Final Oxidation Products Research Articles

Redox reactions between molecular oxygen and tetrachlorohydroquinone at the water∣1,2-dichloroethane interface

The redox process between molecular oxygen, O2, in an aqueous solution, W, and tetrachlorohydroquinone, CQH2, in 1,2-dichloroethane, DCE, at the W ∣ DCE interface was investigated by electrolyses at different potential differences applied between the two phases, Eappl. Three different overall redox reactions were found to occur depending on Eappl. After electrolysis, the final reduction products of O2 in W were H2O as well as a small amount of H2O2 and the final oxidation products of CQH2 in DCE were the tetrachloroquinone anion radical, CQ−, for one range of Eappl, H2O as well as H2O2 and tetrachloroquinone, CQ, for a second range, and H2O2 and CQ for a third range. The redox processes were elucidated taking into account the results obtained by polarography for charge transfer at the W ∣ DCE interface in addition to the results mentioned above. It was also demonstrated that redox reactions between O2 and CQH2 could be realised and controlled, not only by the electrolysis, but also by ion transfer reactions at the interface coupled with the redox reactions.

Oxidation and photooxidation of sulfide and thiosulfate ions catalyzed by transition metal chalcogenides and phthalocyanine complexes

The catalytic and photocatalytic activities of supported cobalt or zinc phthalocyanine complexes, bulk MoS 2, MoS 2 deposited on Al 2O 3, potassium intercalated MoS 2 (K 0.33 H 2O 0.66 MoS 2), CdS and polycrystalline nickel phosphorus trisulfide (NiPS 3) have been investigated in the oxidation of sodium sulfide and Na 2S 2O 3. The phthalocyanine complexes and the metal chalcogenides do not catalyze, in the absence of light, the complete oxidation of the sulfide ion to sulfate ion. The final product of the catalytic oxidation is the formed thiosulfate. No oxidation of Na 2S 2O 3 has been registered in the dark in the presence of any of the catalytic samples. Their activity was enhanced upon irradiation with visible light. Thiosulfate appears to be the final product also of the photooxidation of the sulfide ion catalyzed by metal chalcogenides. They do not catalyze the further photooxidation of Na 2S 2O 3. The only photocatalysts which favour with their presence the oxidation of the sulfide and thiosulfate ions to sulfate ion, are the zinc phthalocyanine complexes. In this case, the photooxidation process involves singlet oxygen.

Destruction of aniline by mediated electrochemical oxidation with Ce(IV) and Co(III) as mediators

Mediated electrochemical oxidation has been employed to test the feasibility of treating soluble organic wastes. We report Ce(IV)- and Co(III)-mediated electrochemical oxidation of aniline at various electrodes in acidic media as an example of organic waste. Aniline was oxidized by an electrogenerated electron transfer mediator, Ce4+ or Co3+, in the anolyte and carbon dioxide was produced as a final oxidation product. Carbon dioxide was collected by bubbling through a barium hydroxide solution. When a powerful oxidizing agent, Ce(IV) or Co(III), was used as an electron shuttling mediator, parameters affecting the coulombic efficiency for aniline oxidation were the standard oxidation potentials of the mediators, their concentrations and the reaction temperature. Intermediate species produced during the oxidation of aniline were identified by cyclic voltammetric and absorption spectroscopic measurements.

Cytotoxicity of 2-butoxyethanol and its oxidation products in vitro

Glycol ethers belong to a group of solvents with a wide spectrum of applications, particularly because of their compatibility to both hydrophilic and lipophilic systems. Especially ethylene glycol monobutyl ether (2-butoxy ethanol, BE) is widely used as a key ingredient in many industrial and consumer cleaning products. From cases of acute BE intoxication in humans it is known that it causes severe toxicity, including coma, metabolic acidosis, hypokalemia and hemoglobinuria. It is currently unknown whether the toxicity is related to BE itself or its oxidation products, butoxyacetaldehyde (BAL) and butoxyacetic acid (BAA). By use of an established kidney epithelial cell line from the proximal tubule (opossum kidney cells), the effects of BE, BAL and BAA on cell viability and the organization of the intracellular cytoskeleton of the cells were investigated and combined with gas chromatographic and mass spectrometric determinations. The experiments were performed with (i) freshly used BE, (ii) BE that had been stored at room temperature for 3 months after use in the original packing and containing 0.5% BAL, (iii) BAL with a purity of 73.5% after catalytic oxidative dehydration of BE with a heated copper meshwork, and (iv) BAA as the final oxidation product of BE. Kidney epithelial cells were unable to oxidize BE via BAL to BAA directly, but formation of BAA after the addition of BAL was observed with a turnover rate of approximately 45%. The tests for cytotoxicity showed no acute toxic effect for freshly used BE and BAA, whereas stored BE containing 0.5% BAL and synthesized BAL caused a dose-dependent decrease of viable cells within 24 h. The EC50 (i.e. the concentration causing a 50% loss in cell viability) was 1 mg/mL for stored BE and 15 μg/mL for BAL. Calculations taking the actual BAL content of stored BE and BAL into account demonstrated that the cytotoxic effects seem to be mainly related to BAL and not to by-products such as butyraldehyde or n-butanol. In accordance with the tests for cytotoxicity were the cytoskeletal alterations of the three-dimensional stress fiber network. Stored BE and synthesized BAL caused a disruption and/or depolymerization of the stress fiber network which points to massive alterations of the intracellular ional milieu. The present study provides evidence that BE and BAA possess only a minor toxic potential, whereas BAL as the corresponding reactive intermediate has marked toxic effects. Thus, the aldehyde might be responsible for the toxic effects observed in studies in vitro and after human intoxication.

A model mechanism of nitric oxide (NO) formation by NO synthase

The recent discovery that nitric oxide (NO) plays a role as a second messenger in biological processes created a growing interest in this radical. NO is synthesized from one of the terminal nitrogen atom of l-arginine by the action of the NO synthase (NOS). The NOS catalyze a five-electron oxidation of l-Arg. We thus propose a reaction mechanism based on the experimental data. Thus, N-methyl guanidine was used as a model compound for Arginine to study the mechanism of oxidative release of NO by means of ab initio molecular computations. The oxidation of this model compound was carried out with the aid of performic acid. In this model mechanism, the released specie is HNO, which can easily form NO. From the values obtained from HOMO–LUMO energy differences, it is clear that the peracid is being reduced, i.e. it is gaining an electron pair and is accommodating that on its HOMO, while l-Arg is being oxidized. This hypothesis is supported by the net charge at the nitrogen atom of guanidine, which undergoes oxidation. On the whole, the reaction mechanism proposed provides a negative Gibbs free energy (−7.33kcalmol−1), suggesting that the reaction is thermodynamically feasible. The reaction mechanism confirmed the previous hyphotesis about the origin of the NO, from the oxidized nitrogen of the guanidine group, and the weakening of the C3–N4 bond that leads to the final product of the second oxidation: HNO. Our results fully agree with the previous experimental findings.

Electrocatalytic mechanism for formaldehyde oxidation on the highly dispersed gold microparticles and the surface characteristics of the electrode

Electrocatalytic mechanism for the electrochemical oxidation of formaldehyde (HCHO) on the highly dispersed Au microparticles electrodeposited on the surface of the glass carbon (GC) electrode in the alkaline Na 2CO 3/NaHCO 3 solution and the surface characteristics of the Au microparticle-modified glass carbon (Au/GC) electrode were studied with in situ FTIR spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). It was found that the final products of HCHO oxidation is HCOO − at the Au/GC electrode and CO 2 at the bulk Au electrode. The difference may be ascribed to the different surface characteristics between the Au/GC electrode and the bulk Au electrode.

KINETIC STUDIES OF THE OXIDATION OF bis[1-HYDROXY-2-(SALICYLIDENEAMINO)-ETHANE]MANGANESE(II) BY MOLECULAR OXYGEN

Oxidation of the bis[l-hydroxy-2-(salicylideneamino)ethane]manganese(II) complex by molecular oxygen has been investigated in aqueous dimethylformamide and acetonitrile solutions. Kinetic EPR experiments indicated rapid oxidation of the metal to Mn(III), followed by a slower step, in which a Mn(IV) complex is formed. This final oxidation product was isolated in 90% yield, and characterized by different techniques. EPR measurements suggested the existence of an unstable binuclear intermediate [L2Mn(II)…Mn(H)L2], preceding the Mn(III) complex formation. Manometric measurements of oxygen uptake also showed two kinetic paths for formation of the Mn(III) species, both of them exhibiting a first-order dependence on the [complex], with rate constants K obs = (1.65 ± 0.08)10-3s−1 and (5.57 ± 0.28)10−6s−1, respectively. Variation of the percentage of water in the reaction solution indicated that the rapid path is dependent on the oxygen concentration, while the slow one is almost [O] independent. The influence of added hydroxide ions and of the solvent were also verified, suggesting competitive substitution reactions. A mechanism involving a monomelic Mn(III) intermediate is proposed and discussed, corroborated by electrochemical and EPR measurements.

Experimental Study of the Oxidation of Gaseous Monomethylhydrazine in a Strictly Monophasic Medium

Intermediate and final products of the oxidation of monomethylhydrazine by dioxygen are studied using an original device which allows analysis of the gaseous medium under strictly monophasic conditions.

Oxidation of guaiacol by myeloperoxidase: a two-electron-oxidized guaiacol transient species as a mediator of NADPH oxidation.

The present study was first aimed at a complete steady-state kinetic analysis of the reaction between guaiacol (2-methoxyphenol) and the myeloperoxidase (MPO)/H2O2 system, including a description of the isolation and purification of MPO from human polymorphonuclear neutrophil cells. Secondly, the overall reaction of the oxidation of NADPH, mediated by the reactive intermediates formed from the oxidation of guaiacol in the MPO/H2O2 system, was analysed kinetically. The presence of guaiacol stimulates the oxidation of NADPH by the MPO/H2O2 system in a concentration-dependent manner. Concomitantly, the accumulation of biphenoquinone (BQ), the final steady-state product of guaiacol oxidation, is lowered, and even inhibited completely, at high concentrations of NADPH. Under these conditions, the stoichiometry of NADPH:H2O2 is 1, and the oxidation rate of NADPH approximates to that of the rate of guaiacol oxidation by MPO. The effects of the presence of superoxide dismutase, catalase and of anaerobic conditions on the overall oxidation of NADPH have also been examined, and the data indicated that superoxide formation did not occur. The final product of NADPH oxidation was shown to be enzymically active NADP+, while guaiacol was generated continuously from the reaction between NADPH and oxidized guaiacol product. In contrast, similar experiments performed on the indirect, tyrosine-mediated oxidation of NADPH by MPO showed that a propagation of the free radical chain was occurring, with generation of both O2(-.) and H2O2. BQ, in itself, was able to spontaneously oxidize NADPH, but neither the rate nor the stoichiometry of the reaction could account for the NADPH-oxidation process involved in the steady-state peroxidation cycle. These results provide evidence that the oxidation of NADPH does not involve a free nucleotide radical intermediate, but that this is probably due to a direct electron-transfer reaction between NADPH and a two-electron-oxidized guaiacol intermediate.

Chemical state and properties of the Nb 5Sn 2Ga grown by the self-component flux method using tin as a solvent

Single crystals of a new compound Nb 5Sn 2Ga were obtained by the flux method using molten tin as a solvent. The crystal structure represents a tetragonal symmetry, space group D 4h 18 I4/ mcm, ordered W 5Si 3-type structure. Lattice parameters are a=1.0586(2) nm and c=0.5177(1) nm. According to XPS study, the peaks of Nb, Sn and Ga 3dXP spectra negatively shift when the three elements of Nb, Sn and Ga form the compound of Nb 5Sn 2Ga. The compound shows superconductivity at T c =1.75 K and Δ T c =140 mK. The residual resistivity ratio, RRR= ρ (293 K)/ ρ (4.2 K), is 12. The Micro-Vickers hardness (MVH) value for the (100) or (110) face is 8.9–8.5 GPa, and for the (001) face is 10.1–9.1 GPa. Oxidation of the Nb 5Sn 2Ga starts at 562°C. Weight gain of the compound heated up to 1200°C in air is 37.3%. Final oxidation product contains NbO 2, Nb 2O 5, Nb 12O 29, SnO 2 and Ga 2O 3.

Growth conditions, electrical resistivity, microhardness and thermal properties of Nb 5Sn 2Ga single crystals synthesized from high-temperature tin solutions

Single crystals of a new ternary intermetallic compound Nb 5Sn 2Ga were prepared from high-temperature tin solutions by a self flux method using Nb and Ga metals as starting materials under a He gas. The crystal was examined by X-ray diffraction and chemical analyses. The ternary single crystals Nb 5Sn 2Ga were generally obtained in the form of prismatic shape extending in the [001] direction, and with (100) and (110) faces. The Vickers microhardness value on (001) plane and (100) or (110) planes of crystals is in the range of 8.5–10.1 GPa. The electrical resistivity determined on crystal is in the range of 253.1–276.7 μΩ cm. The oxidation of Nb 5Sn 2Ga crystal starts at about 562°C. The final oxidation products were NbO 2, Nb 12O 29, Nb 2O 5, SnO 2 and Ga 2O 3.

The photocatalytic autoxidation of sulfur oxoanions by water-soluble porphyrin complexes

The photocatalytic autoxidation of S 4O 6 2− and S 2O 3 2− by water-soluble porphyrin complexes Fe(2-TMPyP), Mn(4-TMPyP), and Co(2-TMPyP) were compared under illumination with a 419 nm visible light in oxygen-saturated aqueous solutions at room temperature. The process involves S 2O 3 2− as an intermediate product and SO 4 2− is the final oxidation product. The trend in photocatalytic activity of the porphyrin complexes for S 4O 6 2− oxidation is Co(2-TMPyP)>Mn(4-TMPyP)>Fe(2-TMPyP). For S 2O 3 2− oxidation, the trend is Co(2-TMPyP)>Fe(2-TMPyP)>Mn(4-TMPyP).

Photooxidation of functionalized perfluorinated polyethers—II

Three functionalized perfluorinated polyethers (PFPE) were exposed to UV light ( λ >300 nm) in air, and chemical changes recorded by FTIR and 1H, 13C, 19F NMR spectrometry. One of the derivatives (ZTET), end-capped by an hydrogenated ether segment and containing four hydroxyl groups per molecule, was found to readily undergo photooxidation as shown by rapid development of carbonyl oxidation species and a corresponding decrease in C–H character. The carbonyl products were assigned to formate and ester groups respectively, which formation can be described by hydroperoxidation of the functionalized chain ends followed by their photolysis. The formate group is produced as a result of beta-scission of the secondary hydroperoxide γ to the perfluorinated chain and is further transformed to a hydroxymethyl chain end group (ZDOL) stabilised by the shielding effect of the adjacent perfluoromethylene group. The ester formation can be accounted for by oxidation of the methylene group α to the perfluorinated chain giving α carboxylic group which then reacts with ZDOL. The final oxidation product of ZTET is thus a polymer made of fluorinated polyether segments linked by an ester bond. The modification of ZTET by urethane or ester end-caps, in order to simulate crosslinks that are present in an eventual isocyanate-cured polyurethane or polyester coating, strongly increased the photostability of the compound. This marked increase in stability was attributed to both chemical and physical modification of the structure.

A DUAL, SEQUENTIAL MECHANISM FOR THE OXIDATION OF PETROLEUM ASPHALTS

ABSTRACT Examination of kinetic data for asphalt oxidative age hardening and the chemistry of the oxidation reactions has led to the proposal of a sequential, dual mechanism for asphalt oxidation. The dual mechanism rationalizes conflicts between results of earlier mechanistic investigations and provides a fundamental explanation for the hyperbolic-like, property-versus-time plots characteristic of asphalt oxidative aging. The oxidation kinetics are in harmony with and provide further confirmation of the microstructural model of asphalt. It is proposed that the rapid oxidation rate at the onset of asphalt oxidation results from reaction of oxygen with a limited amount of highly reactive hydrocarbon components. Final oxidation products of this initial reaction are sulfoxides and most likely aromatized rings. During this initial reaction, a much slower oxidation of benzytic carbons is initiated with both ketones and sulfoxides being formed. The ratio of ketones to sulfoxides formed and the rate of oxidative age hardening were found highly dependent on temperature and oxygen pressure (concentration). Significant differences were found in the oxidation and age hardening kinetics of pressure vessel aged asphalts and asphalts aged at atmospheric pressure. Atmospheric pressure oxidation was significantly more sensitive to variations in temperature and asphalt composition than was pressure vessel oxidation at 100°C.

Photooxidation of [(η 5-C 5H 5)Re(CO) 3], [(η 5-C 5Me 5)Re(CO) 3] and [(η 5-C 5H 5)V(CO) 4] in low-temperature matrices containing O 2

Photolysis of CpRe(CO) 3 or Cp*Re(CO) 3 (Cp=η 5C 5H 5, Cp*=η 5-C 5Me 5) in O 2-doped argon matrices yields in each case Re 2O 7 as the final binary oxide product. The reactions appear to proceed via the organometallic oxides RReO 3 R=Cp or Cp*. Photolysis of CpV(CO) 4 in O 2-doped argon matrices yields a species CpV(CO) 3(η 2-O 2) whose concentration is increased upon irradiation at 500 or 546 nm, but which is reconverted photochromically to the starting material upon irradiation at 436 nm. Continued photolysis generates unidentified oxo-vanadium species.

Characterisation of electrosynthetic l-dopa-melanin films by electrochemical and spectroelectrochemical techniques

Melanin was electrochemically synthesised by the oxidation of l-dopa in aqueous medium. The intermediates of the oxidation process were characterised by cyclic voltammetry and spectroelectrochemistry. The final product of melanin oxidation exhibited properties of a heterogeneous system containing a mixture of quinones and other oxidation intermediates of l-dopa. The charge transport properties of the redox polymeric compound was studied by chronocoulometric techniques using phosphate, perchlorate and sulphonate as electrolytes. The melanin electron transport coefficient (De1/2Ces) estimated for melanin in the various electrolytes are in good agreement with what is reported for other redox polymers. UV spectroelectrochemical reactivities of the melanogenesis process was used to identify some of the intermediate products. The excellent charge transport characteristics of melanin and its redox characteristics suggest that it could be used as an efficient immobilisation matrix for biomolecules in constructing reagentless amperometric biosensors.

Role of endothelins and nitric oxide in hepatic reperfusion injury in the rat.

We determined the functional role of nitric oxide (NO) and endothelins (ET), two potent vasoactive mediator systems in the liver, for the pathogenesis of sinusoidal perfusion failure and lethal hepatocyte injury after low-flow ischemia/reperfusion in the isolated perfused rat liver. NO synthase blockade with Nomega-nitro-L-arginine methyl ester (L-NAME) (10[-3] mol/L) before reperfusion prevented increased N02-/NO3- the final products of NO oxidation, which could be observed in the vehicle group. Epifluorescence microscopy revealed that the decrease in functional sinusoid density during reperfusion was much more profound compared with vehicle. This was associated with a lower surface PO2, a substantially higher number of nonviable hepatocytes, as assessed by in situ propidium iodide staining, and enhanced enzyme release into the perfusate compared with vehicle. In contrast, reperfusion in the presence of the endothelinA+B receptor antagonist bosentan (2 x 10(-4) mol/L) restored functional sinusoid density and surface PO2 to baseline values, resulted in a small reduction in the number of propidium iodide-positive hepatocytes, and caused similar increases in enzyme release as compared with vehicle. This indicates that hepatic generation of NO attenuates sinusoidal perfusion failure and improves liver tissue oxygenation, thus limiting hepatocyte injury during early reperfusion after hepatic low-flow ischemia. In contrast, endothelins counteract the microcirculatory effects of NO, i.e., mediate the no-reflow in hepatic sinusoids; however, the restoration of functional sinusoid density with bosentan resulted only in a small reduction in tissue damage, suggesting that additional components, which are independent of microcirculatory failure, contribute to hepatic reperfusion injury under these conditions.

DNA alkylation by 4,5-dioxovaleric acid, the final oxidation product of 5-aminolevulinic acid.

The heme precursor 5-aminolevulinic acid (ALA) accumulates under pathological conditions, namely, acute intermittent porphyria (AIP) and tyrosinosis, two diseases that are associated with increased liver cancer incidence. This has been previously linked to an enhanced production of reactive oxygen species generated by a metal-catalyzed ALA oxidation process, which was shown to cause DNA single-strand breaks and guanine oxidation within both isolated and cellular DNA. In the present work, we established that the final oxidation product of ALA, 4,5-dioxovaleric acid (DOVA), is an efficient alkylating agent of the guanine moieties within both nucleoside and isolated DNA. Adducts were produced through the formation of a Schiff base involving the N2-amino group of 2'-deoxyguanosine (dGuo) and the ketone function of DOVA, respectively. The modified dGuo nucleosides were characterized, following reduction into stable secondary amines, by extensive NMR, infrared, and mass spectrometry analyses. A method, based on the use of HPLC with electrochemical detection, was then developed for the sensitive measurement of the DOVA-dGuo adducts. Using this assay, we showed that the guanine moieties of isolated DNA can undergo the same reaction as the free nucleoside. The present data provide additional information on the genotoxic potential of ALA and reinforce the hypothesis that AIP may be involved in the induction of primary liver cell carcinoma.

Asphalt Aging: Dual Oxidation Mechanism and Its Interrelationships with Asphalt Composition and Oxidative Age Hardening

The kinetic data and chemistry of asphalt oxidative age hardening suggested a sequential, dual mechanism for asphalt oxidation. The dual mechanism rationalizes conflicts between earlier mechanistic investigations and explains the hyperbolic-like, time-versus-property plots characteristic of asphalt oxidative aging. The oxidation kinetics provide further confirmation of the asphalt microstructural model. It is proposed that the rapid initial oxidation rate of asphalt results from reaction of oxygen with limited amounts of highly reactive hydrocarbons. Final oxidation products of this initial reaction are sulfoxides and, most likely, ring aromatization. During this initial reaction, a slower oxidation reaction of asphalt benzylic carbons is initiated; final products are ketones and sulfoxides. The ratio of ketones to sulfoxides formed and the rate of age hardening were found to be dependent on temperature and oxygen pressure. Low-temperature oxidative aging, as occurs in pavements, was found significantly more sensitive to variations in temperature and asphalt composition than 100°C pressure vessel aging.

Microstructural Investigation of the Oxide Scale on Zr-2.5NB and its Interface with the Alloy Substrate

AbstractOxidized Zr-2.5Nb is being developed as an articular bearing surface for the femoral component in total joint arthroplasty. It has so far demonstrated superior wear performance against ultrahigh molecular weight polyethylene (UHMWPE) with respect to traditional articulating materials such as Co-Cr-Mo alloys. In this investigation, we used thermogravimetric analysis, transmission electron microscopy, and in situ x-ray diffraction techniques to study the microstructure and stress state of the oxide scale grown on Zr-2.5Nb.The oxidation temperature not only determines the kinetics of oxidation but the morphology of the various oxidation products. We have identified the oxidation products of both phases of the two phase alloy and correlated them with the original alloy microstructure. These include not only monoclinic zirconia but also small amounts of tetragonal zirconia and a mixed oxide phase combining both zirconium and niobium. The alloy microstructure both influences the final oxidation products and is reflected in the microstructure of the oxide. The oxide scale itself has a predominantly columnar microstructure which extends from the oxide/metal interface to the outer surface of the oxide. In situ x-ray diffraction measurements revealed that the oxide scale is stressed in compression following cooling and exhibits strong crystallographic texture. The oxide/metal interface is continuous, without pores or voids which might be detrimental to oxide adhesion. In addition, we have identified a phase which develops at the interface between the beta-zirconium grains and the oxide. We have also identified amorphous regions within the oxide scale which serve as sinks for silicon and other impurity elements found in the alloy.