Vanadium compounds redox activity

The insulin-enhancing activity of vanadium compounds is likely to be related to their interactions with cellular redox chemistry and ROS formation, in addition to direct inhibition of PTP-1B and other protein phosphatases as a transition-state analogue [100], Differences in the effects of V (III, IV or V)-dipicolinic acid complexes on blood glucose and absorption of V into serum after chronic oral admin-... 

This book does not follow a chronological sequence but rather builds up in a hierarchy of complexity. Some basic principles of 51V NMR spectroscopy are discussed this is followed by a description of the self-condensation reactions of vanadate itself. The reactions with simple monodentate ligands are then described, and this proceeds to more complicated systems such as diols, -hydroxy acids, amino acids, peptides, and so on. Aspects of this sequence are later revisited but with interest now directed toward the influence of ligand electronic properties on coordination and reactivity. The influences of ligands, particularly those of hydrogen peroxide and hydroxyl amine, on heteroligand reactivity are compared and contrasted. There is a brief discussion of the vanadium-dependent haloperoxidases and model systems. There is also some discussion of vanadium in the environment and of some technological applications. Because vanadium pollution is inextricably linked to vanadium(V) chemistry, some discussion of vanadium as a pollutant is provided. This book provides only a very brief discussion of vanadium oxidation states other than V(V) and also does not discuss vanadium redox activity, except in a peripheral manner where required. It does, however, briefly cover the catalytic reactions of peroxovanadates and haloperoxidases model compounds. 

The vanadium silicalites (with MFI and MEL stmcture) are active oxidation catalyst in gas and liquid phase reactions [180]. As for the titanium silicalites, only the ftamework associated vandium exhibits redox properties [181]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [182]. The catalytically active vanadium species is speculated to be located in non-tetrahedral positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed stmcture of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with SiA ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with V MAS-NMR measurements) that the V sites are isolated in the lattice. 

A brief introduetion to the spectroscopic properties of vanadium(IV) is provided, along with identification of key concepts crucial to the successful application of EPR methods to research questions in the field. Specific discussions of EPR spectroscopies, sueh as continuous wave and pulsed methods, have been recently reviewed [19,20]. The discussion of the use of VO " ions as spin probes by N.D. Chasteen remains a seminal review paper in the field for its complete overview of the speetroseopie properties of the ion and application to biological systems [21]. Numerous reviews on the biochemmical activity of vanadium compounds are available [22-26], and the rationales for development and synthetic routes to new vanadium compounds are also reported [27,28] A reeent study evaluated the in-vitro activity of 22 compounds currently studied in the literature [29]. In this review, EPR s role in the delineation of structure, chemistry, and in-vivo behavior of vanadium compounds will be discussed. The first seetion of die review foeuses on the use of EPR for the description of solution structures, ternary eomplex formation, and redox chemistry of vanadium(IV) and (V) compounds, wifli the general flieme of highlighting in-vitro studies. This seetion is followed by a diseussion of die application of EPR for in-vivo investigations of vanadium cellular uptake, pharmacokinetics, and in-vivo coordination structure. 

The favorable spectral properties of the vanadyl ion have led to its predominant role as a method for the structural characterization of vanadium(IV) antidiabetic complexes. While crystal structures are available for several compounds [11,40-43], flic behavior and structure(s) of the complex in solution must also be elucidated in order to (i) accurately predict interactions in the body, (ii) predict stability to endogenous chelating agents, (iii) determine number and nature (i.e., pH dependence) of structural isomers and solution complexes, and (iv) evaluate the redox activity of die complex. EPR spectroscopy has been used for all of these purposes. 

Most other peroxidases are Fe-heme-containing systems, which function as two-electron redox catalysts (Scheme 8). Dihydrogen peroxide oxidizes the Fe-heme moiety by two electrons, forming Compound 1 (a heme + FeIV=0 species) [97], Compound 1 oxidizes the halide ion, forming the active halogenating species. This mechanism cannot be operative in V-BrPO because the vanadium is already in its highest accessible oxidation state. Moreover, native V-BrPO does not oxidize bromide without an acceptable peroxide source. However, it should... 

At this point it is interesting to compare the evolution of propylene adsorption over catalysts with different surface acid characteristics, i.e. a MoVTeNbO catalyst (active and selective in the partial oxidation of propane to acrylic acid), an alumina-supported vanadium oxide (active in the ODH of propane to propylene), or a MoVNbO mixed oxide (active in the oxidative transformation of propane to propylene and acetic acid). The final products observed in each case were related to the characteristics of the adsorbed intermediates (Fig. 24.7) (i) a ir-allylic compound, interacting with a redox site intermediate in the selective oxidation of... 

---