51V Nuclear Magnetic Resonance Research Articles

Physical and chemical characterization of surface vanadium oxide supported on titania: influence of the titania phase (anatase, rutile, brookite and B)

Different phases of titania were prepared and used to support ca. 1 wt.-% V 2O 5. The different titania phases prepared were: anatase (A22), rutile (R28), brookite (BT110) and B-phase (B18). Physical characterization of the various vanadia-titania catalysts was performed using X-ray photoelectron spectroscopy (XPS), in situ Raman and 51V solid state nuclear magnetic resonance (NMR) spectroscopy. The XPS results reveal that the all the catalysts contain various levels of impurities. In situ dehydration Raman shows, for all the samples, the stretching vibration of the terminal V&z.dbnd;O bond at ca. 1030 cm −1. Solid state 51V NMR spectra of all the samples in the dehydrated state show basically the same powder pattern with a peak maximum around −660 to −670 ppm. The combined Raman and NMR results indicate that the same surface vanadium oxide species is present on all the titania supports irrespective of the crystal structure of the bulk titania phase. Partial oxidation of methanol show similar activity and selectivity for the various vanadia-titania catalysts. The reaction selectivity was primarily to formaldehyde and methyl formate (92–96%). The turnover number for methanol oxidation was essentially the same for all the vanadia-titania catalysts and ranged from 1.4 to 2.8 s −1. These results indicate that the type of titania phase used as the support is not critical for partial oxidation over vanadia-titania catalysts as long as other parameters (e.g. surface impurities ) are similar. Thus, the structure-reactivity studies of the different vanadia-titania catalysts suggest that the specific titania phase is not a critical parameter in determining the physical or chemical nature of the surface vanadia phase.

1H and 51V NMR studies of the interaction of vanadate and 2-vanadio-3-phosphoglycerate with phosphoglycerate mutase.

The formation of complexes of vanadate with 2-phosphoglycerate and 3-phosphoglycerate have been studied using 51V nuclear magnetic resonance spectroscopy. Signals attributed to two 2,3-diphosphoglycerate analogues, 2-vanadio-3-phosphoglycerate and 2-phospho-3-vanadioglycerate, were detected but were not fully resolved from signals of inorganic vanadate and the anhydride formed between vanadate and the phosphate ester moieties of the individual phosphoglycerates. Equilibrium constants for formation of the two 2,3-bisphosphate analogues were estimated as 2.5 M-1 for 2-vanadio-3-phosphoglycerate and 0.2 M-1 for 2-phospho-3-vanadioglycerate. The results of the binding study are fully consistent with non-cooperativity in the binding of vanadiophosphoglycerate to the two active sites of phosphoglycerate mutase (PGM). 2-Vanadio-3-phosphoglycerate was found to bind to the dephospho form of phosphoglycerate mutase with a dissociation constant of about 1 x 10(-11) M at pH 7 and 7 x 10(-11) M at pH 8. Three signals attributed to histidine residues were observed in the 1H NMR spectrum of phosphoglycerate mutase. Two of these signals and also an additional signal, tentatively attributed to a tryptophan, underwent a chemical shift change when the vanadiophosphoglycerate complex was bound to the enzyme. The results obtained here are in accord with these vanadate-phosphoglycerate complexes being much more potent inhibitors of phosphoglycerate mutase than either monomeric or dimeric vanadate. The dissociation constant of 10(-11) M for 2-vanadio-3-phosphoglycerate is about 4 orders of magnitude smaller than the Km for PGM, a result in accordance with the vanadiophosphoglycerates being transition state analogues for the phosphorylation of PGM by 2,3-diphosphoglycerate.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of mutases and isomerases by vanadium

The effects of vanadate and vanadate complexes on the rates of exchange of phosphoryl groups in the reactions catalyzed by the enzymes phosphoglucomutase and the coupled system formed by phosphoglycerate mutase and enolase, and the effects of vanadyl complexes on the interconversion of aldehyde and keto groups catalyzed by the enzymes phosphomannose isomerase, phosphoribose isomerase, and phosphoglucose isomerase, were measured using one-dimensional 31P nuclear magnetic resonance spectroscopy. Chemical exchange was investigated by observing the transfer of magnetization achieved by selective irradiation of resonances using the DANTE pulse sequence. The presence of vanadium stimulated the catalytic activity of the enzymes in vitro, with the exception of enolase whose activity was not affected. Addition of vanadate also increased the rate constants of the interconversion of glucose 6-phosphate and fructose 6-phosphate in hemolysates. 51V nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy were employed to investigate the interactions between ammonium vanadate and sugar phosphates and the formation of vanadium-sugar phosphate complexes that may be involved in the stimulation of the catalytic activity of the isomerases.

Stereochemical requirements for the formation of vanadate complexes with peptides

The condensation reactions occurring between vanadate and a number of amino acids and simple peptides have been studied by 51V nuclear magnetic resonance. Vanadate and ligand stoichiometry has been established for the complexes formed and their formation constants determined. The amino acids were all found to undergo rather weak interactions with vanadate and in general provided two products with 51V chemical shifts near −545 and −555 ppm. The peptides also gave minor products with NMR signals occurring with that of vanadate itself. However, in this case, the major products occur at approximately −510 ppm. These latter complexes are monovanadate, monoligand complexes which require the terminal amino, the carboxylate groups and an unsubstituted nitrogen in the peptide linkage in order for product formation to occur. Sidechains promote product formation, apparently by favouring a more readily complexed peptide conformation. Formation constants vary from about 20 to 50 M−1 depending on the sidechain. The carboxylate group can be replaced by a hydroxymethyl. This, however, leads to a significant decrease in product formation at pH 7. Hydrogen ion concentration studies showed that the complexes, when considered to be formed from VO4H2− and neutral peptide, did not require or give off protons. However, when the carboxylate was replaced by the alcohol functionality a proton was released. On the basis of this study the coordination of the vanadate/peptide complexes has been assigned. Key words: peptide, vanadate, complexes, vanadium magnetic resonance.

Characterization of vanadate-dependent NADH oxidation stimulated by Saccharomyces cerevisiae plasma membranes

Plasma membrane-stimulated vanadate-dependent NADH oxidation has been characterized in Saccharomyces cerevisiae. This activity is specific for vanadate, because molybdate, a similar metal oxide, did not substitute for vanadate in the reaction. Vanadate-dependent plasma membrane-stimulated NADH oxidation activity was dependent on the concentrations of vanadate, NADH, and NADPH and required functional plasma membranes; no stimulation occurred in the presence of boiled membranes or bovine serum albumin. The dependence of membrane-stimulated vanadate-dependent NADH oxidation was not linearly dependent on added membrane protein. The activity was abolished by the superoxide anion scavenger superoxide dismutase and was stimulated by paraquat and NADPH. These data are consistent with the previously proposed chain reaction for vanadate-dependent NADH oxidation. The role of the plasma membrane appears to be to stimulate superoxide radical formation, which is coupled to NADH oxidation by vanadate. 51V-nuclear magnetic resonance studies are consistent with the hypothesis that a phosphovanadate anhydride is the stimulatory oxyvanadium species in the phosphate buffers used at pHs 5.0 and 7.0. In phosphate buffers, compared with acetate buffers, the single vanadate resonance was shifted upfield at both pH 5.0 and pH 7.0, which is characteristic of the phosphovanadate anhydride. Since the cell contains an excess of phosphate to vanadate, the phosphovanadate anhydride may be involved in membrane-mediated vanadate-dependent NADH oxidation in vivo.

Stimulation of human erythrocyte 2,3-bisphosphoglycerate phosphatase by vanadate

The rates of vanadate-stimulated hydrolysis of 2,3-bisphosphoglycerate in metabolically competent erythrocytes and in hemolysates were determined from data on time courses up to 35 min employing 31P nuclear magnetic resonance spectroscopy. The enhanced rate of hydrolysis of the bisphosphate was attributed principally to the activation of the phosphatase activity of 2,3-bisphosphoglycerate synthase both in cell suspensions and in hemolysates. Information on the concentrations of vanadate and vanadyl present in the preparations was obtained employing 51V nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy. Redox reactions involving vanadium ions appeared to be important in establishing the final equilibrium concentrations of the oxy- and oxoions (vanadate and vanadyl, respectively), but the data suggested that the activation of the enzyme resulted from direct action of the vanadium ions on the enzyme and not as a consequence of the alteration in the equilibrium of intracellular oxidants and reductants.

Multinuclear NMR study of the interaction of vanadate with mononucleotides, ADP, and ATP.

The interaction of vanadate with 5'-mononucleotides, ADP, ATP, and various molecules containing some of their chemical moieties was studied in aqueous solution in the pH region of 5-9 using proton, 13C, 31P, and 51V nuclear magnetic resonance (NMR) spectroscopy. All the compounds studied formed noncyclic vanadate esters through interaction of monovanadate or divanadate with the hydroxyl groups of the ribose ring. Noncyclic anhydrides were also formed with the phosphate groups of ribose 5-phosphate, the mononucleotides, ADP, ATP, phosphate, pyrophosphate, and tripolyphosphate. In particular, ADP and ATP analogs resulted from AMP (AMPV and AMPV2) and from ADP (ADPV). Cyclic esters of trigonal bipyramidal geometry resulted from the interaction of vanadate with two ribose ring cis hydroxyl groups. AMP, CMP, and UMP formed two such complexes of 1:1 and 1:2 stoichiometries, similar to what has been observed for uridine and other nucleosides. However, 2'-deoxy-AMP does not yield this type of complexes. ADP and ATP also form similar cyclic ester complexes with vanadate, which does not chelate their pyrophosphate and tripolyphosphate moieties. Nevertheless, the separate pyrophosphate (PP) and tripolyphosphate (PPP) ligands form cyclic anhydrides of octahedral geometry with vanadate. However, their binding to vanadate is weaker than that of the ribose ring of nucleotides. Competition experiments between ethylene glycol and phosphate (P), pyrophosphate (PP), or tripolyphosphate (PPP) show that the relative strength of the interaction of these ligands with vanadate is PP greater than ethylene glycol greater than PPP greater than P.

Interaction of vanadate with monosaccarides and nucleosides: A multinuclear NMR study

Proton, 13C and 51V nuclear magnetic resonance spectroscopy has been used to study the interaction of vanadate with several molecules containing more than one hydroxyl group, including various aldoses and nucleosides. The aldoses D-mannose and D-ribose mainly form tridentate complexes, of trigonal bipyramidal geometry, with vanadate at pH 7. These sugars use three consecutive hydroxyl groups, cis to each other, of their pyranose forms to bind vanadate in those cyclic triesters. Other aldoses, like D-glucose, which do not have this unique structural characteristic, do not form tridentate complexes, but can form weaker bidentate cyclic diesters using two consecutive pyranose cis hydroxyl groups. Of course, the pyranose forms of D-mannose and D-ribose, as well as the furanose form of D-ribose, also yield cyclic diesters of vanadate. All these aldoses form weak monodentate noncyclic monoesters of tetrahedral geometry using a single hydroxyl group. The nucleosides uridine, cytidine and adenosine form two complexes of trigonal bipyramidal geometry with vanadate. In these complexes, having 1:1 and 2:1 ligand-to-metal stoichiometries, the nucleosides form cyclic diesters with vanadate using their C 2′ and C 3′ hydroxyl groups.

The characterization of primary, secondary, and tertiary vanadate alkyl esters by 51V nuclear magnetic resonance spectroscopy

A variety of alkyl vanadates has been studied by 51V nuclear magnetic resonance spectroscopy. It was found that the equilibrium constant for condensation of vanadate with alcohols is insensitive to whether the hydroxyl group is primary, secondary, or tertiary. These products, however, have characteristic vanadium chemical shifts that allow assignment of nmr signals to the appropriate ester. It was also found that chemical shifts are additive in the sense that the chemical shifts of the esters ROVO3H− are one half the chemical shift of the diesters (RO)2VO2− when those shifts are given relative to −559 ppm. This effect is independent of whether the signals are to high or low field of −559 ppm and the additivity extends to mixed ligand systems. This value of −559 ppm is close but not equal to the chemical shift of the vanadate monoanion, H2VO41−, which is at −561 ppm. These results are at variance with arguments concerning the effects of ligand bulkiness on chemical shifts of vanadium(V) complexes.

The dependence of vanadate phenyl ester formation on the acidity of the parent phenols

The condensation of vanadate (VO4H21−) with a variety of substituted phenols in aqueous acetone solution has been studied by 51V nuclear magnetic resonance spectroscopy. The formation constants for the various product esters were determined, as were their respective pKa values. Inductive and resonance contributions to vanadate ester formation were investigated using the dual substituent parameter approach. This procedure did not succeed particularly well, although trends were clearly observed. Correlations with Hammett constants were compared with those of the dual parameter approach. The observation of some linear free-energy relationships was interpreted to mean that the inductive and resonance parameters generally ascribed to the various phenols are not directly applicable to vanadate ester formation where d orbitals are probably involved. The results did indicate that resonance effects have a significantly greater influence on ester formation than do inductive effects.

Vanadium(V) oxyanions. The dependence of vanadate alkyl ester formation on the pKa of the parent alcohols

The interaction of vanadate (VO4H21−) with a variety of alkyl alcohol's (ROH) covering about seven pKa units of acidity have been studied in aqueous solution by 51V nuclear magnetic resonance spectroscopy. A linear free energy relationship between the equilibrium constant for the formation of the vanadate monoester (HOVO3R1−) and the acidity of the alcohol was established at 23 °C. The correlation was found to be [Formula: see text] where K = [HOVO3R1−]/[VO4H21−][ROH]. This correlation was shown not to extend to ligands with π-bonding capability such as phenols or phosphates, where the products are highly favoured relative to the alkyl esters. An interesting correlation between pKa values of the product esters and the pKa of the parent alcohols was also observed. It was found that below a [Formula: see text] of about 15 the pKa values of the esters were essentially constant. However, above this value of 15 the pKa of the ester was found to increase rapidly with an increase in pKa of the alcohol. This result may indicate that the electron accepting ability of the metal is exhausted with the higher pKa alcohols, and the extra electron density is transferred to the oxygens, thus causing an increase in pKa of the ester.

51V nuclear magnetic resonance and magnetic susceptibility study of the electronic structure in Ti-V dihydrides

The 51V nuclear spin-lattice relaxation time T 1, 51V Knight shift K V and magnetic susceptibility χ in nearly stoichiometric Ti 1− x V x H 2 (0.04 ⩽ x ⩽ 0.65) have been measured. It was found that ( T 1 T) v −1 in dilute Ti-V hydrides is twice as large as ( T 1 T) v −1 in VH 2, decreasing gradually with increasing vanadium concentration. The separation of ( T 1 T) v −1 and K v into d spin and d orbital contributions for each vanadium concentration is reported, and the resulting densities N d v ( E F ) of states as well as the d spin and d orbital susceptibilities at vanadium sites are discussed. The results are compared with existing data for binary TiH 2 and VH 2.

2,3-diphosphoglycerate phosphatase activity of phosphoglycerate mutase: stimulation by vanadate and phosphate.

The binding of inorganic vanadate (Vi) to rabbit muscle phosphoglycerate mutase (PGM), studied by using 51V nuclear magnetic resonance spectroscopy, shows a sigmoidal dependence on vanadate concentration with a stoichiometry of four vanadium atoms per PGM molecule at saturating [Vi]. The data are consistent with binding of one divanadate ion to each of the two subunits of PGM in a noncooperative manner with an intrinsic dissociation constant of 4 X 10(-6) M. The relevance of this result to other studies which have shown that the Vi-stimulated 2,3-diphosphoglycerate (2,3-DPG) phosphatase activity of PGM has a sigmoidal dependence on [Vi] with a Hill coefficient of 2.0 is discussed. At pH 7.0, inorganic phosphate has little effect on the 2,3-DPG phosphatase activity of PGM, even at concentrations as high as 50 mM. Similarly, 25 microM Vi has little effect on the phosphatase activity. However, in the presence of 25 microM Vi, a phosphate concentration of 20 mM increases the phosphatase activity by more than 3-fold. This behavior is rationalized in terms of activation of the phosphatase activity by a phosphate/vanadate mixed anhydride. This interpretation is supported by the observation of strong activation of the phosphatase activity by inorganic pyrophosphate. A molecular mechanism for the observed effects of vanadate is proposed, and the relevance of this study to the possible use of vanadate as a therapeutic agent for the treatment of sickle cell anemia is discussed.

Interaction of vanadate with phenol and tyrosine: implications for the effects of vanadate on systems regulated by tyrosine phosphorylation.

The interaction of vanadate with phenol and N-acetyltyrosine ethyl ester in aqueous solution has been studied by using 51V nuclear magnetic resonance spectroscopy. On the basis of these studies, it has been concluded that vanadate rapidly esterifies the hydroxyl group of the aromatic ring to yield a phenyl vanadate. For phenol, the equilibrium constant for this reaction in terms of the convention that the activity of liquid water is 1.0 is K1 = [phenyl vanadate]/[phenol][vanadate] = 0.97 +/- 0.02. This value is well over 4 orders of magnitude larger than estimates from the literature for the corresponding equilibrium constant for the esterification of phenol by phosphate. The equilibrium constant for esterification of the phenol moiety of N-acetyltyrosine ethyl ester is similar to that for esterification of phenol. The relevance of these observations to processes that are regulated by reversible phosphorylation/dephosphorylation of tyrosine residues is discussed, in particular the insulin-like effect of vanadate.

1H and 51V nuclear magnetic resonance study of the precipitation of a titanium hydride phase with a face-centred cubic structure in the TiVH system

The precipitation of a titanium hydride phase with an f.c.c. structure has been studied for hydrides of TiV alloys by means of nuclear magnetic resonance. 1H and 51V wide-line spectra and the spin-lattice relaxation curve of the 1H nucleus suggest the presence of a titanium hydride phase containing no vanadium atoms. The origin of the precipitation and the phase diagram are discussed for the TiVH system.

A vanadium-51 nuclear magnetic resonance investigation of vanadate oxyanions in a lyotropic liquid crystalline bilayer system

The vanadate ion VO4H2− and two of its polymeric forms contained in mixed lyotropic mesophase prepared from potassium dodecanoate/n-alkyltrimethylammonium bromide in water have been investigated by 51V nuclear magnetic resonance spectroscopy. The results indicate that the tetrahedral ion interacts with the micellar surface through the hydrating water associated with this ion. The ion undergoes strong interactions with the carboxylate head group of the dodecanoate as well as with the headgroup of the alkyltrimethylammonium detergent, which of course is of opposite charge. The other two species investigated have previously been assigned by various workers to V2O74− and its cyclic dimer V4O124−. The results obtained here are not consistent with those assignments but rather with more symmetrical compounds such as the tetrahedral uncharged molecule V4O10 and a higher analogue of this, respectively. The nmr spectra from these latter species show that they undergo, at best, extremely weak interactions with the cationic surface but strong interactions with an anionic one. The behaviour of the quadrupole splittings observed is consistent with specific interactions to the carboxylate moiety. The observation of an induced chemical shift separation between vanadium resonances from the same highly symmetric species confirms that binding of this compound is very strong.

51V nuclear magnetic resonance in polycrystalline ferroelastic BiVO 4

Nuclear magnetic resonance studies on polycrystalline ferroelastic BiVO 4 indicate that the 51V electric field gradient asymmetry parameter is an order parameter in the ferroelastic transition. Using ∩ = A( T− T c ) B , B is found to be 0.48(5), in good agreement with earlier studies of this material. Near the phase transition above and below, the vanadium nuclear quadrupole coupling is constant with a value of 4.8(1) MHz.

Nuclear magnetic resonance study of the dihydride phase of the Ti-V-H system

We report the results of nuclear magnetic resonance (NMR) measurements of 1. (1) the line shape, the second and fourth moments and the spin-lattice relaxation of the 1H resonance, and 2. (2) the Knight shift and spin-lattice relaxation of the 51V resonance in the dihydride phase Ti 1− x V x H 2 (0.04 ⩽ x ⩽ 0.65). The X-ray and magnetic susceptibility data are also reported. A two-line system was observed in both the 1H and 51V NMR spectra for x = 0.65 indicating that the range of existence of the single dihydride phase in vanadium-rich dihydrides is much narrower than reported previously. Comparison of the experimental and theoretical values of the linewidth and the second and fourth moments of 1H NMR shows that the dipolar interaction is the main source of the line broadening. The temperature dependence of the 1H value T 1 T and the magnetic susceptibility in dilute vanadium hydrides is interpreted in terms of the transition from an f.c.c. phase to a face-centred tetragonal phase induced by the Jahn-Teller effect. Comparison of the values of vanadium Knight shifts in Ti 1− x V x H 2 with existing data for the titanium Knight shift in TiH 2 strongly suggests that the local densities of states at titanium and vanadium sites in the dilute vanadium hydrides Ti 1− x V xH 2 are different. Opposite dependences of the 1H and 51V spinlattice relaxation rates on the vanadium concentration support this suggestion.

51V and1H nuclear magnetic resonance in vanadium dihydride

Vanadium dihydride has been investigated by 51V and 1H nuclear magnetic resonance measurements. The 51V Knight shift when combined with other related results indicate that the observed Knight shift Kv=-(0.016+or-0.005)% is a result of the strong competition between a negative core polarisation contribution and a positive orbital one. The magnetic susceptibility, in turn, is determined mainly by enhanced d-band spin susceptibility. The proton Knight shift value of KH=-(0.023+or-0.005)% is interpreted in terms of direct contact interaction and assumed indirect polarisation of the hydrogen 1s band by d-like conduction electrons.

Variation of the sublattice magnetisation in antiferromagnetic GdVO4with temperature from the NMR of51V, I=7/2

Gadolinium vanadate (GdVO4) is a tetragonal paramagnet which enters an antiferromagnetic phase below TN=2.4955(5)K, with a simple two-sublattice structure in which the Gd3+ spins are aligned parallel to the tetragonal axis. The authors have studied the 51V (I=7/2) nuclear magnetic resonance between T=0.48K and 1.9K; in zero applied field the seven quadrupole split lines have a width of 0.8 mT and are very intense. The central line lies at a frequency of nearly 12.9 MHz at the lowest temperature, falling to approximately 10.0 MHz at 1.9K, and can be measured to 2 kHz. The frequency can be fitted to the formula nu / nu 0=1-0.2759(T/TN)2-0.1908(T/TN)4... with nu 0=13.03(3) MHz and this is assumed to represent the temperature dependence of the sublattice magnetisation for T<1.9K. The Mossbauer measurements of Cook and Cushion agree closely with their results and extend to the Neel point. A comparison is made with the variation of the sublattice magnetisation as determined by the technique of Hornreich and Shtrikman from measurements of the parallel susceptibility by Cushion et al; the agreement is found to be excellent.