Vanadates protonation

Vanadate, dioxybis(oxamato)-bond-length ratios, 1,57 Vanadate, heptacyano-potassium salt structure, I, 72 Vanadate, hexafluoro-dipotassium salt history, I, 21 potassium salt history, 1,21 tripotassium salt history, 1,21 Vanadate, pentachloro-stereochemistry, 1,40 Vanadate, pentafluorooxy-stereochemistry, I, 50 Vanadates biochemistry, 3,456 calcium/magnesium ATPase inhibition, 6, 567 competition with phosphates physiology, 6,665 protonation, 3,1026 sodium pump, 6, 557 in uranium purification from ore, 6, 899 Vanadates, hexafluoro-, 3. 482,531 Vanadates, oxoperoxo-, 3,501 Vanadates, pentacarbonyl-, 3, 457 Vanadium biology, 6,665 determination, 1. 548 extraction... 

ATPase also catalyzed a passive Rb -Rb exchange, the rate of which was comparable to the rate of active Rb efflux. This suggested that the K-transporting step of H,K-ATPase is not severely limited by a K -occluded enzyme form, as was observed for Na,K-ATPase. Skrabanja et al. [164] also described the reconstitution of choleate solubilized H,K-ATPase into phosphatidylcholine-cholesterol liposomes. With the use of a pH electrode to measure the rate of H transport they observed not only an active transport, which is dependent on intravesicular K, but also a passive H exchange. This passive transport process, which exhibited a maximal rate of 5% of the active transport process, could be inhibited by vanadate and the specific inhibitor omeprazole, giving evidence that it is a function of gastric H,K-ATPase. The same authors demonstrated, by separation of non-incorporated H,K-ATPase from reconstituted H,K-ATPase on a sucrose gradient, that H,K-ATPase transports two protons and two ions per hydrolyzed ATP [112]. 

Protonation, Oligomerization, and Condensation Reactions of Vanadate(V), Molybdate(VI), and Tungstate(VI)... 

PROTONATION, OLIGOMERIZATION, AND CONDENSATION REACTIONS OF VANADATE(V), MOLYBDATE(VI), AND TUNGSTATE(VI)... 

It has been suggested that an increase in the coordination number of vanadium from 4 to 5 already takes place in the second protonation step, i.e. when [H2V04] is formed (21). For reactions (1) and (2), however, the protonation constants and thermodynamic parameters are comparable with those reported for P04 and As04 , providing firm evidence that reaction (2) is not accompanied by incorporation of water in the vanadate ion (15, 17). Further, the estimated thermodynamic quantities for reaction (6), AH° = -39 kJ/mol and AS0 = —51 J/(mol K), obtained by extrapolation from the experimental values for reactions (1) and (2) and those for the three protonation steps of P04 and As04 , are not typical of a simple protonation reaction (17). For such a reaction the entropy change is normally a positive quantity often amounting to 100 50 J/(mol K) and the enthalpy... 

Although no polyanions with nine metal atoms have so far been found in either vanadate(V) or molybdate(VI) solutions the [Mo4V5 O27]3- ion as well as its protonated form, [HMo4V5027]4 with pKa 2.5, have been identified in the mixed solutions (162, 165). 

Both mechanisms discussed above involve an acidic species, and differ primarily in whether that species behaves as a Bronsted or Lewis acid. The vanadic acid approach treats the Y species as a proton donor whereas the oxygen abstraction implies electrophilic attack by V. In either case, it is easy to see how oxygen rich, basic oxides such as MgO function as Y passivators. 

Deng et al. (1993,1998) and Ray et al. (1993) have used V " as an analogue of in an attempt to model the transition state of the hydrolysis of phospho-diesters by ribonuclease A since is assumed to adopt the expected five coordination more readily than Examination of the vibrational spectrum of the vanadate analogue indicates that the terminal V-O bonds are only slightly weakened when bonded to the protein. A quantitative bond valence analysis effectively rules out two proposed mechanisms that involve the protonation of the terminal O atoms. 

Table 1 Comparison of Protonation Constants for Phosphates and Vanadates ...

Other metals such as vanadium have more complicated chemistry. The vanadate ion, VO4-. exists in extremely basic solution (Fig. 16.9). Under very dilute conditions as the pH is lowered, protonation occurs to give monomers ... 

As a result of the short relaxation times of most vanadate species, 51V 2D exchange spectroscopy is limited to dynamic processes that occur within a few tens of milliseconds. This timescale is conveniently lengthened to 1 sec or longer in cases where proton (or other) NMR spectroscopy can be employed, for instance, in ligand exchange reactions. 

Most of the normally encountered applications of proton and carbon NMR spectroscopy have been applied in studies of vanadate complexes, complexation reactions, equilibria, and kinetics. Carbon-13 studies of the influence of complexation... 

There is very little known about the neutral vanadate species, V04H3, because it is, at best, only a minor component in aqueous solution [35], The initial protonation of V04H2 at about pH 3 is accompanied by a second protonation that cannot be separated from the first. The result is the formation of a cationic species. Thermodynamic and spectroscopic evidence [33] suggests that formation of this compound is accompanied by incorporation of water to form the octahedral derivative, V02(H20)4+, commonly referred to as V02+. Theoretical calculations also support the assignment of tetrahedral coordination to the monoanion and octahedral geometry to the cationic form of vanadate [36],... 

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