Hydroxyl radical formation by O-O bond homolysis in peroxynitrous acid

Peroxynitrite decay in weakly alkaline media occurs by two concurrent sets of pathways which are distinguished by their reaction products. One set leads to net isomerization to NO(3)(-) and the other set to net decomposition to O(2) plus NO(2)(-). At sufficiently high peroxynitrite concentrations, the decay half-time becomes concentration-independent and approaches a limiting value predicted by a mechanism in which reaction is initiated by unimolecular homolysis of the peroxo O-O bond, i.e., the following reaction: ONOOH --> (*)OH + (*)NO(2). This dynamical behavior excludes alternative postulated mechanisms that ascribe decomposition to bond rearrangement within bimolecular adducts. Nitrate and nitrite product distributions measured at very low peroxynitrite concentrations also correspond to predictions of the homolysis model, contrary to a recent report from another laboratory. Additionally, (1) the rate constant for the reaction ONOO(-) --> (*)NO + (*)O(2)(-), which is critical to the kinetic model, has been confirmed, (2) the apparent volume of activation for ONOOH decay (DeltaV() = 9.7 +/- 1.4 cm(3)/mol) has been shown to be independent of the concentration of added nitrite and identical to most other reported values, and (3) complex patterns of inhibition of O(2) formation by radical scavengers, which are impossible to rationalize by alternative proposed reaction schemes, are shown to be quantitatively in accord with the homolysis model. These observations resolve major disputes over experimental data existing in the literature; despite extensive investigation of these reactions, no verifiable experimental evidence has been advanced that contradicts the homolysis model.     

Hydroxyl radical induced from hydrogen peroxide by cobalt manganese oxides for ciprofloxacin degradation

Advanced oxidation processes (AOPs) are promising technology to remove organic pollutant in water. However, the main problem in the AOPs is the low generation of hydroxyl radical (^?OH) owing to the low decomposition efficiency of hydrogen peroxide (H₂O₂). Herein, the spinel type cobalt acid manganese (MnCo₂O₄) with flower morphology was fabricated through a co-precipitation method. In situ Fourier transform infrared spectroscopy confirms that the MnCo₂O₄ with the optimal molar ratio of Co and Mn precursors (CM3, Co:Mn = 3) has more Lewis acid sites compared with single metal oxide catalysts (Co₃O₄ and Mn₂O₃), leading to the excellent performances for H₂O₂ decomposition rate constant on CM3, which is about 15.03 and 4.21 times higher than those of Co₃O₄ and Mn₂O₃, respectively. As a result, the obtained CM3 shows a higher ciprofloxacin degradation ratio than that of Co₃O₄ and Mn₂O₃. Furthermore, CM3 shows an excellent stability during several cycles. This work proposes effective catalysts for ciprofloxacin decomposition and provides feasible route for treating practical environmental problems.     

Separation of niobium and tantalum with phenylarsonic acid

Phenylarsonic acid permits satisfactory separation of niobium and tantalum and estimation of tantalum from an oxalate solution containing sulphuric acid up to pH 5.8. For complete precipitation of niobium the pH should exceed 4.8. In mixtures, tantalum is precipitated below pH 3.0 and niobium is then precipitated above pH 5.0. When the oxalate concentration is high, recovery of niobium with cupferron is recommended. When the ratio of Nb₂O₅, to Ta₂O₅ exceeds 2:1, reprecipitation of tantalum is necessary. The effect of interfering ions is studied.     

Hydroxyl radical at the air-water interface

Interaction of the hydroxyl radical with the liquid water surface was studied using classical molecular dynamics computer simulations. From a series of scattering trajectories, the thermal and mass accommodation coefficients of OH on liquid water at 300 K were determined to be 0.95 and 0.83, respectively. The calculated free energy profile for transfer of OH across the air-water interface at 300 K exhibits a minimum in the interfacial region, with the free energy of adsorbtion (DeltaGa) being about 1 kcal/mol more negative than the hydration free energy (DeltaGs). The propensity of the hydroxyl radical for the air-water interface manifests itself in partitioning of OH radicals between the bulk water and the surface. The enhancement of the surface concentration of OH relative to its concentration in the aqueous phase suggests that important OH chemistry may be occurring in the interfacial layer of water droplets, aqueous aerosol particles, and thin water films adsorbed on solid surfaces. This has profound consequences for modeling heterogeneous atmospheric chemical processes.     

Diastereoselective radical mediated alkylation of a chiral glycolic acid derivative

Radical alkylation of 2-(tert-butyl)-2-methyldioxolan-4-one, a chiral equivalent of glycolic acid, occurs with good to high diastereoselectivity that compares favorably with the corresponding enolate alkylation. The importance of the position of the transition state position, early or late, is highlighted.     

Hydroxyl radical in aqueous solution: Computer simulation

Molecular dynamics simulations of hydroxyl radical in water are carried out by use of a classical simple point charge extended (SPC/E) water model and a similar point charge model for hydroxyl radical. Structural and dynamical properties are studied along the coexistence curve of SPC/E water at 298, 373, 473, 573, and 633 K and above its critical point at 683, 733, 783, and 833 K with density fixed at 0.3 g/cm3. Dramatic changes in the diffusion dynamics of water and hydroxyl radical near the critical point are related to the reorganization of the three-dimensional structure of water around hydroxyl radical, as revealed by the study of the spatial distribution functions. This study helps us understand the kinetics of oxidation reactions in high-temperature water.     

Hydroxyl radical,sulfate radical and nitrate radical reactivity towards crown ethers in aqueous solutions

Reaction rate constants of crown ethers (12-crown-4, 15-crown-5, 18-crown-6) and their analogs 1,4-dioxane (6C2) with some important oxidative radicals, hydroxyl radical (OH), sulfate radical (SO₄^?) and nitrate radical (NO₃), were determined in various aqueous solutions by pulse radiolysis and laser photolysis techniques. The reaction rate constants for 6C2 and crown ethers with OH and SO₄^? increase with the number of hydrogen atoms in the ethers, indicating that the hydrogen-atom abstraction is a dominant reaction between crown ethers and these two radicals. The presence of cations in solution has negligible effect on the rate constants of crown ether towards OH and SO₄^?. However, for the NO₃, the rate constants are not proportional to the number of hydrogen atoms in ethers, and 12-crown-4 (12C4) is the most reactive compared with other crown ethers. Except 12C4 and 6C2, the cations in the aqueous solution affect the reactivities of 15-crown-5 (15C5) and 18-crown-6 (18C6). The cations with high binding stability for crown ether would improve the reactivity of 15C5. For the studied crown ethers, the reaction rate constants of these oxidative radicals have the order OH>SO₄^?>NO₃. Furthermore, the formation of radicals after the reaction of crown ethers with sulfate radical could be observed in the range of 260–280 nm using laser photolysis and pulse radiolysis. This is the first report on the kinetic behavior of crown ethers with NO₃, and it would be helpful for the understanding of stability of crown ethers in the processing of spent nuclear fuel.     

Determination of diphenylarsinic acid and phenylarsonic acid,the degradation products of organoarsenic chemical warfare agents,in well water by HPLC–ICP‐MS

Diphenylarsinic acid (DPAA) and phenylarsonic acid (PAA), which were degradation products of organoarsenic chemical warfare agents used as sternutatory gas, were detected in the well water at Kamisu, Ibaraki Prefecture, Japan. The standard material of DPAA was synthesized with aqueous arsenic acid and phenylhydrazine in order to determine organic arsenic compounds in well water. The DPAA showed a protonated ion at m/z 263 [M + H]⁺ and a loss of H₂O ion at m/z 245 [M + H ? H₂O]⁺ from protonated ion by the electrospray ionization time‐of‐flight mass spectrometry. The quantitative analysis of DPAA and PAA was performed by high‐performance liquid chromatography inductively coupled plasma mass spectrometry and the system worked well for limpid liquid samples such as well water. Copyright © 2005 John Wiley & Sons, Ltd.     

Inhibitor mediated protein degradation

The discovery of drugs that cause the degradation of their target proteins has been largely serendipitous. Here we report that the tert-butyl carbamate-protected arginine (Boc(3)Arg) moiety provides a general strategy for the design of degradation-inducing inhibitors. The covalent inactivators ethacrynic acid and thiobenzofurazan cause the specific degradation of glutathione-S-transferase when linked to Boc(3)Arg. Similarly, the degradation of dihydrofolate reductase is induced when cells are treated with the noncovalent inhibitor trimethoprim linked to Boc(3)Arg. Degradation is rapid and robust, with 30%-80% of these abundant target proteins consumed within 1.3-5 hr. The proteasome is required for Boc(3)Arg-mediated degradation, but ATP is not necessary and the ubiquitin pathways do not appear to be involved. These results suggest that the Boc(3)Arg moiety may provide a general strategy to construct inhibitors that induce targeted protein degradation.     

The isomerization of methoxy radical: intramolecular hydrogen atom transfer mediated through acid catalysis

The catalytic ability of water, formic acid, and sulfuric acid to facilitate the isomerization of the CH(3)O radical to CH(2)OH has been studied. It is shown that the activation energies for isomerization are 30.2, 25.7, 4.2, and 2.3 kcal mol(-1), respectively, when the reaction is carried out in isolation and with water, formic acid, or sulfuric acid as a catalyst. The formation of a doubly hydrogen bonded transition state is central to lowering the activation energy and facilitating the intramolecular hydrogen atom transfer that is required for isomerization. The changes in the rate constant for the CH(3)O-to-CH(2)OH isomerization with acid catalysis have also been calculated at 298 K. The largest enhancement in the rate, by over 12 orders of magnitude, is with sulfuric acid. The results of the present study demonstrate the feasibility of acid catalysis of a gas-phase radical isomerization reaction that would otherwise be forbidden.     

Hydroxyl radical measurements in a photochemical reactor by laser-induced fluorescence

Measurements of hydroxyl radical (HO) concentrations in ambient air by the technique of laser-induced fluorescence have been recently reported. The present study was undertaken to provide an independent test of the validity of those measurements. A photochemical reactor was used to provide a source of HO, and the concentration of HO in the reactor was determined by the laser-induced fluorescence technique. The HO concentration was also deduced from measured hydrocarbon decay rates in the reactor. There was agreement between the HO concentrations obtained by these two different methods, thus providing further validation of the fluorescence method. Some studies of HO fluorescence efficiency as well as of possible interferences with the fluorescence measurements are reported.     

Hydroxyl radical reactivity with nitrobenzene in subcritical and supercritical water

The bimolecular rate constants of the addition reaction between hydroxyl radical (*OH) and nitrobenzene (C(6)H(5)NO(2)) were measured in subcritical and supercritical water (SCW) at temperatures between ambient and 390 degrees C. The measured bimolecular rate constants showed distinctly non-Arrhenius behavior (i.e., essentially no increase with temperature) from ambient to 350 degrees C, but increased in the slightly subcritical and supercritical region between 350 and 390 degrees C. These data were modeled reasonably well over the entire temperature range with a three-step reaction mechanism, originally proposed by Ashton et al.(1) This model includes the formation of a pi-complex intermediate as the precursor of the nitrohydroxycyclohexadienyl radical.     

Cholesterol degradation in archaeological pottery mediated by fired clay and fatty acid pro-oxidants

Cholesterol is generally absent in animal fat residues preserved in archaeological ceramic vessels. It is known from edible oil refining that during bleaching with activated clay sterols are degraded, largely via oxidation. Laboratory heating experiments using fired clay from replica pottery vessels promoted rapid degradation of cholesterol via oxidation. Furthermore, heating cholesterol with fatty acids (saturated and unsaturated) revealed additional degradation to occur independently of the ceramic matrix. As both conditions are met in archaeological pottery during animal (and plant) product processing involving heating, the very rare detection of sterols in organic residues can be explained.     

Complexes of diphenylarsinic acid and phenylarsonic acid with thiols: a 1H and 13C NMR study

The high toxicity of diphenylarsinic acid, found in ground water and well water as a probable consequence of the inappropriate disposal of warfare agents, prompted us to study the reaction, monitored by 1H and 13C NMR spectroscopy, of the compound and its monophenyl analogue, phenylarsonic acid, with cellular thiols as represented, in particular, by glutathione. Glutathione reduced the phenylarsenic acids to trivalent forms and complexed them: diphenylarsinic acid to a monoglutathione adduct and phenylarsonic acid to a diglutathione adduct. The complexes were characterized by 1H and 13C NMR spectroscopy and mass spectrometry. The NMR spectra showed the diastereotopic nature of the two phenyl groups in the diphenylarsinic acid-glutathione compound, and of the two glutathione residues in the phenylarsonic acid-diglutathione complex. The stereochemistry of thiol compounds of phenylarsonic acid was further explored by 1H and 13C NMR spectroscopy of the L-cysteine complex. The diphenylarsinic acid-glutathione complex was stable below pH 12 but at higher pH the complex dissociated into diphenylarsinous acid and reduced glutathione. The released diphenylarsinous acid then oxidized to diphenylarsinic acid with a half-life of about 7 h at pH 13 and at room temperature.     

Degradation of phenylarsonic acid and its derivatives into arsenate by hydrothermal treatment and photocatalytic reaction

The degradation of phenylarsonic acid (PA) and its derivatives by hydrothermal treatment (HTT) was examined, especially focusing on the effect of adding H₂O₂ upon the degradation efficiency. The degradation was assessed by the generation of arsenate resulting from the cleavage of As C bonds in the PA derivatives. When PA (without substituents) was subjected to an HTT with H₂O₂ (H₂O₂‐HTT; 0.5–1% H₂O₂) at 175–200 °C, PA was almost completely degraded into arsenate, whereas an HTT with NaOH (NaOH‐HTT; 3 M NaOH) at the temperatures provided almost no degradation. The H₂O₂‐HTT also worked well for the degradation of PA derivatives with hydroxy and/or nitro groups on the phenyl ring. However, the degradation of aminophenylarsonic acids was not favorably performed by the H₂O₂‐HTT. The effect of the structure of PA derivatives upon the degradation susceptibility was discussed. A photocatalytic reaction using TiO₂ was also attempted for the degradation of PA derivatives. Copyright © 2005 John Wiley & Sons, Ltd.