Vanadate complex formation

The automated spectrophotometric procedure [47] for the determination of total nitrogen in soils discussed in section 12.5.1.2 has also been applied to the determination of 0-0.75% phosphorus in soils. The method is based on the formation of the yellow molybdate-vanadate complex which has an absorption maximum at 420nm. 

Jaswal, J.S. and A.S. Tracey. 1991. Stereochemical requirements for the formation of vanadate complexes with peptides. Can. J. Chem. 69 1600-1607. 

Funahashi, S., K. Haraguchi, and M. Tanaka. 1977. Reactions of hydrogen peroxide with metal complexes. 2. Kinetic studies on the peroxo complex formation of nitrilo-triacetatodioxovanadate(V) and dioxo(2,6-pyridinedicarboxylato)vanadate(V). Inorg. Chem. 16 1349-1353. 

FIGURE 9.5 The formation constants (V04H2 + H2L V02L ) for various diethanolamine vanadate complexes are shown as a function of the pKj of the ligand. The solid line is provided as a guide and has no theoretical significance. The values are for 0.40 M KC1 solutions at room temperature and are taken from the work of Crans and Boukhobza [21]. 

The reactions discussed entail coordination of simple aquo ions such as Cu2+, Mg2+, etc. Metal-containing oxo-anions such as VO, MoO2 and WO2 also form stable complexes, especially with cis-diols13. These reactions are quite rapid vanadate, for example, has bimolecular complex formation rate constants on the order of 104 M 1 s"114). 

All three oxidation states of vanadium found in living systems form stable complexes with a wide variety of mono- and multidentate ligands. Ions of these three oxidation states form complexes rapidly they are therefore labile. As mentioned on page 142, bimolecular complex formation rate constants for monomeric vanadate species are approximately 104 M V1. Water exchange rate constants are 280s 1 for V3+, and 500s 1 for V02+ at 25 °C52. ... 

The reduction of vanadate can be slowed, as vanadium(V) is efficiently coordinated by multifunctional thiols such as dithiothreitol, (H4dtt) (Figure 2.17). Reduction of vanadate by dithiothreitol occurs within ca 90 min, allowing for the identification of the intermediately formed vanadate complexes, such as V2(Hdtt), with a formation constant [Equation (2.16)] of = 10 at pH 7. Since this dinuclear complex gives rise to two resonance signals in the NMR spectrum, there are two distinct vanadium sites. A possible structure is depicted in Figure 2.17.P ]... 

In the vanadate-dependent haloperoxidases, an 0x0 group of the vanadate centre is exchanged for peroxide prior to 0x0 transfer, referred to as an activation of peroxide, and 0x0 transfer to a substrate such as halide or R2S is afforded by nucleophilic attack of the substrate to the activated (hydro)peroxide as shown in Figure 4.18. Generally, peroxovanadium complexes can thus be considered to model the active catalytic site in the haloperoxidases. A mechanism for the peroxovanadium complex formation as depicted in Equation (4.14), proposed by Pecoraro and co-workersl on the basis of... 

New results from MO calculations on the VO ion are in better agreement with experimental data than those obtained previously/ There have been reports of vanadate reduction by ascorbic acid/ and complex formation in aqueous solution/ ... 

Complex formation of vanadate ions by edta and (15) has been studied by the... 

Goda T, Sakurai H, Yashimura T. 1988. Structures of oxovanadium(IV)-glutathione complexes and rednctive complex formation between glutathione and vanadate (+5 oxidation state). Nippon KagakuKaishi 654—661. 

Vanadium. The interactions betwen vanadate, uridine, and imidazole have been studied using C, 0, and V NMR spectroscopy. Complex formation between HSCH2CHOHCHOHCH2SH and vanadate has been investigated using V NMR spectroscopy. V NMR spectroscopy has been used to study the interaction of vanadate oligomers with a 90 kDa heat shock protein, Hsp90. The interaction of vanadate with aquatic humic substance has been studied using V NMR spectroscopy. ... 

The colour sequence already described, for the reduction of van-adium(V) to vanadium(II) by zinc and acid, gives a very characteristic test for vanadium. Addition of a few drops of hydrogen peroxide to a vanadate V) gives a red colour (formation of a peroxo-complex) (cf. titanium, which gives an orange-yellow colour). 

Uptake by FeoB is inhibited by the energy poisons 1 111 FCCP, DCCD, and vanadate. Both Fe3+ and Fe2+ serve equally well as iron sources, but Fe3+ provided as a citrate salt is immediately reduced to Fe2+, as demonstrated by the formation of the magenta-coloured Fe2+-ferrocine complex. 

The limit test for phosphate is based upon the formation of ayellow colour reaction with molybdovanadic reagent (combination of ammonium vanadate and ammonium molybdate) in an acidic medium. However, the exact composition of the molybdovanadophosphoric acid complex is yet to be established. 

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