Orthovanadates

Linalool can also be made from nerol and geraniol by the orthovanadate-catalyzed isomerization. Because linalool is lower boiling than nerol and geraniol, the isomerization can be mn under distillation conditions to remove the linalool overhead while continually adding nerol and geraniol to the distillation kettie for further isomerization (56). 

Determination of vanadium as silver vanadate Discussion. Vanadates are precipitated by excess of silver nitrate solution in the presence of sodium acetate after boiling, the precipitate consists of silver orthovanadate. The following reactions occur with a solution of a metavanadate ... 

It has been stated that the results obtained by precipitation of vanadate as silver orthovanadate Ag3V04 are not altogether satisfactory. Better results are obtained by precipitation at pH 4.5 as silver metavanadate AgV03 the precipitate is weighed after drying at 100-105 °C. 

Vanadate (sodium orthovanadate or peroxovanadate) exhibits insulin-like effects in vitro (activation of insulin receptor tyrosine kinase, PI 3-kinase, Akt) and in vivo (diabetic rats, humans). These effects can be explained at least in part by the inhibition of phosphotyrosine phosphatases which deactivate the INSR tyrosine kinase. 

Useful serine/threonine protein phosphatase inhibitors include microcystin-LR (which inhibits protein phosphatases 1, 2A, and 2C, and related enzymes) and /1-glycerophosphate. Sodium fluoride may also be employed. Sodium orthovanadate inhibits protein tyrosine phosphatases. 

The orthovanadate ion [V04]3- occurs only at very high pH. It is such a strong base that the first step in its protonation, forming [HV04]2 , is already complete at pH —12. When the pH is gradually lowered to —1 successive protonation ultimately leads to the formation of the pale yellow cationic species, usually formulated as V02... 

Orthovanadate(V) is a potent inhibitor of many phosphate metabolizing enzymes. In what ways might -mediated enzymes resemble or differ from PO -mediated enzymes N. D. Chasteen, J. K. Grady and C. E. Holloway, Inorg. Chem. 25, 2754 (1986). 

It is obvious that the narrow band thermoluminescence 8, 9) influenced Carl Auer von Welsbach in developing his mantle between 1884 and 1892, but as discussed in the next chapter, the optimized conditions for white gas-light rather involve another t cpe of excited states of cerium(IV). On the other hand, the cathodo-luminescence in narrow bands discovered by William Crookes and carefully studied by Urbain [12) corresponds to internal transitions in the partly filled shell. Thus, the excited state of 4/ europium(III) produces the red emission (important for colour television) in the orthovanadate [13) Yi Eux VO 4 and in the oxysulphide (74) by transitions to " Fz, and Fq. Certain... 

Only the NaCl region has been measured for magnesium and calcium orthovanadates (57) thus no data are available for either of the bending vibrations. Previously the IR spectrum of Ca3(V04)2 had been discussed with incorrect crystallographic data. An X-ray structural analysis has only recently been reported 54) which shows that in the lattice crystallographicaUy non-equivalent VO4 groups are present. Also in the case of Mg3(V04)2 it was not previously possible to differentiate between two possible site symmetries, C2 or Cj 51). A recently reported crystallographic study shows that the VO4 ions sit on C, sites (55). 

There is only little information available for orthovanadates with monovalent cations, as seen in Table 2. The IR spectra for Li3V04 and Li3V04 show clearly the bands associated with the VOl" polyhedron. The high value of vi for TI3VO4 is certainly an error. 

The IR spectra of orthovanadates with trivalent cations are also well known as seen in Table 3 59, 60). In the IR spectra of the orthovanadates from Ce to Gd only one band less is observed as predicted and in no case is the vn band observed to split. On the contrary, for LaV04 more bands appear than are predicted from site symmetry rules due to strong correlation field effects. For the same reasons the Raman spectrum of this compound is difficult to interpret 60). 

It is well known that vanadium can replace phosphorus or arsenic in the apatite lattice. Most of the orthovanadates which crystallize in this form have been investigated by IR and Raman spectroscopic techniques 62—64). The results of these studies are summarized in Table 4. The space group of all of these compounds is certainly C ft with the VO4 ion lying on a C site. Therefore, one should always find a nine band spectrum. Phosphorus apatites behave similarly, as seen in Ref. (65). 

Some orthovanadates which crystallize in the spodiosite lattice with the general formula M2VO4X have been spectroscopically investigated, but the spectra have not been fully discussed (66, 67). 

All the orthovanadates with zircon structure and formula LnV04 where Ln = Ce, Pr, Nd, Sm, Eu, and Gd, which have been studied up to now, have very similar spectra. The vs vibration which is predicted to spUt into two bands from the site symmetry treatment appears in the IR as one strong, broad band with a weak shoulder. On the other hand, Vi appears as a sharp band. CaCr04, which has the same structure, possesses a very similar spectrum. 

Vanadates For the isostructural strontium and barium ortho-vanadates the V-—0 vibrational frequencies decrease with increasing effective nuclear charge [51]. Similar orthovanadates with the apatite structure have for a given halogen ion decreasing frequencies of the VO4 ion in the order Ca>Sr >Ba ((62) and Table 4]. However, it is impossible to see any trend towards the tiivalent lanthanide cations (59). The IR bands are so wide in this case that small changes cannot be measured. 

Several interesting examples of the possibilities of this technique are provided by the systems Ba3(V04)2/Ba3(P04)2 and Sr3(V04)2/Sr3(P04)2 (775). Here, for the first time it was possible to determine clearly the position of the vi Ai) vibration for Ba- and Sr-orthovanadate and to prove that the two bands observed for pure orthovanadates can be treated as the two predicted components of the vsiFz) vibration.Thus, the mixed crystal studies also showed that the vi band lies between the two vs components and that it is not observed in the pure substance, not only because of its weaker intensity but also because it is overlapped by the very broad E component of the vs vibration. 

Table 1. IR data (in cm t) for orthovanadates with divalent cations... 

SELENOPHOSPHATE SYNTHETASE STARCH PHOSPHORYLASE SUCCINYL-CoA SYNTHETASE SUCROSE PHOSPHORYLASE TUBULIN.TYROSINE LIGASE ORTHOPHOSPHATE CONTINUOUS ASSAY Orthovanadate,... 

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