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PubMed2006

Synthesis and characterizations of cyclic octanuclear mixed-valence vanadium(IV,V) clusters with polyoxometalate counterions.

Hydrothermal reaction of Na2WO4, VOSO4, 2,2'-bpy and H3PO4 has afforded in high yield the compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PW11V(V)O40](bpy).12H2O (1). Compound 1 contains a novel octanuclear mixed valence V(IV,V) cluster, [V(IV)2V(V)6O14(bpy)8(PO4)2]4+, with [PW11V(V)O40]4- as counterion. In the vanadium cluster, four V(V) centers are localized and the remaining two V(IV) and two V(V) ions are disordered over four crystallographically equivalent positions. The isostructural compound [V(IV)2V(V)6O14(bpy)8(PO4)2][PMo11V(V)O40](bpy).3H2O (2) has also been synthesized. Thermodiffractometry experiments indicate that 2 is stable up to 360 degrees C. Redox activities for both the vanadium and molybdenum centers have been observed by solid-state electrochemical measurements performed on mechanically attached microparticles of 2. Magnetic measurements performed on have shown the occurrence of weak ferromagnetic interactions between the V(IV) centres (J = +0.34 cm(-1), H(ex) = -JS1 x S2), and combined with DFT calculations, have allowed to propose a localization of the two V(IV) centers on two of the four equivalent crystallographic sites. Finally high field electron paramagnetic resonance has evidenced the magnetic axial anisotropy of the paramagnetic centers (g(x) = g(y) = 1.975(3); g(z) = 1.939(4)).

Crystallography, X-Ray↗
PubMed1998

[Synthesis and anti-influenza virus activities of heteropoly compounds containing rare earth elements].

Tungstoborate and tungstogermanate heteropoly compounds containing rare earth elements with the general formula K15[Ln(BW11O39)2].nH2O and K13[Ln(GeW11O39)2].nH2O, where Ln = La, Ce, Pr, Nd, Sm, have been synthesized and characterized by elemental analysis, IR, 183W NMR spectra. The results of experiments on cytotoxicity and anti-influenza virus activities using cell-cultivation method showed that four compounds among them exhibited good inhibitory effect on influenza virus.

Animals↗
PubMed1994

Metal oxyanion stabilization of the rat glucocorticoid receptor is independent of thiols.

The ability of sodium molybdate, both to stabilize the steroid binding activity of glucocorticoid receptors and to prevent the activation of receptor-steroid complexes to a DNA binding species, has long been thought to involve thiols. Two receptor thiols in particular, Cys-656 and Cys-661 of rat receptors, have been suspected. The requirements for the action of molybdate, as well as two other metal oxyanions (tungstate and vanadate) known to exert the same effects as molybdate, have now been examined with receptors in which these thiols, or a third cysteine in the steroid binding cavity (Cys-640), have been mutated to serine. No mutation prevented any metal oxyanion from either stabilizing steroid-free receptors or blocking the activation of complexes for binding to nonspecific or specific DNA sequences. Thus, Cys-640, Cys-656, and Cys-661 are not required for any of the effects of molybdate, tungstate, or vanadate with rat glucocorticoid receptors. Studies with hybrid receptors, and with a 16-kDa steroid binding core fragment containing only 3 cysteines at positions 640, 656, and 661, indicated that no cysteine of the rat receptor was needed to maintain responsiveness to molybdate. Even when all of the thiol groups in crude cytosol were blocked by reaction with excess methyl methanethiol-sulfonate, each metal oxyanion was still able to stabilize the steroid binding of receptors. These results argue that molybdate, tungstate, and vanadate each interact with the receptor or an associated nonreceptor protein(s) in a manner that does not require thiols. An indirect mechanism of molybdate action was evaluated in light of the recent report that the whole cell actions are mediated by increased levels of intracellular cGMP. Under cell-free conditions, however, the effects of molybdate could not be reproduced by cGMP derivatives. Evidence consistent with a direct effect was that molybdate, tungstate, or vanadate each modified the kinetics of proteolysis of wild type receptors at 0 degrees C by trypsin, presumably due to induced conformational changes of the receptor. This alteration of trypsin digestion constitutes yet another effect of metal oxyanions on the glucocorticoid receptor.

Affinity Labels↗
PubMed2002

Regulation of amino acid uptake in conifers by exogenous and endogenous nitrogen.

Although an accumulating amount of research clearly indicates that plants are capable of taking up exogenous amino acids, the actual importance of such organic N sources for plant N nutrition is under debate. In this study, we show that amino acid uptake by Scots pine (Pinus sylvestris L.) is significantly decreased by elevated internal NH(4)(+) levels, while it increases following exposure to exogenous amino acids. Furthermore, amino acid uptake is larger in N-deficient plants than in plants grown with a large access of N. The regulatory pattern of amino acid uptake shows important similarities to the regulation of NO(3)(-) and NH(4)(+) transport as well as to the regulation of yeast amino acid transporters. In addition, our data suggest that uptake may be regulated by factors not originating from N metabolism. The up-regulation of uptake in response to N deficiency suggests that amino acid uptake may be a significant contributor to the N economy of P. sylvestris.

Adaptation, Physiological↗
PubMed1997

Purification of a factor that enhances the antibacterial activity of beta-lactams against methicillin-resistant Staphylococcus aureus: its identification as undecaphosphotungstate.

Previously, a factor (Factor T) was found in aged mixtures of tungstate and phosphate, which greatly enhances the antibacterial effects of beta-lactam antibiotics on methicillin-resistant Staphylococcus aureus. Factor T was purified and identified as undecaphosphotungstate ([PW11O39]7-). Undecasilicotungstate ([SiW11O39]8-), a compound closely related to undecaphosphotungstate, showed a similar enhancing effect.

Anti-Bacterial Agents↗
PubMed2002

Alkaline phosphatase revisited: hydrolysis of alkyl phosphates.

Escherichia coli alkaline phosphatase (AP) is the prototypical two metal ion catalyst with two divalent zinc ions bound approximately 4 A apart in the active site. Studies spanning half a century have elucidated many structural and mechanistic features of this enzyme, rendering it an attractive model for investigating the potent catalytic power of bimetallic centers. Unfortunately, fundamental mechanistic features have been obscured by limitations with the standard assays. These assays generate concentrations of inorganic phosphate (P(i)) in excess of its inhibition constant (K(i) approximately 1 muM). This tight binding by P(i) has affected the majority of published kinetic constants. Furthermore, binding limits k(cat)/K(m) for reaction of p-nitrophenyl phosphate, the most commonly employed substrate. We describe a sensitive (32)P-based assay for hydrolysis of alkyl phosphates that avoids the complication of product inhibition. We have revisited basic mechanistic features of AP with these alkyl phosphate substrates. The results suggest that the chemical step for phosphorylation of the enzyme limits k(cat)/K(m). The pH-rate profile and additional results suggest that the serine nucleophile is active in its anionic form and has a pK(a) of < or = 5.5 in the free enzyme. An inactivating pK(a) of 8.0 is observed for binding of both substrates and inhibitors, and we suggest that this corresponds to ionization of a zinc-coordinated water molecule. Counter to previous suggestions, inorganic phosphate dianion appears to bind to the highly charged AP active site at least as strongly as the trianion. The dependence of k(cat)/K(m) on the pK(a) of the leaving group follows a Brønsted correlation with a slope of beta(lg) = -0.85 +/- 0.1, differing substantially from the previously reported value of -0.2 obtained from data with a less sensitive assay. This steep leaving group dependence is consistent with a largely dissociative transition state for AP-catalyzed hydrolysis of phosphate monoesters. The new (32)P-based assay employed herein will facilitate continued dissection of the AP reaction by providing a means to readily follow the chemical step for phosphorylation of the enzyme.

Alkaline Phosphatase↗
PubMed2006

W(CO)5(L)-catalyzed tandem intramolecular cyclopropanation/cope rearrangement for the stereoselective construction of bicyclo[5.3.0]decane framework.

Utilizing the biscarbene character of electrophilically activated alkynes, a novel tandem intramolecular cyclopropanation/Cope rearrangement of 3-siloxy-1,3,9-trien-7-ynes catalyzed by W(CO)5(L) for the stereoselective construction of bicyclo[5.3.0]decane framework is achieved. When 3-siloxy-1,3,9-trien-7-ynes were treated with a catalytic amount of W(CO)6 under photoirradiation, bicyclo[5.3.0]decanes were obtained in good yield stereoselectively. In this reaction, the Cope rearrangement of the divinylcyclopropane intermediates, generated by the intramolecular cyclopropanation of 3-siloxy-1,3,9-trien-7-ynes based on the W(CO)5(L)-catalyzed electrophilic activation of alkynes, occurs to give synthetically useful functionalized bicyclo[5.3.0]decane derivatives stereoselectively.

Alkynes↗
PubMed1999

Inhibitors of green crab (Scylla serrata) alkaline phosphatase.

Green crab (Scylla serrata) alkaline phosphatase (EC 3.1.3.1) is a metalloenzyme which catalyzes the nonspecific hydrolysis of phosphate monoesters. The effects of several inhibitors on its activity have been studied. The results show that Na2HPO4, Na2HAsO4 and Na2WO4 are competitive inhibitors, L-phenylalanine is an uncompetitive inhibitor, while L-cysteine is a mixed-type inhibitor. The equilibrium constants were determined for inhibitor binding. The order of potency was Na2HPO4 (KI = 0.95 mM) > Na2AsO4 (1.10 mM) > Na2WO4 (1.55 mM).

Alkaline Phosphatase↗