Oxidation Of Bromide Research Articles

Evidence for the presence of a CmuA methyltransferase pathway in novel marine methyl halide‐oxidizing bacteria

Marine bacteria that oxidized methyl bromide and methyl chloride were enriched and isolated from seawater samples. Six methyl halide-oxidizing enrichments were established from which 13 isolates that grew on methyl bromide and methyl chloride as sole sources of carbon and energy were isolated and maintained. All isolates belonged to three different clades in the Roseobacter group of the alpha subdivision of the Proteobacteria and were distinct from Leisingera methylohalidivorans, the only other identified marine bacterium that grows on methyl bromide as sole source of carbon and energy. Genes encoding the methyltransferase/corrinoid-binding protein CmuA, which is responsible for the initial step of methyl chloride oxidation in terrestrial methyl halide-oxidizing bacteria, were detected in enrichments and some of the novel marine strains. Gene clusters containing cmuA and other genes implicated in the metabolism of methyl halides were cloned from two of the isolates. Expression of CmuA during growth on methyl halides was demonstrated by analysis of polypeptides expressed during growth on methyl halides by SDS-PAGE and mass spectrometry in two isolates representing two of the three clades. These findings indicate that certain marine methyl halide degrading bacteria from the Roseobacter group contain a methyltransferase pathway for oxidation of methyl bromide that may be similar to that responsible for methyl chloride oxidation in Methylobacterium chloromethanicum. This pathway therefore potentially contributes to cycling of methyl halides in both terrestrial and marine environments.

The bromine electrode Part III: reaction kinetics at highly boron-doped diamond electrodes

Bromide oxidation and bromine reduction were investigated at boron-doped diamond (BDD) electrodes, in acidic media. Both the anodic and the cathodic reactions of the bromine redox couple were found to take place through a mechanism in which the Volmer step is rate-determining, as a result of a very poor stabilization of intermediate radical species. Accordingly, exchange current densities at BDD and polycrystalline Pt differ by more than five orders of magnitude. Finally, from the analysis of CV data, estimations of the anodic and cathodic transfer coefficients, as well as of heterogeneous rate constants, were obtained.

The bromine electrode Part II: reaction kinetics at polycrystalline Pt

The kinetics of bromide oxidation and Br3− reduction were studied at polycrystalline Pt electrodes in acidic media. The electrochemical behavior of equimolar Br3− and Br− solutions was investigated, in a concentration range of the electroactive species between 0.1 and 1.0 M. E–log j plots did not exhibit linear segments, most probably because of extensive adsorption of bromine radicals. Further analysis supported the hypothesis of a Volmer–Tafel mechanism, with the chemical recombination step as rate determining. Electrosorption isotherms for Br radicals were found to be of the Frumkin type. The kinetics of Br3− reduction was controlled by the surface dissociation of the Br2 molecules.

Synthetic and computational modeling of the vanadium-dependent haloperoxidases

Abstract Vanadium-dependent haloperoxidases that catalyze the halogenation of organic substrates using hydrogen peroxide to oxidize halides are a rare class of enzymes which have an absolute requirement for vanadium. In this article, we describe studies using synthetic, small-molecule analogs of the vanadium(V) active site to functionally mimic the oxidation of bromide and thioethers. In addition, we describe computational studies using density functional theory that help describe the mechanism of catalysis.

The (Schiff Base)vanadium(V) Complex Catalyzed Oxidation of Bromide — A New Method for the in situ Generation of Bromine and Its Application in the Synthesis of Functionalized Cyclic Ethers.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A mechanistic study of the electro-oxidation of bromide in acetonitrile and the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide at platinum electrodes

The oxidation of bromide has been investigated by linear sweep and cyclic voltammetry at platinum electrodes in the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide, ([C 4mim][NTf 2]), and the conventional aprotic solvent, acetonitrile, (MeCN). Similar voltammetry was observed in both solvents, despite their viscosities differing by more than an order of magnitude. DigiSim ® was employed to simulate the voltammetric response. The mechanism is believed to involve the direct oxidation of bromide to bromine in a heterogeneous step, followed by a homogenous reaction to form the tribromide anion: 2 Br - → Br 2 + 2 e - Br 2 + Br - ⇌ k b k f Br 3 - K eq Athigher potentials, the tribromide anion dissociates to bromine, Br 2, and bromide, Br −, which is immediately oxidised, leading to the emergence of a second anodic wave. Irreversible electrode kinetics were inferred from Tafel analysis and removal of the first electron is believed to be the rate-determining step in the formation of bromine. The equilibrium constant, K eq, where K eq = [ Br 3 - ] [ Br 2 ] [ Br - ] was found to be 3 × 10 3 and 9 × 10 6 M −1 in the ionic liquid and in acetonitrile, respectively. An equilibrium constant, K complex for the complexation of 1-ethyl-3-methylimidazolium bromide in acetonitrile was found to be 35.1 M −1.

Photoinduced halophenol formation in the presence of iron(III) species or cadmium sulfide

Phenol photo-induced decomposition has been investigated in the presence of iron(III) oxide or iron(III) ions at different pH, paying particular attention to the evolution of intermediates. The addition of halides causes a modest decrease in phenol degradation rate and starts additional transformation pathways. Chloride induces the formation of chlorinated compounds (2- and 4-chlorophenol), whose concentrations are strongly dependent on pH. Analogously, bromide ions lead to the formation of bromophenols. Similar investigations have been performed by adopting cadmium sulfide as a photocatalyst, whose valence band edge potential ( E VB°) allows the oxidation of bromide but not chloride. The addition of chloride does not lead to chlorophenols, while bromide causes the formation of bromophenols, thus confirming the possibility of obtaining selective abiotic halogenation of phenol.

Determination of bromide ions in seawater using flow system with chemiluminescence detection

A simple flow-based procedure with chemiluminescence (CL) detection is proposed for bromide ion determination in seawater. The procedure was based on the oxidation of bromide to bromine by chloramine-T followed by the reaction of bromine with luminol resulting in CL emission. Since no significant reaction within chloramine-T and luminol was observed, the detection was carried out without bromine extraction from the oxidant medium. The proposed flow system had a sampling rate of 40 determinations per hour, reagents consumption of 100 μg luminol and 60 μg chloramine-T per determination, a limit of detection of 0.5 mg l −1 bromide ions, a linear concentration range ( r = 0.999 and n = 7) between 0 and 100 mg l −1, and a coefficient of variance better than 2.5% (for 10 measurements of a 10 mg l −1 Br − solution) were achieved. The analytical system was applied for the determination of bromide in seawater and estuarine-water samples, obtaining an analyte recovery ranging from 94 to 102% and comparing the results with a reference spectrophotometric method no significant difference was observed in 95% confidence level.

The (Schiff base)vanadium(V) Complex Catalyzed Oxidation of Bromide − A New Method for the in situ Generation of Bromine and Its Application in the Synthesis of Functionalized Cyclic Ethers

Abstract(Schiff base)vanadium(V) complexes 5 with tridentate imine auxiliaries served as catalysts for the oxidation of Br− with tert‐butyl hydroperoxide (TBHP) in nonaqueous solvents. This reaction has been applied for the conversion of substituted 4‐penten‐1‐ols into 5‐exo‐bromo‐cyclized products, including a diastereomerically pure heterocyclic precursor used in a synthesis of the all‐trans‐configured 2,3,4,5‐substituted tetrahydrofuran 2‐epi‐magnosalicin. Treatment of ψ‐substituted bis(homoallylic) alcohols with the reagent combination of pyHBr, TBHP, and a vanadium(V) catalyst 5 afforded 6‐endo‐cyclized products, i.e. brominated tetrahydropyrans, as major compounds. The results from 51V NMR, ESI‐MS, and supporting reactivity‐selectivity studies indicated that the mechanism of the new bromination reaction consists of vanadium‐dependent and vanadium‐independent steps. A (Schiff base)vanadium(V) compound 5 is required for activation of TBHP via in situ formation of the corresponding tert‐butylperoxy complex. This reagent oxidizes Br−, which under the reaction conditions provides Br2 as the active brominating reagent. The molecular bromine generated thus is released into the solution at a steady rate and serves as a reagent for the synthesis of β‐brominated cyclic ethers from bis(homoallylic) alcohols in a second, vanadium‐independent step. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Minimizing Bromate Concentration by Controlling the Ozone Reaction Time in a Full-Scale Plant

ABSTRACT The latest European Directive 98/83/CE (5 December 1998), concerning the quality of water intended for human consumption, has set a two-stage parametric value for bromate. Bromate concentration will comply with 25 μg/L after December 25, 2003, and with 10 μg/L after December 25, 2008. Bromate formation in water is generally due to bromide oxidation during the ozonation stage. Due to higher temperatures, this latter parametric value is often exceeded in summer. Minimizing bromate levels is thus a crucial problem for drinking water producers. A bromate-minimizing strategy consists of shortening the reaction time between ozone and water. This can be done by neutralizing dissolved ozone residual with bisulfite at the exit of the ozone reactor chamber and/or by managing the introduction of ozone in different chambers depending on the water flow rate. This is only possible if, in our case, the disinfection goal for ozone is respected toward bacteria and viruses. The CT value must comply with 1.6 mg/min/L. In our plants, this value could be very large due to high contact time in and after leaving the ozone reactors.

Oxidation of Benzylic Bromides by DMSO in the Presence of Zinc Salts: A New Variant of Kornblum′s Method.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The Complex Synergy of Water in the Metal/Bromide Autoxidation of Hydrocarbons Caused by Benzylic Bromide Formation

AbstractOne of the most active and selective catalysts in homogeneous liquid phase oxidation using molecular oxygen (O2) is a mixture of cobalt, manganese and bromide salts in acetic acid. It has been used to produce hundreds of different carboxylic acids in high yield and purity including the commercial production of terephthalic acid from p‐xylene. Water is normally a by‐product in these reactions and it is shown here that its concentration is an important reaction variable. In anhydrous acetic acid, with reagents with sufficiently strong electron‐withdrawing substitutents (toluene, 4‐carboxytoluene, 4‐chlorotoluene), all of the active bromide becomes inactive via benzylic bromide formation. The Co/Mn/Br catalyst is therefore converted to a Co/Mn catalyst which is dubbed ‘catalyst failure’ because of its undesirable characteristics of lower activity, decreased selectivity especially towards over‐oxidation and color formation. For 4‐chlorotoluene, increasing the water concentration to 5 weight % initially decreases the rate of reaction but eventually is more active and selective because the oxidation and hydrolysis of the benzylic bromide allows for sufficient active catalytic bromide. It is shown that benzylic bromides do not ‘promote’ the reaction and that both oxidation and solvolysis of the benzylic bromide occurs during autoxidation. During polymethylbenzene oxidation, benzylic bromide formation occurs only with the most reactive methyl group. The complex factors during metal/bromide autoxidation – some favored by increased water concentration and others detrimental – are outlined.

Hydroperoxyl radical (HO2•) oxidizes dibromide radical anion (•Br2−) to bromine (Br2) in aqueous solution: Implications for the formation of Br2 in the marine boundary layer

The release of photoactive halogen species such as Br2 from sea‐salt particles, snowpack, and sea‐ice can have significant effects on chemistry in the marine boundary layer (MBL). Although the reaction of hydroperoxyl radical (HO2•) with dibromide radical anion (•Br2−) might be a key step in the formation and release of Br2, there is currently no consensus on whether this reaction produces bromide (Br−) or molecular bromine (Br2). To address this question, we measured the formation of gaseous and aqueous oxidized bromide (primarily Br2) in illuminated bromide solutions as a function of pH. Results from these two sets of experimental data are best explained by kinetic models where HO2• oxidizes •Br2− to Br2. Using this reaction in a simple aerosol model reveals that the hydroxyl radical‐induced oxidation of particulate bromide, followed by reaction of •Br2− with HO2•, could be an important source of Br2 in the MBL.

Kinetic characterization of active site mutants Ser402Ala and Phe397His of vanadium chloroperoxidase from the fungus Curvularia inaequalis

Site-directed mutagenesis was performed on two conserved active site residues of vanadium chloroperoxidase (VCPO) from the fungus Curvularia inaequalis. Mutation of an active site residue Ser402 to Ala (S402A), a residue proposed to be brominated during turnover, caused a decrease in its activity, however it still catalyses efficiently the oxidation of both chloride and bromide. The K m values for chloride and bromide of S402A at the optimal pH 4.5 were 3.2 mM and 20 μM, respectively. The active site residues of VCPO and vanadium bromoperoxidases (VBPO) from the seaweeds Ascophyllum nodosum and Corallina officinalis show very high similarity. A prominent difference in the active site architecture of VCPO and VBPO is the presence of a second histidine in VBPO, a residue substituted by a phenylalanine in VCPO. The mutation of Phe397 to His (F397H) resulted in the enhancement of bromination activity under certain conditions. However, inactivation of F397H by halide especially at low pH was observed during turnover. Kinetic parameters and characteristics of these mutants are discussed in this report. A detailed kinetic analysis of the pH dependence of the chlorinating and brominating activity of VCPO and the S402A yielded an interesting difference between the two activities. The results show that the K m for Cl − was pH dependent whereas the K m for Br − was hardly pH dependent. The data confirm that protonation of an active site residue or the bound peroxide is essential for chloride oxidation.

Substituent Effects in Arylseleninic Acid-Catalyzed Bromination of Organic Substrates with Sodium Bromide and Hydrogen Peroxide

Arylseleninic acids were found to be catalysts for the oxidation of bromide with hydrogen peroxide in a two-phase mixture of ether and pH 6 phosphate buffer. Benzeneseleninic acid (3) and 4-methoxyphenylseleninic acid (4) were more efficient catalysts than 3,5-bis(trifluoromethyl)phenylseleninic acid (1), 4-nitrophenylseleninic acid (2), 4-dimethylaminophenylseleninic acid (5), and 2,4,6-trimethylphenylseleninic acid (6). A variety of organic substrates were efficiently brominated under these conditions using 5−10 mol % of commercially available benzeneseleninic acid (3).

A new synthesis of the 2,2,3,5,6,6-substituted tetrahydropyran aplysiapyranoid A and its 5-epimer

The vanadium(V)-catalyzed oxidation of bromide in the presence of methyl ( E)-2-(1-hydroxy-1-methylethyl)-5-phenyl-4-hexenoate furnished 5,6- trans-5-bromo-6-phenyl-2,2,6-trimethyl-3-methyloxycarbonyltetrahydropyran, which was converted into the marine natural product aplysiapyranoid A and its 5-epimer, via a short sequence of decarboxylative bromination and transition metal-based procedures for transforming a phenyl into a chlorovinyl substituent.

Bromide oxidation and bromine reduction in propylene carbonate

Bromide oxidation and bromine reduction in propylene carbonate (PC) +1 M LiClO 4 was investigated voltammetrically using polycrystalline platinum rotating disc electrode. The corresponding voltammograms were compared to those of iodide oxidation and HCl decomposition. The oxidation 2Br −→Br 2+2e − was found to proceed through two steps, which are caused by the formation of a stable intermediate Br 3 − ion. The stability constant of tri-bromine ion was estimated to be 10 5.5 M. The diffusion coefficient of Br − ion in this solution was determined to be 3.41×10 −6 cm 2 s −1. In the case of bromine reduction, a time dependence of the shape of the voltammogram was observed, which showed that one is dealing with a direct bromination of PC.

An Exceptionally Simple and Catalytic Method for Regeneration of Carbonyl Functionality from the Corresponding 1,3‐Oxathiolanes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Increased Affinity and Stability of an Anti-HIV-1 Envelope Immunotoxin by Structure-based Mutagenesis

HIV-infected cells are selectively killed by an immunotoxin in which a truncated form of Pseudomonas exotoxin A is joined to the variable region of a broadly neutralizing antibody (3B3) that recognizes the viral envelope glycoprotein (Env). To improve the efficacy of this molecule, we used three-dimensional structural information and phage selection data to design 23 single and multiple point mutations in the antibody variable region sequences that contact Env. Substituting an aromatic residue for an aspartate in the third complementarity-determining region of V(H) increased the potency of the immunotoxin by approximately 10-fold in a cell-killing assay. Detailed analysis of one such mutant, N31H/Q100eY, revealed both a higher affinity for monomeric and cell surface Env and an increased stability against aggregation compared with the starting immunotoxin. Conversion to a disulfide-linked two-chain format further stabilized the protein. N31H/Q100eY retained the ability to bind to Env from multiple viral isolates, to inhibit Env-mediated cell fusion, and to limit spreading viral infection in peripheral blood mononuclear cells. Such site-directed mutants may increase the utility of immunotoxins for reducing or eradicating persistent HIV-1 infection in humans.

An exceptionally simple and catalytic method for regeneration of carbonyl functionality from the corresponding 1,3-oxathiolanes

A wide variety of 1,3-oxathiolanes 1 can be chemoselectively deprotected to the corresponding carbonyl compounds 2 in good yields by employing V2O5–H2O2 catalyzed oxidation of ammonium bromide in a CH2Cl2–H2O mixture at 0–5 °C. Some of the major advantages of this procedure are its mild conditions, and that it is highly selective and efficient, high yielding, and cost-effective. Furthermore, no brominations occur at the double bond or allylic position or α to the keto position or aromatic ring.