Vanadate decavanadates

It is evident from Fig. 22.2 that only in very dilute solutions are monomeric vanadium ions found and any increase in concentrations, particularly if the solution is acidic, leads to polymerization. nmr work indicates that, starting from the alkaline side, the various ionic species are all based on 4-coordinate vanadium(V) in the form of linked VO4 tetrahedra until the decavana-dates appear. These evidently involve a higher coordination number, but whether or not it is the same in solution as in the solids which can be separated is uncertain. However, it is interesting to note that similarities between the vanadate and chromate systems cease with the appearance of the decavanadates which have no counterpart in chromate chemistry. The smaller chromium(VI) is apparently limited to tetrahedral coordination with oxygen, whereas vanadium(V) is not. 

Vanadate-catalyzed photocleavage of the Ca -ATPase was observed after illumination of sarcoplasmic reticulum vesicles or the purified Ca -ATPase with ultraviolet light in the presence of 1 mM monovanadate or decavanadate [104]. Two sites... 

In a.p. ZV(a) and ZV(i) samples, broad bands arising from hydrated vanadates were detected in the 800-1100 cm-i region. Metavanadate-like species (band at 920 cm i) prevailed on ZV samples with V-content < 1.5 atoms nm 2 and decavanadates (bands at 850-880 cm i and 960-990 cm- ) in the range 1.5-3 atoms nm 2. A.p. ZV(acac) samples showed bands from CH3 and C=0, suggesting the adsorption of VO(acac)2 as such (spectra not reported). 

Other forms of vanadium have been implicated in the stimulation of the plasma membrane vanadate-dependent NAD(P)H oxidation reaction. Decavanadate has been shown to be a more potent stimulator of the vanadate-dependent NADH oxidation activity than added orthovanadate [30,31], Interestingly, decavanadate reductase activity has been found to be an alternative activity of an NADP-specific isocitrate dehydrogenase [32], Diperoxovanadium derivatives have also been shown to be involved in this type of reaction [33,34], Decavanadate may play a role in the biological role of vanadium, as it is found in yeast cells growing in the presence of orthovanadate [8] and is a potent inhibitor of phosphofructokinase-1, the control step of glycolysis, and other metabolic reactions [35],... 

The role of these interesting plasma membrane-dependent, vanadate-stimulated NAD(P)H oxidation reactions in cellular metabolism remains to be elucidated, although multiple interactions with cellular metabolism and components are possible including interactions with xanthine oxidase and lipid peroxidation [24], Decavanadate has been shown to enhance cytochrome c reduction [31], and cytochrome c release from mitochondria is associated with initiation of apoptosis. Perhaps the reduced cytochrome c is more readily released from the mitochondria. With increasing emphasis on the redox properties of vanadium being important in its pharmacological effects, it is quite possible that these reactions, either protein dependent or not, may play a role in therapeutic actions of vanadium. 

Five-coordinate pentavalent vanadium has never been actually demonstrated for any enzyme, nor is the state of protonation of the bound vanadate known. While monomeric orthovanadate, or one of its protonated forms, appears to be the species responsible for enzyme inhibition, in a few instances the decavanadate may be involved27,28. In general, caution must be exercised when attributing inhibition to a particular species of vanadate. Which vanadate species are present in solution depends markedly on pH and on concentration and, moreover, equilibrium is usually only slowly achieved. 

Sodium and potassium vanadates. The authors [68] investigated mechanochemical synthesis of sodium and potassium decavanadates starting from V Oj and conesponding carbonates or metavanadates. They have shown that varying the initial ratio of components and the time of mechanical activation of the mixtures, the synthesis of different vanadates, including decavanadates, occurs, characterized by high rate of reactions. 

Two metastable aqueous polyvanadates have been identified. Acidification of neutral vanadate solutions yields a solution with the expected V and O NMR spectra of the decavanadate anion and with additional resonances (two V, 1 12 three O, 1 1 1) that decay with a half-life of 80 min at 25 °C. The spectra are consistent with either a protonated a-Keggin anion or its e-isomer, [II12V13O40]3, 8 9 Reaction of V02+ with 50% H202 and subsequent treatment with tetramethyl-ammonium ion leads to black (NMe -jI I6[V, 5O4J in 36% yield. The anion has a trans bicapped Keggin structure analogous to that of [PV 4042]9 (see below) and has the expected four-line 51V NMR spectrum (1 2 8 4). At pH 3.5, solutions of the anion are stable for a week at room temperature, and exhibit an initial reduction potential of 0.23 V vs. Ag/AgCl. Acidification leads irreversibly to VO2+.X0... 

Acidification of vanadate(V) solutions yields yellow solutions of decavanadate n = 0-3, depending on the pH) or, for pH values <2, the colourless hydrated dioxovanadium monocation, [V02(H20)4]+. If these V -containing solutions are reduced, gradual formation of vanadyl (blue), V + (green) and V + (violet)... 

Other condensed vanadates, e.g. trivanadate, hexavanadate, dodecavanadate and pentadecavanadate, are known, but play minor roles in the aqueous systems (tri-and hexavanadate) or are thermodynamically unstable with respect to decavanadate (polyoxovanadates with more than 10 vanadium centres). An interesting case from the structural point of view is the tridecavanadate [Hj2Vi304q], included in Table 2.3 I 1... 

Vanadium(V), which does not contain d electrons, obviously is restricted to intra-ligand and LMCT absorptions. Simple compounds such as vanadate are colourless, because the LMCT bands lie in the UV region. Decavanadate, and also vanadate-dependent haloper-oxidases (which contain vanadate additionally coordinated to a side-chain imidazole of the protein matrix), are yellow, because the LMCT tails from the UV into the violet range. More complex vanadium(V) complexes can be very colourful when the LMCT... 

V NMR spectroscopy has revealed that the sub fragment 1 of the muscle protein myosin interacts with the various vanadates present at sufficiently high concentrations, preferentially so with tetravanadate " ] and decavanadate " ] (see the previous section). If the myosin subfragment 1 is irradiated with UV light in the presence of vanadates, cleavage occurs and vanadate is reduced to vanadyl, which is not active as an energy transmitter in photo-cleavage. ]... 

Vanadate-induced photo-oxidation of serine and subsequent cleavage of the protein were also observed for ribulose-l,5-diphoshate carboxylase/oxygenase from spinach leaves. Activity could be restored by treatment with sodium boronate, which reduces formylglycine back to serine.Similarly, vanadate-induced UV oxidation of threonine (Thr353, at the phosphorylation site) in the sarcoplasmatic reticulum (SR) Ca +-ATPase can be restored by reduction with Na[BH4]. For monovanadate to become catalyti-cally active it is essential that Mg + and ADP bind to the phosphorylation site of the enzyme in its Ca +-receptive conformation, suggesting the formation of an active MgADPV as proposed for the myosin fragmentation. In contrast, decavanadate catalyses the photo-oxidation of a serine (Seri 86) residue in the SR ATPase in the absence of ADP.P bl... 

As an example, gelation of vanadate systems depends on the solution concentration. Decavanadic acid, [Vio028-x(OH)x] , predominates below 10 molL At higher concentrations, polymeric species are observed (Mw = 2xl0 ), and gelation occurs if the vanadium concentration exceeds 0.1 mol Decavanadic acid and fibrous polymeric species coexist at the gel point. ... 

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