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PubMed2002

Photocatalytic reduction and recovery of copper by polyoxometalates.

A series of polyoxometalates PW12O40(3-), SiW12O40(4-), and P2Mo18O62(6-) have been used as photocatalysts for recovery of copper and production of fine metal particles. The process involves absorption of light by polyoxometalates, oxidation of an organic substrate, for instance, propan-2-ol as sacrificial reducing reagent, and reoxidation of the reduced polyoxometalates by Cu2+ ions, closing the photocatalytic cycle. Copper(II) ions are reduced to copper(I) and finally to zero-state particles in a 2-electron process, as also suggested by the half-order dependence. Increase of catalyst or propan-2-ol concentration, or both, accelerates the photodeposition of copper until a saturation value is reached. The method is operational at a wide range of copper concentrations varying from 3 to 1300 ppm, leading to very low final concentrations (<0.2 ppm). The presence of dioxygen suppresses the initiation of copper recovery, though the process is equally effective after dioxygen is consumed. The process is independent of pH within the range 0.3-5.0. Addition of ClO4-, NO3-, or CH3COO- has no effect on the removal of copper ions. Chloride ions retard the enhancement of copper precipitation through stabilization of copper(I). This homogeneous, polyoxometalate-based process exhibits some benefits in comparison with the semiconductor-based (heterogeneous) recovery of metals: The final zero-state metal particles are obtained in pure form. No separation from the catalyst is needed, and moreover, the process is catalytic as the photodeposited metal particulates do not hinder the photocatalytic action of polyoxometalate anions.

Acetates↗
PubMed2001

Use of WO(4)(2-) on layered double hydroxides for mild oxidative bromination and bromide-assisted epoxidation with H(2)O(2).

Tungstate, exchanged on a (Ni,Al) layered double hydroxide, is applied as a heterogeneous catalyst in the oxidation of bromide with H(2)O(2) and the ensuing electrophilic bromination of olefins. The high halogenation activity of the catalyst in essentially neutral conditions mimicks the activity of V-bromoperoxidase enzymes. In water, aromatic and aliphatic olefins are selectively converted to bromohydrins; in methanol, methoxybromides are produced. In appropriate solvent conditions, the bromohydroxylation of geminally di-, tri-, and tetrasubstituted olefins proceeds via dehydrobromination to the epoxide. Evidence for this mechanism is provided by kinetic and labeling experiments. This one-pot alternative for the two-step halohydrin epoxidation process is enabled by the mild pH conditions; bromide is effective in substoichiometric, catalytic amounts. All new catalytic procedures are characterized by a high oxidative stability of the catalyst, high productivity of the catalyst on weight basis, high W turnover frequencies in ambient conditions (up to 50 mol of product per W per h), and high chemo-, regio-, and stereoselectivities.

Alkenes↗
PubMed2006

Observation of current reversal in the scanning tunneling spectra of fullerene-like WS2 nanoparticles.

Current-voltage characteristics measured using STM on fullerene-like WS2 nanoparticles show zero-bias current and contain segments in which the tunneling current flows opposite to the applied bias voltage. In addition, negative differential conductance peaks emerge in these reversed current segments, and the characteristics are hysteretic with respect to the change in the voltage sweep direction. Such unusual features resemble those appearing in cyclic voltammograms, but are uniquely observed here in tunneling spectra measured in vacuum, as well as in ambient and dry atmosphere conditions. This behavior is attributed to tunneling-driven electrochemical processes.

Electric Conductivity↗
PubMed1987

Comparative induction of nitrate reductase by nitrate and nitrite in barley leaves.

The comparative induction of nitrate reductase (NR) by ambient NO3- and NO2- as a function of influx, reduction (as NR was induced) and accumulation in detached leaves of 8-day-old barley (Hordeum vulgare L.) seedlings was determined. The dynamic interaction of NO3- influx, reduction and accumulation on NR induction was shown. The activity of NR, as it was induced, influenced its further induction by affecting the internal concentration of NO3-. As the ambient concentration of NO3- increased, the relative influences imposed by influx and reduction on NO3- accumulation changed with influx becoming a more predominant regulant. Significant levels of NO3- accumulated in NO2(-)-fed leaves. When the leaves were supplied cycloheximide or tungstate along with NO2-, about 60% more NO3- accumulated in the leaves than in the absence of the inhibitors. In NO3(-)-supplied leaves NR induction was observed at an ambient concentration of as low as 0.02 mM. No NR induction occurred in leaves supplied with NO2- until the ambient NO2- concentration was 0.5 mM. In fact, NR induction from NO2- solutions was not seen until NO3- was detected in the leaves. The amount of NO3- accumulating in NO2(-)-fed leaves induced similar levels of NR as did equivalent amounts of NO3- accumulating from NO3(-)-fed leaves. In all cases the internal concentration of NO3-, but not NO2-, was highly correlated with the amount of NR induced. The evidence indicated that NO3- was a more likely inducer of NR than was NO2-.

Cycloheximide↗
PubMed1989

Inhibition of endotoxin-induced bacterial translocation in mice.

The primary functions of the gut are to absorb nutrients and exclude bacteria and their products. However, under certain circumstances the gut may lose its barrier function and serve as a reservoir for systemic microbial infections. These experiments were performed to determine the mechanisms whereby endotoxin causes bacteria to escape (translocate) from the gut. Bacteria translocated from the gut to the mesenteric lymph nodes of mice challenged with nonlethal doses of Escherichia coli 026:B6 or E. coli 0111:B4 endotoxin. Physical disruption of the gut mucosal barrier appears to be the primary mechanism whereby endotoxin promotes bacterial translocation. Mucosal injury and endotoxin-induced bacterial translocation were reduced by inhibition (allopurinol) or inactivation (tung-sten diet) of xanthine oxidase activity (P less than 0.01), but were not affected by the platelet-activation factor antagonists, SRI 63-441 or BN 52021. Because the inhibition or inactivation of xanthine oxidase activity reduced both the extent of mucosal injury and endotoxin-induced bacterial translocation, the effect of endotoxin on the gut appears to be mediated, at least to some degree, by xanthine oxidase-generated, oxygen-free radicals.

Allopurinol↗
PubMed1995

Novel assay system favorable for the study of cell-to-cell transmission of HIV-1 and its application to the evaluation of anti-HIV drugs.

The cell-to-cell transmission of human immunodeficiency virus type 1 (HIV-1) was studied using MOLT-4 cells chronically infected with a variant strain of HIV-1SF-2 (MOLT-4/HIV-1SF-2H) and CD4+ human lymphoid MT-4 cells. MOLT-4/HIV-1SF-2H cells produced less than 1 TCID50 infectious particles per day as determined by the cytopathogenicity in MT-4 cells. However, the expression of envelope glycoproteins gp120 and gp41 on the MOLT-4/HIV-1SF-2H cell membrane was satisfactory for syncytium formation with the uninfected MOLT-4 cells. When MOLT-4/HIV-1SF-2H and MT-4 cells were co-cultured, severe cytopathogenicity was observed in MT-4 cells without being accompanied by the formation of multi-nucleated cells. Thus, the system consisting of MOLT-4/HIV-1SF-2H and MT-4 cells is convenient for exclusive study of the mechanism of cell-to-cell transmission of HIV-1. Using various compounds, it was confirmed that cell-to-cell transmission required both gp120/gp41-CD4 binding and de novo DNA synthesis.

Acquired Immunodeficiency Syndrome↗
PubMed1989

Inhibitory effects of sulphur compounds, copper and tungsten on nitrate reduction by mixed rumen micro-organisms.

1. The inhibitory effects of inorganic and organic sulphur-containing compounds, copper and tungsten on nitrate reduction by mixed rumen micro-organisms were investigated in two in vitro studies. 2. Coarsely strained rumen fluid from nitrate-adapted (Expt 1) or non-adapted (Expt 2) Suffolk Down wethers maintained on lucerne (Medicago sativa) cubes was used as an inoculum. In Expt 1, anaerobic incubation was carried out for 24 h for each medium supplemented with 10 mM-sodium nitrate and the following chemicals: 0, 1, 2, 3, 5, 8 and 10 mM-sodium sulphide, 1 and 10 mM-sodium sulphite, 1 and 10 mM-sodium sulphate, 1 and 10 mM-L-cysteine, 1 and 10 mM-DL-methionine, 1 mM-sodium tungstate and 1 mM-copper sulphate. In Expt 2, 1 and 10 mM-Na2S, 1 and 10 mM-L-cysteine, 1 mM-Na2WO4, and 1 mM-CuSO4 were added to incubation media to test for chemical inhibition of microbial reduction of nitrate. 3. In Expt 1, the amount of nitrite formed decreased with increasing concentration of sulphide-S added. The additions of L-cysteine, W and Cu suppressed nitrite formation in media from both nitrate-adapted and non-adapted sheep. 4. In contrast to the effects of sulphide, L-cysteine and W counteracted, to some degree, nitrate-induced reduction of volatile fatty acid (VFA) production. Addition of Cu to the media resulted in a further depression of VFA production.

Animals↗
PubMed2005

Temperature dependent electrochemical investigations of molybdenum and tungsten oxobisdithiolene complexes.

To achieve a better understanding why thermophilic and hyperthermophilic organisms use tungsten instead of molybdenum within the active sites of their molybdopterin dependent oxidases, electrochemical investigations of model complexes for the active sites of enzymes belonging to the DMSO reductase (molybdenum) and the aldehyde oxidoreductase (tungsten) family have been undertaken. Cyclic voltammetry and differential pulse voltammetry of four pairs of molybdenum and tungsten oxobisdithiolene compounds show huge differences in the response of their redox potentials to rising or decreasing temperatures, depending on the substituents at the dithiolene group. The mnt2- compounds (1a, 1b) respond with decreasing redox potentials E(1/2) to rising temperatures whereas all other compounds show positive gradients deltaE/deltaT. In every case the values for the gradients for the tungsten compounds are greater than those for the molybdenum compounds. Six of the investigated compounds are known in the literature and two compounds were newly synthesized. These two new compounds include the pyrane subunit of the native molybdopterin ligand and should therefore be even better models for the active site of the molybdopterin containing enzymes. The molybdenum/tungsten pair with these new ligands shows a remarkably small difference for the redox potentials of the transition M(IV) <--> M(V) of only 30 mV at 25 degrees C and the reversion of the usual order with higher potentials for the molybdenum than the tungsten compound at a temperature of 70 degrees C; a temperature that is in the range where usually tungsten containing enzymes instead of molybdenum containing ones are found.

Binding Sites↗
PubMed1996

Insulin-like vs. non-insulin-like stimulation of glucose metabolism by vanadium, tungsten, and selenium compounds in rat muscle.

The direct impact of vanadate, tungstate, selenate, and selenite on glucose metabolism of isolated rat soleus muscle was investigated. All compounds stimulated glucose transport, but only vanadate exerted an insulin-like effect on glycogen synthesis (mumol glucose into glycogen*g-1*h-1: control 1.43 +/- 0.11 vs. 1 mmol/l vanadate, 2.08 +/- 0.11, p < 0.0001), which was more distinct in the presence of 1 mmol/l H2O2 (control, 1.44 +/- 0.13 vs. 1 mmol/l vanadate, 3.49 +/- 0.12, p < 0.001). Glucose handling of muscles exposed to tungstate, selenate, or selenite resembled that of hypoxic muscle, i.e. the induced rise in glucose uptake was inhibited by dantrolene and associated with high rates of glycolysis and rapid glycogen depletion (glycogen content after incubation, mumol glucosyl units/g: control, 16.2 +/- 0.7 vs. hypoxia, 2.7 +/- 0.5, p < 0.0001; control, 17.0 +/- 0.5 vs. 100 mmol/l tungstate, 5.5 +/- 0.4, p < 0.001; control, 16.2 +/- 0.7 vs. 100 mmol/l selenate, 1.5 +/- 0.3, and vs. 300 mumol/l selenite, 1.7 +/- 0.3, p < 0.0001 each). The results suggest that vanadate (and more pronounced it's peroxides) exerts true insulin-like action on isolated muscle glucose metabolism, whereas tungsten and selenium salts trigger glucose transport in association with a catabolic response, which may represent an unspecific response to toxic/osmotic stress.

Androstadienes↗
PubMed2003

Magnetic properties of cationic tungsten(IV) half sandwich compounds: experimental and theoretical study of a solvent and ligand stabilized singlet ground state leading to a thermally induced singlet-triplet spin state interconversion.

A series of novel neutral tungsten(III) and cationic tungsten(IV) complexes with disubstituted 4,4'-R,R-2,2'-bipyridyl (R(2)-bpy) ligands of the type [CpW(R(2)-bpy)Cl(2)](n+) (n = 0,1) were prepared and characterized by X-ray crystallography. Susceptibility measurements of the tungsten(IV) complexes revealed an intrinsic paramagnetism of these compounds and evidenced different magnetic properties of the dimethylamino and methyl (R = NMe(2), Me) substituted tungsten(IV) compounds in solution and in the solid state. In dichloromethane solution, singlet ground states with thermally populated triplet states were observed, whereas triplet (R = Me) and singlet ground states (R = NMe(2)) were observed in the solid state. Using both experimental and theoretical techniques (DFT) allowed to establish solvation and ligand effects to account for the different magnetic behavior. Thermodynamic parameters were derived for the spin equlibria in solution by fits of the temperature dependent (1)H NMR shifts to the Van Vleck equation and were found to be in excellent agreement with the DFT calculations.

Journal Article↗
PubMed1984

The relationship of Mo, molybdopterin, and the cyanolyzable sulfur in the Mo cofactor.

Reconstitution of the apoprotein of the molybdoenzyme nitrate reductase in extracts of the Neurospora crassa mutant nit-1 with molybdenum cofactor released by denaturation of purified molybdoenzymes is efficient in the absence of exogenous MoO2-4 under defined conditions. Evidence is presented that this molybdate-independent reconstitution is due to transfer of intact Mo cofactor, a complex of Mo and molybdopterin (MPT), the organic constituent of the cofactor. This complex can be separated from denatured protein by gel filtration, and from excess MoO2-4 by reverse-phase HPLC. Sulfite oxidase, native xanthine dehydrogenase, and cyanolyzed xanthine dehydrogenase are equipotent Mo cofactor donors. Other well-studied inactive forms of xanthine dehydrogenase are also shown to be good cofactor sources. Using xanthine dehydrogenase specifically radiolabeled in the cyanolyzable sulfur, it is shown that this terminal ligand of Mo is rapidly removed from Mo cofactor under the conditions used for reconstitution.

Animals↗
PubMed2006

Circumambulatory rearrangement with characteristics of a 2:1 covalent molecular bevel gear.

anti-W(CO)(5)-complexed 9-methyl-9-phosphabicyclo[6.1.0]nonatriene represents a covalently interlocked molecular bevel gear. Correlated movement of the phosphorus atom and the eight-membered ring by way of a "walk" rearrangement makes gear slippage impossible. The gearing motion is transferred to the four-toothed W(CO)(5) propeller connected to the rotating phosphorus atom, enabling a gearing ratio of 2:1 according to B3LYP and Car-Parrinello Molecular Dynamics calculations. Methyl substitution of the eight-membered ring tempers the gearing process, with the PMeW(CO)(5) entity passing the substituted carbon atom only at temperatures above 50 degrees C.

Bridged Bicyclo Compounds↗
PubMed2003

Utilization of Folin-Ciocalteu phenol reagent for the detection of certain nitrogen compounds.

To determine in more detail the reaction of Folin-Ciocalteu phenol reagent with nitrogen compounds, a number of hydroxylamine-related compounds and a large number of guanidine-containing compounds were tested. In general, guanidine compounds did not react strongly unless they were hydroxyamino or hydrazino derivatives. The non-hydroxyamino paralytic shellfish poison saxitoxin, however, reacted to a significant extent. This may be due to the presence of a five-membered ring structure and its analogy to 2-aminopurines, which react strongly. A number of simpler amines were also tested. Tertiary aliphatic amines, but not primary, secondary, or quaternary amines, reacted strongly with the reagent. Primary, secondary, and tertiary aromatic amines all reacted strongly with the reagent. Reactivity was extended to pyrroles and indole derivatives but not to imidazole and benzimidazole derivatives. Defining the reactivity of Folin-Ciocalteau phenol reagent with nitrogen compounds extends the usefulness of the reagent for the detection and determination of certain nitrogen compounds in basic extracts by colorimetric means and by thin-layer chromatography.

Amides↗