Vitro Studies of Vanadium Speciation and Redox Chemistry

IN VITRO STUDIES OF VANADIUM SPECIATION AND REDOX CHEMISTRY 

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. 

Despite the utility of the additivity rule, without supporting data occasionally it can be difficult to narrow down possible solution structures to one possibility based on EPR data alone. EPR is frequently paired with other techniques, most often po-tentiometry [41,42,44-48], to detect the number of species and to use the additivity rule to give insights into the first coordination sphere donor atoms of the moieties in solution. This information can then be used in the fitting of potentiometric titration curves, which reports on the absolute and relative thermodynamic stabilities of species in solution. EPR spectroscopy is also used as a complementary technique to V NMR [49-51], allowing for characterization of both V(IV) and V(V) complexes in solution. Other techniques include UV-Vis spectroscopy [41,44,46], circular dichroism [48] and neutron activation analysis [52-56]. 

Solution Structures of Insulin-Enhancing Vanadium(IV) Complexes 

Crans and coworkers utilized EPR, V NMR, and absorption spectroscopy to observe the solution chemistry of their series of insulin-enhancing complexes based on the acetylacetone ligand (acac) [46]. Solutions of VO(acac)2 were found to be a mixture of three species, labeled A, B, and C. The B species was observed as a weak shoulder on the resonances attributed to the dominant A EPR signals. Over time, the EPR signal of A(-l-B) converted to C, and this conversion was studied by EPR and absorption spectroscopy. NMR was used to quantify any oxidation of the complex to a V(V) derivative. Quantification of the EPR signal showed fliat 90% of the total vanadium was present as A and B, while 10% was EPR silent as a dimer. After 24 days, the solution components had changed to 25% A(+B) and 65% C. This transformation was later shown to be a pH-dependent process. Preparation of VO(acac)2 at a reduced pH produced flie C-type EPR spectrum quantitatively [59]. 

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