Vanadate speciation

Traditionally, the principal tools for the study of vanadate speciation in aqueous solution were UV/vis and electrochemistry. Unfortunately, the complex chemistry associated with vanadate has rendered much, but certainly not all, of the earlier work obsolete. The reaction solutions often contained numerous products that, a priori, could not be specified. Properly describing the chemistry was somewhat like doing a jigsaw puzzle without knowing what the pieces looked like or how many there were. Only with the advent of 51V NMR spectroscopy in high field NMR spectrometers was there a tool in place that allowed a coherent picture of V(V) chemistry to be fully developed. The combination of potentiometry with NMR spectroscopy has proven a certain winner. Additionally, x-ray diffraction studies have provided an invaluable source of information, but it is information that, in all cases, must be used with extreme caution when attempting to describe the chemistry in solution. 

Elvingson, K., A.G. Baro, and L. Pettersson. 1996. Speciation in vanadium bioinorganic systems. 2. An NMR, ESR, and potentiometric study of the aqueous H+-vanadate-maltol system. Inorg. Chem. 35 3388-3393. 

Elvingson, K., M. Fritzsche, D. Rehder, and L. Pettersson. 1994. Speciation in vanadium bioinorganic systems. 1. A potentiometric and 51V NMR study of aqueous equilibria in the H+-vanadate(V)-L-a-alanyl-L-histidine system. Angew. Chem., Int. Ed. Engl. 48 878-885. 

The active sites of these enzymes can have a nitrogen ligand, usually as histidine (acid phosphatases and some protein phosphatases), a nucleophilic serine residue (alkaline phosphatases), a cysteine residue in which the thiol group can form a covalent species with the phosphate ester (protein phosphatases), or an aspartate-linked phosphate (plasma membrane ion pumps). The inhibitory form of vanadium is usually anionic vanadate V(V), but cationic vanadyl V(IV) has also shown strong inhibition of some types of phosphorylase reactions. Above neutral pH, speciation of vanadyl ions produces anionic V(IV) species capable of inhibition of enzymes in the traditional transition-state analogue manner [5],... 

The speciation and stabihty constants of the Cu(n), Ni(II), Zn(II) and VO(II) complexes of mimosine (9) were determined . The Cu(II) and VO(II) complexes adopt a coordination chemistry analogous to the those of maltol (discussed below). The Zn(II) complex of the deprotonated mimosine analogue 48 has a 1 2 distorted square-pyramidal structure with trans configuration and a water molecule at the apex (49) . Pb(II) forms a 1 2 complex with 48, which further interacts with one enolate of an adjacent Pb(II) center via Pb-0 bridges (2.64 A) to yield a dinuclear Pb complex (50). Vanadate (with a -1-5 oxidation state on V) forms with 48 the 1 1 [V02(48 )(0H)(H20)] and 1 2 [V02(48 )2] complex anions, as shown by potentiometry, cyclic voltammetry and NMR and EPR spectroscopies, with V NMR peaks at —502 and —476 ppm, respectively, similar to the maltol complexes (peaks at —509 and —496 ppm) discussed in the next section. 

The aqueous speciation of the vanadate-maltol system has been characterized, describing complexes which chelate the vanadium(V) in a bidentate manner (16,17).131 The formation of the corresponding vanadium(V) complex of 2-hydroxycyclohexanone (analogous to 17) has been proposed as an intermediate in the oxidation of cyclohexanone.132 Squaric acid forms a series of mono- and dinuclear complexes with vanadium(IV) and (V).133 Subtle differences in the structure are observed depending on the oxidation state of the vanadium, and are induced by the rigid ligand structure. 

Speciation diagrams for aqueous vanadate solutions (mole fraction Xy vs pH) for c(V) = 1 mM (left) and 1 p,M (right calculated), /Na(ci) = O.lSmM, 25°C. V+ = [V02(H20)4]+ for the notations of vanadates used in the diagram on the left, see Table 2.3. Species comprising less than 3% are not shown. Courtesy of L. Pettersson, Umea University, Sweden. 

Structural formulae of L-alanyl-L-serine (ala-ser) and L-alanyl-L-histidine (ala-his) in their zwitterionic representation, and proposed structures of the predominant vanadate complexes. For the ala-ser complexes, see also the speciation diagram in Figure 2.16. 

CD can be a versatile tool in speciation analyses of aqueous solutions containing vanadate or VO + and chiral ligands.P l and for assigning a chiral compound the correct configuration. Figure 3.19 illustrates the latter situation for the optical antipodes of the complex [VO(naph-tyr)], where naph-tyr is the Schiff base formed from o-hydroxynaphthaldehyde and tyrosine. 

The speciation in the aqueous system containing vanadate, H2O2 and L-alanyl-L-histidine (ala-his) as a ligand modelling the VFlPOs active site histidine (see also Figure 2.15) has... 

The (metabolic) pathways of dietary vanadium, such as vanadate [H2V04], can be expressed as illustrated in Scheme 5.1 after oral uptake, vanadate reaches the gastrointestinal tract, where it is partially reduced and precipitated to vanadyl (VO ) hydroxides, which are excreted with the faeces. Another portion is absorbed and circulated in the blood, where it undergoes redox speciation and complexation by the serum proteins transferrin and albumin. Vanadate and vanadyl are finally incorporated into cells, mainly those of the liver, spleen and kidney. Excretion is achieved via the urine. Part of the vanadium is taken up by bones, where the mean retention time is comparatively long. 

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