Vanadyl-phosphate complexes

Evidence for the formation of vanadyl-phosphate complexes in aqueous solution relevant for physiological systems has been obtained from electron paramagnetic... 

This model study confirmed bone uptake of vanadium(IV) compounds could lead to formation of a highly stable vanadyl-phosphate complex, and that complex degradation must take place prior to incorporation of the vanadyl ion into the bone matrix. HYSCORE spectroscopy was shown to be very powerful for the deconvolution of complicated ID ESEEM spectra without the 2D pulse sequence, researchers were only able to confirm the presence of nuclei [71]. 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

Correlation of the observed rates with the concentrations of the substrate species (10) indicates that the metal ion does not catalyze the hydrolysis of the monoanionic form of salicyl phosphate. Combination of the monoanionic form of the substrate with the vanadyl ion would result in an unreactive complex having a neutral carboxyl group. Shift of the proton to the phosphate group could not take place in accordance with the requirements of the general reaction mechanism illustrated in Figure 3. Thus the vanadyl ion would be expected to catalyze the hydrolysis of only the di- and trinegative forms of the substrate. 

Figure 6. Probable structures of vanadyl complexes of salicyl phosphate...

Vanadate is reduced in red cells to vanadyl ion by intracellular glutathione following uptake through a phosphate transport system.1072,1073 Free vanadyl is normally unstable with respect to oxidation, but appears to be stable when complexed with intracellular proteins or smaller molecules.1074 Vanadyl ion is a relatively powerful inhibitor of (Na+, K+)-ATPase. For pure fractions of the enzyme, inhibition was nearly complete at less than 5 pmol dm-3 vanadyl ion.1075 The state of vanadyl ion at pH 7 is somewhat uncertain, but may involve a hydroxylated species. Vanadyl ion also inhibits alkaline phosphatase more effectively than does vanadate.1076... 

Vanadyl and copper(n) ions catalyse the /J-elimination reaction of O-phospho-threonine in the presence of pyridoxal.429 Equilibrium spectroscopic studies of the threonine-metal ion-pyridoxal system have identified a metal-ion complex of the amino-acid-pyridoxal Schiff base. The catalytic effect of the metal is ascribed to its electron-with drawing effecCIt was suggested that the specific catalytic effect of Cu2 + and V02+ arises from their reluctance to co-ordinate the phosphate in an axial position. Other metal ions such as nickel can also form the Schiff base complex but probably stabilize the phosphothreonine system by chelate formation. 

Figure 3. Three basic strategies for the incorporation of multiply bonded metal-metal guest species into vanadyl and zirconium phosphate host layers, (a) The direct intercalation of solvated M—— M cores into the native layered phosphate host structure, (b) Incorporation of M—— M complexes with ancillary ligands containing a Lewis basic site, (c) Coordination of M—— M cores with ligands provided from modified phosphate layers.

Vanadate enters cells by an active transport process that depends on the structural and electronic similarity to phosphate ion transport, since the isoelectronic species, HZ04 and H2Z04, are dominant species for Z = P or V at physiological pH. Neutral vanadyl complexes such as... 

Speciation of the pyrimidone complex 19 in blood as revealed by EPR and NMR, based on ref. 36a. Extra refers to blood serum, intra to the intracellular medium of red blood cells. 21a enters the cells by diffusion and vanadate through phosphate channels. VO + stands for any vanadyl complex formed with cytosolic constituents after intracellular reductive degradation of 21a or reduction of vanadate. 

Analysis of the principal values of the chemical shift tensor derived from the solid-phase P n.m.r. spectrum of 5 -AMP adsorbed on Zn +-exchanged bentonite clay shows that the zinc ion is co-ordinated directly to an oxygen atom of the phosphate. Multinuclear n.m.r. studies, including P n.m.r., have been used to investigate the nature and co-ordination pattern of the complexes formed by ATP with aluminium(m) ions and with vanadyl ions at different pH values. At least four different complexes could be distinguished with Al + ions, and three with VO + ions. The perturbation of the P n.m.r. spectrum of ATP, bound in the high-affinity ATP-binding site of nitrated... 

The 1 1 dipyridyl-Cu(II) chelate was found to have no catalytic activity toward the hydrolysis of [II] and [III], This result is interpreted as being due to the formation of a mixed ligand chelate compound in which the carboxylate group is bound to the metal ion and is thus prevented from attacking the phosphate group, A similar interpretation is offered for the failure of the 1 1 Cu( II)-N-hydroxyethylethylenediamine chelate to show catalytic activity. However, the 1 1 vanadyl complexes of 3,5-disulfopyrocatechol, 5-sulfo--8-hydroxyquinoline, and 5-sulfosalicylic acid which do not form stable mixed ligand chelates with salicyl phosphate, were found to have considerable catalytic activity. 

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