomposition increases with increasing surface area of the oxide, but the number or efficiency of reactive sites rather than the total surface area may have the dominant role. Very efficient scavengers for OH radicals in the bulk liquid are not able to prevent formation of molecular oxygen, the main H2O2 gaseous decay product, suggesting that decomposition occurs on the oxide surfaces. The decomposition of H2O2 in the γ-radiolysis of water is enhanced by the addition o...

Investigations on the Iron-Catalyzed Asymmetric Sulfide Oxidation

Abstract: The development of an enantioselective sulfide oxidation involving a chiral iron catalyst and aqueous hydrogen peroxide as oxidant is described. In the presence of a simple carboxylic acid, or a carboxylate salt, the reaction affords sulfoxides with remarkable enantioselectivities (up to 96 % ee) in moderate to good yields. The influence of the structure of the additive on the reaction outcome is reported. In the sulfoxide-to-sulfone oxidation a kinetic resolution (with s = 4.8) occurs, which, however, plays only a negligible role in the overall enantioselective process. Furthermore, a positive nonlinear relationship between the ee of the product and that of the catalyst has been found. On the basis of these observations, a possible catalyst structure is proposed.

A role for ETR1 in hydrogen peroxide signaling in stomatal guard cells.

Abstract: Signaling through the redox active molecule hydrogen peroxide (H2O2) is important for several processes in plants, such as stomatal closure, root growth, gravitropism, and responses to pathogen challenge ([Neill et al, 2002][1]; [Laloi et al, 2004][2]) Although oxidative modification of reactive

In Situ Detection of Hydrogen Peroxide in PEM Fuel Cells

Abstract: An electrochemical technique using thin Pt wires as working electrodes was employed to determine the concentration of peroxide within the proton exchange membrane (PEM) of operating fuel cells. The existence of hydrogen peroxide was clearly observed, and its concentration depended primarily on membrane thickness, with thinner membranes displaying higher peroxide concentrations. H 2 O 2 is most likely formed on the anode side of the cell through reduction of O 2 .