Attack Of H2O Research Articles

Competition between Water and Hydrogen Peroxide at Ti Center in Titanium Zeolites. An ab Initio Study

A combined Car−Parrinello molecular dynamics blue moon sampling approach has been adopted to study the competitive attack of H2O and H2O2 at a tetracoordinated Titanium site in a Ti−zeolite. The results indicate that, although the attack of water to form a trigonal bipyramidal center is thermodynamically more stable, the attack of hydrogen peroxide to form a similar adduct is kinetically favored. In both cases, solvent cooperation is effective in the formation of the adducts. The relevance of such a result in relation to the catalytic properties of Ti−zeolites is discussed.

Polysaccharide Fragmentation Induced by Hydroxyl Radicals and Hypochlorite

AbstractFor Abstract see ChemInform Abstract in Full Text.

Convenient Synthesis of t‐Butyl Z‐3‐Substituted Glycidates under Conditions of Phase‐Transfer Catalysis.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Convenient synthesis of t-butyl Z-3-substituted glycidates under conditions of phase-transfer catalysis

Reactions of mixtures of t-butyl E- and Z-3-substituted glycidates 1a– h with 50% aq. sodium hydroxide and a catalyst, benzyltriethylammonium chloride, TEBAC in dichloromethane (phase-transfer catalysis, PTC) led to preferential hydrolysis of the E-isomers to afford pure (90–98%) t-butyl Z-3-substituted glycidates 1a– i in good yields; PTC cleavage of glycidates additionally substituted at C-2, 1g or C-3, 1h, i suggests that an aryl group in the Z isomers hampers attack of HO − on the carbonyl carbon atom. As described in the literature, the diastereoselective PTC synthesis of Z-3-substituted glycidates and glycidonitriles consists of fast hydrolysis of E isomers present in mixtures with Z ones.

Proton inventory study of the base-catalyzed hydrolysis of formamide. Consideration of the nucleophilic and general base mechanisms.

An NMR study of the rates of hydroxide-promoted hydrolysis of formamide in aqueous media of varying mole fraction D(2)O (n) was performed at [LO(-)] = 1.42 M, T = 25 degrees C, to shed light on whether the mechanism involves a nucleophilic attack of HO(-) on the C=O or HO(-) acting as a general base to remove a proton from an attacking water. The solvent deuterium kinetic isotope effect under these conditions is inverse, k(OH)/k(OD) = 0.77 +/- 0.02 or k(OD)/k(OH) = 1.30 +/- 0.03. Proton inventory analysis of the k(n)() versus n data was undertaken through NLLSQ fits to equations representing four possible mechanisms encompassing nucleophilic and general base ones with waters of solvation on the attacking hydroxide, and with or without waters of solvation on the developing amide hydrate oxyanion. Both nucleophilic and general base mechanisms can be accommodated, but there are restraints on each that are discussed. The preferred mechanism is a nucleophilic one proceeding through a transition state having two solvating waters remaining on the attacking hydroxide and three additional waters attached to the developing amide hydrate oxyanion.

Iron(III) photoinduced degradation: Catalytic method for 4 ,4′‐diaminostilbene‐2 ,2′‐disulfonic acid removal from water

The Fe(III)‐photoinduced degradation of 4 ,4′‐diaminostilbene‐2 ,2′‐disulfonic acid (DSD), used as a model compound of fluorescent whitening agents, was investigated in aqueous solution. The mixing of DSD and iron(III) solutions immediately led to DSD degradation into two oxidation products 5‐amino‐2‐ formyl‐benzene sulfonic acid (1) and DSD epoxide (2) together with the formation of Fe(II). The behaviour of the latter solution was investigated under monochromatic excitation at 365 nm. The irradiation of the solution resulted in a photoredox process that yielded Fe(II) and HO• radicals; being the most photoactive species. The disappearances of the oxidation products were shown to only involve the attack of HO• radicals; the quantum yields of disappearances are wavelength dependent as the formation of the hydroxyl radicals is. For prolonged irradiation, the total mineralisation of the oxidation products was achieved with the formation of sulphate and ammonium ions. The half‐lives of products 1 and 2 when submitted to such a process under solar irradiation were estimated to be respectively 12 and 11 min in the mixture with 100% of monomeric species . Under these conditions, the total mineralisation is reached within 2 days of exposure.

Production of Hydroxyl Free Radical in the Xanthine Oxidase System with Addition of 1-methyl-3-nitro-1-nitrosoguanidine

We have examined the mechanism of 1-methyl-3-nitro-1-nitrosoguanidine (MNNG)-induced gastric cancer with respect to the production of hydroxyl free radical (OH). Nucleophilic attack by H2O2 on the nitroso group of MNNG produces 1-methyl-3-nitroguanidine (MNG) and the intermediate peroxynitric acid (ONOOH), which splits into hydroxyl free radical (OH) and nitrogen dioxide leading to the formation of nitric and nitrate ions in water. Xanthine oxidase (XO) induces the production of O2.- or H2O2 from molecular oxygen, depending on the overall level of enzyme reduction. In this study, we examined OH production by the reaction of MNNG with H2O2 derived from the XO-HX system containing XO and the purine substrate hypoxanthine by ESR using the spin trapping reagent 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO). OH was produced in the XO-HX-DMPO system with addition of MNNG (the MNNG-XO-HX-DMPO system) under aerobic conditions, but was not in the XO-HX-DMPO system, and production of OH was inhibited by catalase but not by superoxide dismutase, suggesting that OH was produced by the reaction of MNNG with H2O2 derived from the XO-HX system. The production of OH was significantly increased with increase in the reducing activity of XO, though that of O2.- was not, also suggesting the O2(.-)-independent .OH production. The productions of nitrite ion and MNG in the MNNG-XO-HX system were determined by the colorimetric method and HPLC, respectively. Based on these findings, we conclude that .OH was produced by homolytic split of the intermediate ONOOH formed by nucleophilic attack of H2O2 derived from the XO-HX system on MNNG.

Effect of quercetin and its conjugated metabolite on the hydrogen peroxide-induced intracellular production of reactive oxygen species in mouse fibroblasts.

To clarify the antioxidative role of quercetin metabolites in cellular oxidative stress, we measured the inhibitory effects of the quercetin aglycon and quercetin 3-O-beta-D-glucuronide (Q3GA), which is one of the quercetin metabolites in the blood after an intake of quercetin-rich food, on the production of hydrogen peroxide (H2O2)-induced intracellular reactive oxygen species in mouse fibroblast 3T3 cultured cells. When the cells were exposed to H2O2 in the presence of quercetin or Q3GA, Q3GA was found to be less effective than quercetin. In the case of a pretreatment with quercetin or Q3GA before the exposure, Q3GA, but not the quercetin aglycon, exerted an inhibitory effect, although its cellular uptake was unlikely. The quercetin aglycon appeared to fail in its antioxidative effect due to metabolic conversion into isorhamnetin conjugates, with substantial oxidative degradation resulting from the pretreatment. It is, therefore, suggested that quercetin metabolites take part in the protection of intracellular oxidative stress induced by the extraneous attack of H2O2.

Investigation of the intermediates formed during the degradation of Malachite Green in the presence of Fe3+ and H2O2 under visible irradiation

The photodegradation of Malachite Green (MG) was investigated in the presence of Fe3+ and H2O2 under visible irradiation (λ > 470 nm) and the intermediates produced during the degradation were determined by FTIR, GC-MS and 1H NMR. The mineralized products of nitrogen atoms in the dye molecule were analyzed by ammonia ion-selective electrode and ion chromatography. N,N-dimethylaniline, 4-dimethylamino phenol, 4-methylamino benzophenone, 4-dimethylamino benzophenone, benzeneacetic acids and 4-hydroxybenzoic acid were found during the photodegradation of MG. It can be concluded that the dye underwent two different principal degradation pathways in the initial period of the photodecomposition, one is that the dye molecule is splittingly decomposed into small species by attack of HO• radicals on the central carbon, another is that the N-methyl groups are oxidized by HO• radicals, leading to demethylation. Nitrogen atoms in the dye molecules are finally transformed into ammonium ions. A possible degradation mechanism is proposed.

The role of nitric oxide in the innate resistance to microfilariae of Litomosoides sigmodontis in mice.

Nitric oxide (NO) has been shown to be an important effector mechanism in the defence against various pathogens, including filariae. The production of NO, as well as H2O2, is induced by the Th1 cytokine IFN-gamma. Therefore, the microfilariae (mf) of filarial nematodes, which are known to elicit the release of IFN-gamma, may be a target of NO release. In this study, we found that mf of the filarial species Litomosoides sigmodontis were resistant to the attack of H2O2, but vulnerable to NO exposure in vitro by a chemical NO donor, as well as activated macrophages. Adult worms were considerably less affected by exposure to NO. In-vivo production of NO following injection of mf, in this and previous studies, suggested a central role in the defence to filariae. However, neither pharmaceutical inhibition of nitric oxide synthesis, nor genetic knockout of the gene for inducible nitric oxide synthase (iNOS), abrogated resistance to circulating mf in mice. Interestingly, however, iNOS-KO mice showed higher interleukin (IL)-2 responses and lower IL-10 production, compared to their wild-type counterparts. In conclusion, despite its effectiveness in vitro and the observed production of NO by ex vivo cells following infection, nitric oxide seems not to be an important factor in elimination of mf of L. sigmodontis in vivo. However, it may have a regulatory role in the immune response.

Structure and Reactivity of Dinitrogen Pentoxide in Small Water Clusters Studied by Electronic Structure Calculations

High level electronic structure calculations have been used to investigate the mechanism of hydrolysis of dinitrogen pentoxide in small neutral water clusters containing one to six solvating water molecules. The calculations clarify some of the current uncertainties in the hydrolysis mechanism. Increasing the size of the solvating water cluster leads to strong polarization and distortion of the N2O5 entity producing incipient (but not preexisting) NO2+, thus enhancing the electrophilicity of the nitrogen atom. The reaction mechanism involves nucleophilic attack of H2O on strongly ionized N2O5 followed by proton transfer to a neighboring water and does not involve the H2ONO2+NO3- ion pair. The solvating waters actively participate in the hydrolysis mechanism. The hydrolysis products, molecular nitric acid (HONO2) and ionized (H3O+NO3-) nitric acid are found to be stable in two different types of structures containing five and six water molecules. For the cluster containing six water molecules, which has a ...

Cationic Ring-Opening Polymerization of Seven-Membered Cyclic Carbonate with Water−Hydrogen Chloride through Activated Monomer Process

Controlled ring-opening polymerization of a seven-membered cyclic carbonate, 1,3-dioxepan-2-one (7CC), with H2O/HCl·Et2O initiator system was investigated. The molecular weight (Mn) of the obtained polymer could be controlled by the amount of H2O in the range of 103−104 maintaining a low polydispersity ratio (Mw/Mn = 1.11−1.17). The second portion of the monomer after completion of the first polymerization was converted quantitatively to give the corresponding polymer with a low Mw/Mn. The kinetic and 1H NMR spectroscopic studies suggested that the nucleophilic attack of H2O to the monomer activated with hydrogen chloride afforded α-hydroxyl-ω-carbonic acid, followed by decarboxylation reaction to give α,ω-dihydroxyl compound. It was suggested that the chain growth in this system was the attack of the terminal hydroxyl group to the monomer activated with hydrogen chloride.

Substituent effect on the photoinduced electron-transfer reaction of para-substituted triphenylphosphines sensitised by 9,10-dicyanoanthracene

The photoinduced electron-transfer reaction of para-substituted triphenylphosphines sensitised by 9,10-dicyanoanthracene (DCA) occurred in acetonitrile containing 2 vol% water to form the corresponding triphenylphosphine oxide. Transient absorption spectral measurements were carried out during 355 nm laser flash photolysis of a mixture of the phosphine and DCA. The electron transfer from the phosphine to singlet excited DCA initiated the reaction. A para substituent on the benzene ring affects the quantum yields of the phosphine radical cation and phosphine oxide. The back electron transfer from the DCA radical anion to the phosphine radical cation governed the quantum yield of the phosphine radical cation. The quantum yield of the phosphine oxide was dependent on the conjugation between the π-electron of the benzene ring and the n-electron of the phosphorus atom in the phosphine radical cation. The phosphine oxide forms through nucleophilic attack of H2O toward the phosphorus atom of the phosphine radical cation, producing the phosphoranyl radical.

Mechanism-Based Inhibition of Human Folylpolyglutamate Synthetase: Design, Synthesis, and Biochemical Characterization of a Phosphapeptide Mimic of the Tetrahedral Intermediate

Folylpolyglutamate synthetase (FPGS) catalyzes anATP-dependent ligation reaction that results in the synthesis of poly(γ-glutamate) metabolites of folates and some antifolates. We have synthesized and characterized the prototype of a new class of mechanism-based FPGS inhibitor in which a phosphonate moiety mimics the tetrahedral intermediate formed during the ligation reaction.This phosphonate, 4-amino-4-deoxy-10-methyl-pteroyl-l-glutamyl-γ-[Ψ{P(O)(OH)-O}]glutarate (4-NH2-10-CH3-Pte-l-Glu-γ-[Ψ{P(O)(OH)-O}]glutarate), is not a substrate for human FPGS, but is a linear, competitive inhibitor (Kis= 46 nM) with respect to methotrexate as the variable substrate. Inhibition is not time-dependent and preincubation of FPGS with this phosphonate does not increase the degree of inhibition, suggesting that it is not a slow, tight-binding inhibitor involving a time-dependent isomerization, EI → EI*. Substructures containing the phosphonate moiety but lacking the pterin are much less inhibitory to FPGS, indicating that a significant portion of the inhibitor binding energy is derived from the pterin moiety, a feature also observed in substrate binding. 4-NH2-10-CH3-Pte-l-Glu-γ-[Ψ{P(O)(OH)-O}]glutarate is also an analog of a proposed tetrahedral intermediate in the reaction catalyzed by γ-glutamyl hydrolase (γ-GH), another enzyme of importance in controlling folate homeostasis in cells. This intermediate would arise from direct attack of H2O on the dipeptide, 4-NH2-10-CH3-Pte-l-Glu-γ-l-Glu. The fact that 4-NH2-10-CH3-Pte-l-Glu-γ-[Ψ{P(O)(OH)-O}]glutarate is not an inhibitor of γ-GH strongly suggests that hydrolysis of poly-γ-glutamates catalyzed by γ-GH does not involve the direct attack of water at the scissile amide bond. Methotrexate, its γ-glutamyl dipeptide metabolite, and 4-NH2-10-CH3-Pte-l-Glu-γ-[Ψ{P(O)(OH)-O}]glutarate are equipotent as inhibitors of human dihydrofolate reductase (the primary target of methotrexate), but the phosphonate does not significantly inhibit another important folate-dependent enzyme, thymidylate synthase. Thus, the phosphonate moiety in this analog represents an important new lead in the development of FPGS inhibitors.

Mediation of oxidative DNA damage by nickel(II) and copper(II) complexes with the N-terminal sequence of human protamine HP2.

The potential of Ni(II) and Cu(II) complexes with Arg-Thr-His-Gly-Gln-Ser-His-Tyr-Arg-Arg-Arg-His-Cys-Ser-Arg-amide (HP2(1-15)), a peptide modeling the N-terminal amino acid sequence of human protamine HP2, to mediate oxidative DNA damage was studied by measurements of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) generation from 2'-deoxyguanosine (dG) and calf thymus DNA and by formation of double-strand breaks in calf thymus DNA. The concentrations of reagents were 0.1 mM dG and the metal-HP2(1-15) complex, 1 mM H2O2, 1.5 mM DNA (per phosphate group), 100 mM phosphate buffer, pH 7.4, ambient O2. Samples were incubated at 37 degrees C for 16-24 h. The Cu(II)-HP2(1-15) complex was found to be an effective promoter of the formation of 8-oxo-dG from both dG and DNA with ambient O2 (approximately 13- and 3-fold increase versus the oxidant alone, respectively) and H2O2 (approximately 25-fold increase in either case). The Ni(II)-HP2(1-15) complex was ineffective with O2 versus 8-oxo-dG production from both substrates but markedly enhanced the attack of H2O2 on dG and DNA (approximately 5-fold increase of 8-oxo-dG production in either case). Both Cu(II)- and Ni(II)-HP2(1-15) equally promoted double-strand scission by H2O2 in calf thymus DNA. The promotion by the complexes of dG and DNA oxidation with H2O2 was accompanied by oxidative damage to the complexes themselves, consisting of decreasing contents of their His (to approximately 50% of control in either complex) and especially Tyr (down to 48% of control in Cu(II)- and 19% in Ni(II)-HP2[1-15]) residues, as well as appearance of aspartic acid, the known oxidation product of His residues in peptides (up to 22% vs Gly for Cu(II)- and 10% for Ni(II)-HP2(1-15)). The above results provide a novel chemical mechanism of Cu(II) and Ni(II) toxicity and may have wide implications for reproductive and transgenerational effects of metal exposure.

Endothelial cells protect vascular smooth muscle cells from H2O2 attack.

Endothelial-dependent vascular responses are altered in ischemic acute renal failure. Oxidants formed during reperfusion of ischemic kidneys injure the renal microvasculature and prevent recovery of renal function. To determine whether endothelial cells (EC) modulate oxidant attack on vascular smooth muscle cells (VSMC), rat mesenteric artery VSMC were grown on coverslips and then coincubated with bovine pulmonary artery EC grown in wells. In the absence of EC, H2O2 caused time- and concentration-dependent increases in VSMC injury as indicated by release of [3H]adenine. In contrast, addition of EC reduced H2O2-mediated (5 mM, 1 h) VSMC adenine release from 63.8 +/- 4.5% to 28.6 +/- 2.9% (P < 0.001). The protective effect of EC did not occur when H2O2 was added to the surface of VSMC unopposed to EC and was partially reversed when EC were treated with aminotriazole to inactivate catalase (41.7 +/- 2.7%). To determine whether EC nitric oxide (NO) modified H2O2 attack on VSMC, EC were treated with N omega-nitro-L-arginine (L-NNA). The protective effect of EC was partially abrogated with L-NNA (53.8 +/- 4.3%). Treatment of EC with interleukin-1 beta (IL-1 beta) for 24 h prior to coincubation with VSMC enhanced the protective effect of EC. IL-1 beta-induced protection was reversed with L-NNA. No protection was observed when VSMC were treated with 8-bromoguanosine 3',5'-cyclic monophosphate, forskolin, or phorbol 12-myristate 13-acetate. Our conclusions are as follows. VSMC are protected by EC from luminal but not contraluminal oxidant attack. The protective effect of EC is mediated by catalase- and NO-dependent inactivation of oxidants. EC dysfunction could account for renal injury caused by oxidants formed during reperfusion of ischemic kidneys.

Kinetics of the reaction between hydrogen peroxide and hypobromous acid: Implication on water treatment and natural systems

HOBr can be formed in various oxidation processes in engineered and natural systems. The rate of HOBr reduction by H 2O 2 is decisive to avoid formation of brominated organic compounds and bromate during ozone or hydrogen peroxide-based advanced or naturally occurring oxidation processes. From the pH dependence of this rate that was determined by stopped-flow measurements we conclude that either OBr − and H 2O 2 or HOBr and HO − 2 react with each other. Assuming that either one of the reactions takes place the corresponding second-order rate constants were determined to be k OBr −·H 2O 2 = (1.2 ± 0.2)·10 6M −1s −1 and k HOBr·HO − 2 = (7.6 ± 1.3)·10 8M −1s −1. Mechanistic considerations lead to the conclusion that a nucleophilic attack of HO − 2 on HOBr must be the dominant reaction in the system. From the determined rate constants it can be estimated that the half-life for HOBr is less than a few seconds for a H 2O 2 concentration of 0.1 mg L −1 (3 μM) at pH 8. However, at a lower pH of 5, as encountered in cloud waters, the half-life of HOBr is several hours for the same H 2O 2 concentration.

Influence of a small amount of Al2O3 addition on the transformation of Y2O3-partially stabilized ZrO2 during annealing

Tetragonal-to-monoclinic phase transformations in 2 and 3 mol% Y2O3–ZrO2 ceramics and their composites with 5 vol% Al2O3 during annealing in water and in vacuum at 353–623 K were investigated to explore the effect of a small quantity of Al2O3 addition on the transformation. The dispersion of Al2O3 particles into the ZrO2(Y2O3) matrix was found to be effective to suppress the transformation directly induced by the attack of H2O during annealing in water, even though the amount was as small as 5 vol%. However, the transformation predominantly caused by thermal activation during annealing in vacuum was not affected by the limited amount of Al2O3 addition. The effect of suppression of Al2O3 on the water-induced phase transformation was considered to be realized through the hydroxydation of Al2O3 particles, by which the sample surface was effectively "protected" from further attack of H2O, which accelerated the low-temperature degradation transformation.

Application of electrospray ionisation mass spectrometry to the study of dilute aqueous oligomeric anions and their reactions

For the first time, electrospray ionisation mass spectrometry has been carried out on purely aqueous solutions of anions, specifically dilute (10–3 mol dm–3) isopolytungstates, peroxotungstates and heteropolymolybdates and tungstates. Many new species have been detected that have not previously been reported in water, including [W6O19]2– and [W2O7]2–. To demonstrate its investigative potential, the method was used to identify the complex and previously uncharacterised reaction products from the reaction of H2O2 on powdered tungsten metal, where hexametalates form with up to four peroxo ligands, and also to track kinetic processes such as the course of metal-ion exchange between phosphododecametalates, and the attack of H2O2 on paratungstate B.

Biosynthesis of Dodecano‐4‐lactone in Ripening Fruits: Crucial Role of an Epoxide‐Hydrolase in Enantioselective Generation of Aroma Components of the Nectarine (Prunus persica var. nucipersica) and the Strawberry (Fragaria ananassa)

AbstractEarly steps in the biosynthesis of dedecano‐4‐lactone in ripening nectarines (Prunus persica var. nucipersica) and strawberries (Fragaria ananassa) were studied using cis‐9,10‐([18O]epoxy)heptadecanoic acid (5), instead of the natural C18‐precursor epoxystearic acid 1. In nectarines, an epoxide hydrolase catalyzed, in a highly regio‐ and enantioselective reaction, the attack of H2O at C(9) of epoxy acid 5 yielding preferentially (9R,10R)‐dihydroxyheptadecanoic acid 6a as an early intermediate and, after further metabolization, undecano‐4‐lactone 7a. In strawberries, the same transformation exhibited only a moderate regio‐ and enantioselectivity. Besides the previously established function of phytogenic epoxide hydrolases involved in generating oxygenated fatty‐acid derivatives, which are used in cutin monomer production and plant defence, a new rǒle for these enzymes now emerges in the enantioselective production of aroma components of ripening fruits. The cis‐9,10‐([18O]epoxy)heptadecanoic acid (≥ 98% 18O; 5) was obtained in good overall yield (68%) from isopropyl (9Z)‐heptadec‐9‐enoate using 18O2 gas in a free‐radical chain reaction in the presence of 2‐methylbutanal.