Vanadate Anion Research Articles

Oxidation of 2-Propanol and Cyclohexane by the Reagent “Hydrogen Peroxide−Vanadate Anion−Pyrazine-2-carboxylic Acid”: Kinetics and Mechanism

The vanadate anion in the presence of pyrazine-2-carboxylic acid (PCA [identical with] pcaH) efficiently catalyzes the oxidation of 2-propanol by hydrogen peroxide to give acetone. UV-vis spectroscopic monitoring of the reaction as well as the kinetics lead to the conclusion that the crucial step of the process is the monomolecular decomposition of a diperoxovanadium(V) complex containing the pca ligand to afford the peroxyl radical, HOO(.-) and a V(IV) derivative. The rate-limiting step in the overall process may not be this (rapid) decomposition itself but (prior to this step) the slow hydrogen transfer from a coordinated H2O2 molecule to the oxygen atom of a pca ligand at the vanadium center: "(pca)(O=)V...O2H2" --> "(pca)(HO-)V-OOH". The V(IV) derivative reacts with a new hydrogen peroxide molecule to generate the hydroxyl radical ("V(IV)" + H2O2 --> "V(V)" + HO(-) + HO(.-)), active in the activation of isopropanol: HO(.-) + Me2CH(OH) --> H2O + Me2C(.-)(OH). The reaction with an alkane, RH, in acetonitrile proceeds analogously, and in this case the hydroxyl radical abstracts a hydrogen atom from the alkane: HO(.-) + RH --> H2O + R(.-). These conclusions are in a good agreement with the results obtained by Bell and co-workers (Khaliullin, R. Z.; Bell, A. T.; Head-Gordon, M. J. Phys. Chem. B 2005, 109, 17984-17992) who recently carried out a density functional theory study of the mechanism of radical generation in the reagent under discussion in acetonitrile.

Selective Sorption of Metal Oxoanions from Dilute Solution by Bead Cellulose Sorbent

Bead sorbent Perloza MT 50 was used for selective removal of metal W, Mo, V, Ge, and Sb oxoanions. All experiments were carried out by dynamic column sorption. Sorption of tungstate and molybdate anions was successful. The sorption capacity decreased with increasing concentration of accompanying anions (chlorides, sulphates) and with increasing pH (3.5–5.5). Sorption of vanadate anion was possible but the sorption capacity was very low. Sorption of Ge(IV) and Sb(III) oxoanion was negligible.

Anion Binding Tripodal Receptors as Structural Models for the Active Site of Vanadium Haloperoxidases and Acid Phosphatases

In an attempt to mimic the active sites of the anion-binding enzymes vanadium haloperoxidase and acid phosphatase, two tripodal receptors have been shown to bind phosphate and vanadate anions in organic solvents through H-bonding interactions.

The Effects of Drug Complexation on the Stability and Conformation of Human Serum Albumin: Protein Unfolding

We report different analytical methods used to study the effects of 3\'-azido-3\'-deoxythymidine, aspirin, taxol, cisplatin, atrazine, 2,4-dichlorophenoxyacetic, biogenic polyamines, chlorophyll, chlorophyllin, poly(ethylene glycol), vanadyl cation, vanadate anion, cobalt-hexamine cation, and As2O3, on the stability and secondary structure of human serum albumin (HSA) in aqueous solution, using capillary electrophoresis, Fourier transform infrared, ultraviolet visible, and circular dichroism (CD) spectroscopic methods. The concentrations of HSA used were 4% to 2% or 0.6 to 0.3 mM, while different ligand concentrations were 1 microM to 1 mM. Structural data showed drugs are mostly located along the polypeptide chains with both specific and nonspecific interactions. The stability of drug-protein complexes were in the order K(VO(2+)) 1.2 x 10(8) M(-1) > K(AZT) 1.9 x 10(6) M(-)1 > K(PEG) 4.1 x 10(5) M(-1) > K(atrazine) 3.5 x 10(4) M(-1) > K(chlorophyll) 2.9 x 10(4) M(-1) > K2,4-D 2.5 x 10(4) M-1 > K(spermine) 1.7 x 10(4) M(-1) > K(taxol) 1.43 x 10(4) M(-1) > K(Co(3+)) > 1.1 x 10(4) M(-1) > K(aspirin) 1.04 x 10(4)i(-1) > K(chlorophyllin) 7.0 x 10(3) M(-1) > K(VO(3)(-)) 6.0 x 103 M(-1) > K(spermidine) 5.4 x 10(3) M(-1) > K(putrescine) 3.9 x 10(3) M(-1) > K(As(2)O(3)) 2.2 x 10(3) M(-1)> K(cisplatin) 1.2 x 10(2) M(-1). The protein conformation was altered (infrared and CD results) with major reduction of alpha-helix from 60 to 55% (free HSA) to 49 to 40% and increase of beta-structure from 22 to 15% (free HSA) to 33 to 23% in the drug-protein complexes. The alterations of protein secondary structure are attributed to a partial unfolding of HSA on drug complexation.

Mononuclear metavanadate catalyses gas phase oxidation of methanol to formaldehyde employing dioxygen as the terminal oxidant

Multistage mass spectrometry experiments reveal a sequence of gas phase reactions for the oxidation of methanol to formaldehyde with a mononuclear oxo vanadate anion as the catalyst and dioxygen as the terminal oxidant.

A Density Functional Theory Study of the Mechanism of Free Radical Generation in the System Vanadate/PCA/H2O2

Experimental studies by Shul'pin and co-workers have shown that vanadate anions in combination with pyrazine-2-carboxylic acid (PCA identical with pcaH) produce an exceptionally active complex that promotes the oxidation of alkanes and other organic molecules. Reaction of this complex with H2O2 releases HOO* free radicals and generates V(IV) species, which are capable of generating HO* radicals by reaction with additional H2O2. The oxidation of alkanes is initiated by reaction with the HO* radicals. The mechanism of hydrocarbon oxidation with vanadate/PCA/H2O2 catalyst has been studied using density functional theory. The proposed model reproduces the major experimental observations. It is found that a vanadium complex with one pca (PCA identical with pcaH) and one H2O2 ligand is the precursor to the species responsible for HOO* generation. It is also found that species containing two pca ligands and an H2O2 molecule do not exist in the solution, in contradiction to previous interpretations of experimental observations. Calculated dependences of the oxidation rate on initial concentrations of PCA and H2O2 have characteristic maxima, the shapes of which are determined by the equilibrium concentration of the active species. Conversion of the precursors requires hydrogen transfer from H2O2 to a vanadyl group. Our calculations show that direct transfer has a higher barrier than pca-assisted indirect transfer. Indirect transfer occurs by migration of hydrogen from coordinated H2O2 to the oxygen of a pca ligand connected to the vanadium atom. The proposed mechanism demonstrates the important role of the cocatalyst in the reaction and explains why H2O2 complexes without pca are less active. Our work shows that the generation of HOO* radicals cannot occur via cleavage of a V-OOH bond in the complex formed directly from the precursors, as proposed before. The activation barrier for this process is too high. Instead, HOO* radicals are formed via a sequence of additional steps involving lower activation barriers. The new mechanism for free radical generation underestimates the observed rate of hexane oxidation by less than an order of magnitude; however, the calculated activation energy (67-81 kJ/mol) agrees well with that determined experimentally (63-80 kJ/mol).

Effect of acetonitrile on the catalytic decomposition of hydrogen peroxide by vanadium ions and conjugated oxidation of alkanes

The rate of hydrogen peroxide decomposition in acetonitrile in the presence of a vanadate anion and pyrazine-2-carboxylic acid decreases remarkably when alkane (cyclohexane, n-heptane, isooctane) is added to the reaction solution. The alkane added is oxidized by this system to alkyl hydroperoxide. This is explained by the fact that much more hydrogen peroxide molecules are consumed to acetonitrile oxidation with formation of the final products, which is suppressed considerably by additives of necessary amounts of alkane, than those consumed to the oxidation of cyclohexane to form cyclohexyl hydroperoxide. In an organic solvent, H2O2 decomposes in a non-chain radical process.

Decavanadate as a biochemical tool in the elucidation of muscle contraction regulation

Recently reported decameric vanadate (V 10) high affinity binding site in myosin S1, suggests that it can be used as a tool in the muscle contraction regulation. In the present article, it is shown that V 10 species induces myosin S1 cleavage, upon irradiation, at the 23 and 74 kDa sites, the latter being prevented by actin and the former blocked by the presence of ATP. Identical cleavage patterns were found for meta- and decavanadate solutions, indicating that V 10 and tetrameric vanadate (V 4) have the same binding sites in myosin S1. Concentrations as low as 50 μM decavanadate (5 μM V 10 species) induces 30% of protein cleavage, whereas 500 μM metavanadate is needed to attain the same extent of cleavage. After irradiation, V 10 species is rapidly decomposed, upon protein addition, forming vanadyl (V 4+) species during the process. It was also observed by NMR line broadening experiments that, V 10 competes with V 4 for the myosin S1 binding sites, having a higher affinity. In addition, V 4 interaction with myosin S1 is highly affected by the products release during ATP hydrolysis in the presence or absence of actin, whereas V 10 appears to be affected at a much lower extent. From these results it is proposed that the binding of vanadate oligomers to myosin S1 at the phosphate loop (23 kDa site) is probably the cause of the actin stimulated myosin ATPase inhibition by the prevention of ATP/ADP exchange, and that this interaction is favoured for higher vanadate anions, such as V 10.

Synthesis of a new vanadyl(IV) complex with trehalose (TreVO): insulin-mimetic activities in osteoblast-like cells in culture.

Vanadium compounds show interesting biological and pharmacological properties. Some of them display insulin-mimetic effects and others produce anti-tumor actions. The bioactivity of vanadium is present in inorganic species like the vanadyl(IV) cation or vanadate(V) anion. Nevertheless, the development of new vanadium derivatives with organic ligands which improve the beneficial actions and decrease the toxic effects is of great interest. On the other hand, the mechanisms involved in vanadium bioactivity are still poorly understood. A new vanadium complex of the vanadyl(IV) cation with the disaccharide trehalose (TreVO), Na(6)[VO(Tre)(2)].4H(2)O, here reported, shows interesting insulin-mimetic properties in two osteoblast cell lines, a normal one (MC3T3E1) and a tumoral one (UMR106). The complex affected the proliferation of both cell lines in a different manner. On tumoral cells, TreVO caused a weak stimulation of growth at 5 microM but it inhibited cell proliferation in a dose-response manner between 50 and 100 microM. TreVO significantly inhibited UMR106 differentiation (15-25% of basal) in the range 5-100 microM. On normal osteoblasts, TreVO behaved as a mitogen at 5-25 microM. Different inhibitors of the MAPK pathway blocked this effect. At higher concentrations (75-100 microM), the complex was a weak inhibitor of the MC3T3E1 proliferation. Besides, TreVO enhanced glucose consumption by a mechanism independent of the PI3-kinase activation. In both cell lines, TreVO stimulated the ERK phosphorylation in a dose- and time-dependent manner. Different inhibitors (PD98059, wortmannin, vitamins C and E) partially decreased this effect, which was totally inhibited by their combination. These results suggest that TreVO could be a potential candidate for therapeutic treatments.

Kinetics and mechanism of alkane hydroperoxidation with tert-butyl hydroperoxide catalysed by a vanadate anion.

tert-Butyl hydroperoxide oxidizes alkanes in acetonitrile at 60 degrees C if the soluble vanadium(v) salt, n-Bu4NVO3, is used as a catalyst. Alkyl hydroperoxides are formed as main products which decompose during the course of the reaction to produce the more stable corresponding alcohols and ketones. Turnover numbers (ie. numbers of moles of products per one mole of a catalyst) attained 250. The kinetics and selectivity of the reaction have been studied. The mechanism proposed involves the formation of a complex between the V(V) species and t-BuOOH (K5 was estimated to be 5 dm3 mol(-1)) followed by decomposition of this complex (k6 = 0.2 s(-1)). The generated V(IV) species reacts with another t-BuOOH molecule to produce an active t-BuO* radical which attacks the hydrocarbon.

Aerobic hydroxylation of hydrocarbons catalysed by vanadate ion

Vanadate anion catalyses aerobic hydroxylation of hydrocarbons in acetonitrile in the presence of solid ascorbic acid or zinc and with obligatory participation of pyridine, pyrazine-2-carboxylic acid and acetic acid as mediators of proton and electron transfer. If sufficient amount of water is present in the reaction mixture, ascorbic acid is dissolved in aqueous acetonitrile and no hydroxylation occurs in this case. The dependencies of the product yields on the concentrations of the reactants have been studied and a mechanism of the formation of hydroxyl radicals has been proposed. These systems mimic generation of hydroxyl radicals in certain vanadium-dependent biological processes.

Vanadium Interactions with Chitosan: Influence of Polymer Protonation and Metal Speciation

Chitosan is very efficient at removing vanadium from dilute solutions: sorption capacity can reach 400−450 mg V g-1 under optimum experimental conditions, which correspond to pH 3. In acidic solutions, the chitosan's amine groups are protonated, and vanadate anions can be exchanged with counterions bound to −NH3+ sites. A correlation is observed between the speciation diagram and the sorption isotherms, and it appears that decavanadate species are more favorable to sorption than other anionic vanadate species. Selecting experimental conditions under which decavanadate ions are the predominant form of vanadium results in enhanced sorption capacities and improved sorption kinetics. The results obtained in the study of vanadate desorption confirm the high affinity of chitosan for these polyoxoanions.

Organic vanadium chelators potentiate vanadium-evoked glucose metabolism in vitro and in vivo: establishing criteria for optimal chelators.

Several ligands, when complexed with vanadium, potentiate its insulinomimetic activity both in vivo and in vitro. We have recently found that L-Glu-gamma-monohydroxamate (HXM) and L-Asp(beta)HXM were especially potent in this regard. In the present study, we used vanadium-enriched adipose cells and cell-free experimental systems to determine the features of L-Glu(gamma)HXM and L-Asp(beta)HXM that turn these ligands into optimal-synergizing vanadium chelators. We found that L-Glu(gamma)HXM and L-Asp(beta)(HXM) possess the following characteristics: 1) They associate with vanadium(+5) at pH 7.2 within a narrow range of an apparent formation constant of 1.3 to 1.9 x 10(2) M(-1); 2) they have nearly the same binding affinity for the vanadyl(+4) cation and the vanadate(+5) anion at physiological pH values; and 3) they form intense ultraviolet absorbing complexes upon associating with vanadium(+4) at 1 and 3 M stoichiometry, respectively, at pH 3.0. Vanadium ligands lacking any of these three defined criteria synergize less effectively with vanadium to activate glucose metabolism.

Polyvanadate-exchanged hydrotalcites as catalysts for selective oxidation of cyclohexene with molecular oxygen

Catalytic activity of polyvanadate-exchanged Mg,Al-, Zn,Al-, and Zn,Cr-hydrotalcites in the selective oxidation of cyclohexene with dioxygen has been investigated and discussed with reference to the composition of the brucite layer, type of the intercalated vanadate anion, vanadium loading and distribution of reaction products.

Oxidations by the reagent `O 2–H 2O 2 – vanadate anion – pyrazine-2-carboxylic acid'. : Part 10 . Oxygenation of methane in acetonitrile and water

The oxidation of methane by a combination of air and hydrogen peroxide is effectively catalyzed in solution by a system composed of vanadate and pyrazine-2-carboxylic acid (PCA). In acetonitrile solution, containing the vanadate anion as tetrabutylammonium salt, the reaction gives, over a temperature range of 25 to 50°C, methanol, carbon monoxide, formaldehyde, formic acid and carbon dioxide, the latter three compounds, however, being partially due to the oxidation of the acetonitrile used as the solvent, especially at higher temperatures. In aqueous solution, containing the vanadate anion in the form of the sodium salt, the reaction affords, over a temperature range of 40 to 70°C, selectively methyl hydroperoxide within 4 h. The yield of CH 3OOH attains 24%, based on H 2O 2, after 24 h at 50°C, the catalytic turnover number being 480. The process seems to involve hydroxyl radicals, generated by the catalyst from H 2O 2 even at low temperatures. At 120°C, methane is oxidized by O 2 and H 2O 2 to give formaldehyde and formic acid, even in the absence of the catalyst, presumably due to the formation of HO · radicals from H 2O 2 in the presence of very low concentrations of metal ions from the autoclave under high temperature conditions.

Aluminium Anodic Oxide Films Formed in Nonaqueous Solutions.

Anodization of aluminum was examined in nonaqueous solutions containing a small amount of water, and the resul-tant anodic oxide films were characterized by SEM, TEM, AES, XPS, XANES, and FT-IR. The film properties were dependent on not only the kind of nonaqueous solutions but also the amount of water in the solutions. When the water content in nonaqueous solutions was decreased below 1∼0.1 %, anions and/or organic solvents were incorporated into the films and very different anodic films were formed. A phthalate/γ-butyrolactone solution gave a fibrous porous-type film accompanying solution decomposition, whereas in a maleate/γ-butyrolactone solution fast and efficient film formation was observed and a barrier-type film containing maleate anions as an oxygen source was obtained. A vanadate/ethylene glycol solution provided a very thick film due to the incorporation of the solvent rather than vanadate anions. The optimum content of water to give the composite oxide films seems to depend on the viscosity of the solutions.

Oxidation by a ?H2O2-vanadium complex-pyrazine-2-carboxylic acid? reagent

Lower alkanes (ethane, propane,n-butane,n-pentane) are readily oxidized in acetonitrile solvent by H2O2 with vanadate anion — pyrazine-2-carboxylic acid (PCA), as the catalyst at 75 °C and pressures of −3 atm to produce predominantly or exclusively ketones (aldehydes). Isobutane is transformed selectively intotert-butyl alcohol. The oxidation of cyclohexane at 26 °C in acetone or acetic acid is less efficient than in acetonitrile. The reaction does not occur intert-butyl alcohol.

Interaction of Decavanadate Polyanions with Proteins

The binding of polymeric decavanadate anion [V10O28]6− with bovine serum albumin and gelatin was studied at pH 4.0 and 3.0, the region of thermodynamic stability of oligomeric vanadate anion. The binding of decavanadate anion at pH 4.0 with bovine serum albumin (BSA) and gelatin was found to be 9 and 32 gmol of decavanadate per gram mole of the proteins. The binding at pH 3.0 was found to be 12 and 38 gmol, respectively. Freshly formed BSA decavanadate precipitate was particulate in nature while that of gelatin-decavanadate made a gummy mass. This indicates a different mode of binding of decavanadate anions with globular and fibrillar proteins. Infrared spectra of the adducts endorses electrostatic binding between proteins and decavanadate. Scanning electron microscopy micrographs reveal extended crosslinked binding between decavanadate and gelatin and aggregation of the uncharged BSA-decavanadate molecules to make a granular adduct. The mode of binding was also correlated with the structure of decavanadate anions, BSA, and gelatin.

XAS study of transition metal ions in hydrotalcite-like compounds

EXAFS and XANES studies have been performed on hydrotalcite-like materials containing vanadium species in the layers, as V3+ cations, or in the interlayer space as vanadate anions. It has been found that in the former sample V3+ isomorphically substitutes Mg2+ in the octahedral sites, while in the second case anions in the interlayer are as decavanadate clusters. Amorphous decomposition products obtained after calcination of the V3+ hydrotalcite contain V5+ cations as isolated VOa3− units.

The chemistry of peroxovanadium compounds relevant to insulin mimesis.

The inorganic coordination chemistry of peroxovanadium compounds relevant to insulin mimesis is reviewed. The structure and kinetic reactivity of solutions of vanadate anion, vanadyl complexes and peroxovanadate complexes are briefly compared. Peroxovanadium compounds contain an oxo group, one or two peroxo ligands (O2(2-)) and an ancillary ligand which is usually bidentate. These compounds approximate a trigonal bipyramidal structure which can be divided conceptually into a polar 'oxo' half and a relatively non-polar organic half. This presents a number of interesting design variations which are discussed with respect to the development of a rudimentary structure-activity correlation of insulin mimetic ability.