We
hope the above discussion has clearly shown the importance of model
studies for a deeper understanding of some fundamental aspects related
to the relatively complex biochemistry of vanadium. These studies have
revealed valuable information concerning vanadium metabolism, toxicity,
detoxification and catalytic activity. They have also extended the
coordination chemistry of vanadium, especially that of the vanadium(IV)
and vanadium(V) oxocations, as a direct consequence of the use of a
variety of new ligand types. Nevertheless, there are a number of very
important and interesting problems that remain open.
A
better understanding of vanadium metabolism requires new efforts to
comprehend its transport in both anionic and cationic forms, and for
the initial reduction processes of vanadium(V) to VO2+ in the gastrointestinal tract. It would also be interesting to obtain crystalline VO2+
complexes with glutathione and related ligands and to explore further
the interaction of this oxocation with serum albumin, transferrin and
ferritin. A better chemical characterization of excreted vanadium
species also seems very important.
More
detailed speciation studies, including the determination of stability
constants, are necessary for a wider characterization of the VO2+/nucleotide systems.
Other
efforts should be directed to structural information on oxovanadium(IV)
saccharide complexes. It has not yet been possible to obtain single
crystals adequate for crystallographic studies,69 therefore EXAFS studies, like those recently performed on iron-saccharide complexes,120 should be attempted. Studies on the interaction of the VO2+ cation with polysaccharides should also be expanded, as these systems are of particular biological importance.
The
possible use of phosphonates and related ligands, as well as the
combination of appropriate reducing/chelating systems for use in
vanadium detoxification merits further exploration.
Most
of the model systems so far investigated, are also good candidates for
theoretical studies, which may be very useful for a better
understanding of stability, electronic structures, and reactivity. Some
recent examples of this type of work are the molecular modeling of
vanadium peroxides121 and studies of the electronic structures of Amavadine models.122
Acknowledgements
It
is a great pleasure to acknowledge the contributions of the colleagues
and collaborators whose names appear in the references. Work from our
laboratory reported here was supported by the Consejo Nacional de
Investigaciones Científicas y Técnicas de la República Argentina
(CONICET), the Comisión de Investigaciones Científicas de la Provincia
de Buenos Aires and the Agencia Nacional de Promoción Científica y
Tecnológica. The author is a member of the Research Career of CONICET.
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