Vanadium compounds, peroxovanadates

Experiments on the proliferative effects of vanadium compounds are not always clear. Results on the effect of vanadium compound administration on erythropoiesis, or development of red blood cells, have been mixed. A recent report has shown that orthovanadate stimulated erythropoiesis by stimulating the maturation of red blood cell precursors [78], Peroxovanadate addition is reported to stimulate neurite outgrowth in PC12 cells [79], whereas vanadate causes growth inhibition in these cells [70],... 

Inorganic (top row) and organic (bottom row) vanadium compounds which are of (historical) interest as insulin mimics (sodium) vanadate (3) vanadyl (sulfate) (4) peroxovanadate (mixtures of vanadate and H2O2) (5) a peroxo-picolinato complex (6) bis(maltolato) complexes 7 R = CH3 BMOV = [VO(ma)2] = C2H5 BEOV 7b R = iPr 7c and the allixinato complex 7d and... 

Many peroxovanadates have potent insulin-mimetic properties [1,2]. Apparently, this functionality derives from the ability of these compounds to rapidly oxidize the active site thiols found in the group of protein tyrosine phosphatases that are involved in regulating the insulin receptor function [3], The discovery of vanadium-dependent haloperoxidases in marine algae and terrestrial lichens provided an additional stimulus in research toward obtaining functional models of peroxidase activity, and there is great interest in duplicating the function of these enzymes (see Section 10.4.2). 

Studies of the oxidation of organic sulfides with amino acid-derived ligands in acetonitrile revealed very little difference between the mechanism of their oxidation and that of halides, except for one major exception. Despite the fact that acid conditions are still required for the catalytic cycle, hydroxide or an equivalent is not produced in the catalytic cycle, so no proton is consumed [48], As a consequence, there is no requirement for maintenance of acid levels during a catalyzed reaction. Peroxo complexes of vanadium are well known to be potent insulin-mimetic compounds [49,50], Their efficacy arises, at least in part, from an oxidative mechanism that enhances insulin receptor activity, and possibly the activity of other protein tyrosine kinases activity [51]. With peroxovanadates, this is an irreversible function. Apparently, there is no direct effect on the function of the kinase, but rather there is inhibition of protein tyrosine phosphatase activity. The phosphatase regulates kinase activity by dephosphorylating the kinase. Oxidation of an active site thiol in the phosphatase prevents this down-regulation of kinase activity. Presumably, this sulfide oxidation proceeds by the process outlined above. 

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. 

Three general classes of vanadium-containing compounds are of interest for their utility as insulin-mimetic agents (1) inorganic vanadium salts, both anionic (vanadates [V04] ) and cationic (vanadyl VO +) (2) complexes resulting from combination of vanadium(V) and hydrogen peroxide(s) (mono- and di-peroxovanadates, [V0(02 )(H2O)2(L-0f ( =0,1,2) and [V0(02)2 (L-Oj ( = 1,2, 3, and L = e.g., bipyridil, oxalate, phenanthroline, picoli-nate), and (3) chelated vanadium(IV) complexes (Scheme 5.8) [160], Vanadyl,... 

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