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Other Models of Biochemical Interest
- Model studies related to vanadium biochemistry: recent advances and perspectives

A great number of other vanadium-containing complexes and systems are also of direct biochemical interest. In this section we restrict the information only to some aspects of systems involving nucleotides, phosphates and carbohydrates and some closely related ligands.

3.1. Interaction of VO2+ with nucleotides and related materials

The coordination behavior of both vanadates(V) and oxovanadium(IV) with nucleotides and their constituents is of great interest in relation to competitive processes in the regulation of the ATP-ases, ribonucleases and similar systems, as well as with regard to the possible cancerostatic action of vanadates.

As the field of VO2+/nucleotide interactions has been recently reviewed,57 only the most important conclusions are summarized here: in acid media all types of phosphate nucleosides (mono-, di- and tri-phosphates) interact with the cation, but only through the phosphate groups; at high pH interaction takes place only through the deprotonated OH-groups of D-ribose; at neutral pH di- and tri-phosphate nucleosides generate VO(nucl)2 complexes at high ligand-to-metal ratios; at lower ratios, participation of N atoms of the nucleic acid bases occurs. The behavior of monophosphate nucleosides is more complex. Important ligand rearrangements take place with increasing pH. Phosphate groups, together with OH-groups of D-ribose, participate in coordination.

Interaction of the VO2+ cation with the nucleic bases58 and with nucleosides59 has also been investigated.

Even though different solid VO2+/nucleotide complexes have been reported,60 but they have not been well characterized. Our own experience in these systems has shown that the isolation and purification of such complexes is not easy.12

Simple and complex phosphates deserve special attention, not only to the presence of vanadium in nucleotides but also due to its participation in a wide range of biological systems and processes.12

D-ribose-5-phosphate (Rib-5P) shows a similar solution behavior to the monophosphate nucleosides61 and three powdered solid oxovanadium(IV) complexes containing this species were isolated and characterized.62 The spectroscopic analysis of the light blue species [VO(Rib-5P)(OH)(H2O)2].2H2O and [VO(Rib-5P)(H2O)3Cl shows coordination through oxygen atoms of the phosphate group which is bidentate in the first case and monodentate in the second. In contrast, in the green Na6[VO(Rib-5P)2].6H2 O complex, coordination takes place through pairs of two adjacent deprotonated OH-groups of the sugar moiety.

Phytic acid and thiamine diphosphates are other biologically interesting phosphates. From the nutritional point of view, phytic acid (mio-inositol hexaphosphate) appears especially interesting because of its important effects on the bioavailability of essential trace metals.63 Depending on the pH, the VO2+ cation interacts with phytate forming both soluble and insoluble complexes.64

Regarding thiamine diphosphate (cocarboxylase, TDP), a coenzyme that catalyzes the decarboxylation of a-ketoacids, we have found that in the pH-range 3-4 a 1:1 complex with VO2+ is formed, and this involves only O-phosphate bonds. The participation of the N(1) atom of the pyrimidine ring in bonding has been suggested at higher pH.65 A solid complex of composition [VO(TDP)Cl].7H2O, was precipitated with ethanol from an aqueous solution at pH 3.5. In this complex, the terminal PO3 group of TDP is bidentate.66

Another complex, relevant to a better understanding of the VO2+/phosphate interactions, is the recently investigated trimer species Na6[(VO)3(P2O7)3].7H 2O.67

3.2. VO2+ complexes of carbohydrates

As carbohydrates are the most abundant class of compounds in the biosphere,12,68 the study of their interaction with relevant vanadium species is of great interest. It is well known that sugars interact with metal ions either as reductants and/or chlelators.12,68 Most of them reduce vanadates(V) to oxovanadium(IV) and complex this cation. This field of vanadium biochemistry has also been recently reviewed69 and therefore we shall reduce the discussion only to its most relevant aspects.

Due to its strong hydrolytic tendency, the VO2+ cation usually needs the presence of additional donor groups (e.g. carboxylates) in the sugar molecule. Once bonded, it can easily deprotonate the OH-groups and strongly coordinate up to four of them. Oxovanadium(IV) complexes coordinated by pairs of doubly deprotonated sugar moieties usually display a very characteristic, three band, electronic absorption spectrum.69,70

Oxovanadium(IV) coordination is favored in basic media and only occurs with sugar molecules provided with pairs of adjacent OH-groups.69,71

A great number of VO2+/monosaccharide complexes have been reported in recent years. Their stoichiometries are summarized in Table 1. All these complexes are green-colored powders and are usually hygroscopic and very soluble in water.

Only five oxovanadium complexes with disaccharides as ligands have been so far reported. These are sucrose,74 turanose,74 maltose73,77 and lactose,78 with the following stoichiometries:

Na3[VO(D-Suc)2OH].H2 O
Na3[VO(D-Tur)2OH].3H2 O
Na5[VO(D-Mal)2].CH3 OH
Na4[VO(Lact)2].3H2 O

Even though a number of oxovanadium(IV) complexes with some carboxylate derivatives of carbohydrates and sugar phosphates, and with polysaccharides, has been also investigated, most of these compounds remains poorly characterized.69


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