Oxovanadium(IV) and oxovanadium(V) complexes relevant to biological systems

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Abstract

Different aspects of the coordination chemistry of oxovanadium(IV) and oxovanadium(V), relevant to the bioinorganic chemistry of vanadium, are presented. Some of the investigated complexes are good models for different aspects of the metabolism and detoxification of vanadium or for a better characterization and understanding of the structural and electronic peculiarities of the coordination spheres of VO2+ and VO2+ in biomolecules. Their structural, spectroscopic and magnetic properties are briefly discussed. The investigated systems include ligands such as reduced and oxidized glutathione, l-ascorbic acid, nucleotides and related systems, carbohydrates, phosphates, carboxylic acids, oxine derivatives and some others.

Introduction

The essentiality, biodistribution, and toxicology of vanadium, like its biological and pharmacological activity, are areas of increasing research and widespread interest. Although numerous biochemical and physiological functions have been suggested, and despite the magnitude of the knowledge so far accumulated, vanadium still does not have a clearly defined role in higher organisms [1], [2], [3], [4], [5], [6], [7], [8], [9].

Like molybdenum, vanadium assumes an exceptional position among the biometals because its anionic and cationic forms can participate in biological processes [8], [9], [10]. As regards its anionic forms (vanadates(V)), it actually resembles phosphates, but in its cationic forms — mainly as VO2+, and sometimes as V(III) — it behaves like a typical transition metal ion, which competes with other metal cations in coordination with biogenic ligands and compounds. This duality, together with the facility with which it changes coordination environments and oxidation states, may be responsible for the very peculiar and somewhat unparalleled behavior of this biometal, the characteristics of which have just begun to emerge.

Different aspects of the coordination chemistry of vanadium relevant to its presence and activity in biological systems have been recently reviewed [2], [4], [7], [8], [9], [10], [11], [12], [13], [14]. As VO2+ is probably the most relevant species present in biological systems, we have carried out a number of systematic model studies, on the interaction of this oxocation with different biomolecules and other ligands of biological and/or pharmacological interest in order to contribute to a better understanding of its possible roles and functions in living organisms. Some of these studies have also included the corresponding vanadium(V) oxocation, VO2+.

In this paper, we present some of the most representative results of our studies, which cover different aspects of the bioinorganic chemistry of vanadium.

Section snippets

Systems related to vanadium metabolism

Although information about the metabolism of physiological amounts of vanadium in higher forms of life is scarce, during the last 15 years an increasing amount of data has been accumulated from laboratory animal experiments. Thus, some general aspects related to the absorption, transport, biological transformations, toxicity and excretion of vanadium could be understood [15], [16]. Dietary vanadium probably occurs mainly as H2VO4 and may enter cells through the phosphate transport mechanism.

Complexes of nucleotides and related ligands

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

The field of VO2+/nucleotide interactions has been recently reviewed [48]. Therefore, only a brief summary of them, together with some newer results shall be presented

VO2+/phosphate systems

Simple and complex phosphates deserve special attention in relation to vanadium biochemistry not only due to its presence in nucleotides but also due to its participation in a wide variety of biological molecules and systems, and taking into account the preference of the oxovanadium(IV) cation for oxygen donors.

A great number of VO2+/phosphate complexes of different stoichiometries are well known. Some of them are very stable and have been well characterized [48], [53], [66].

In the case of the

VO2+ complexes of carbohydrates

As carbohydrates are the most abundant class of compounds in the biosphere [75], the study of their interaction with relevant vanadium species is of great interest. It is well known that sugars interact with metal ions by acting either as reductants and/or chelators [16], [75]. Actually, most of them can reduce vanadates(V) to oxovanadium(IV) and complex this cation.

Due to its strong hydrolytic tendency, the VO2+ cation usually needs the presence of additional donor groups (e.g. carboxylates)

Model complexes that mimic biological sites

In order to attain an insight into the structural, electronic and spectroscopic characteristics of different vanadium environments present in natural systems, a number of complexes with organic and inorganic ligands, adequate to mimic such types of environments and metal-to-ligand interactions, have been investigated. The usefulness of such types of model studies arises from the fact that they allow relationships to be established between structural and spectral features of well-defined

Complexes with ligands of pharmacological interest

The pharmacological activity of different vanadium compounds is well known [4], [5], [8], [9]. The insulino-mimetic effects [5], [143], [144], [145], [146] and the antitumoral properties [5], [147], [148] have especially been investigated in recent years.

Studies on the interaction of simple vanadium species with drugs also constitute a field of increasing interest. For example, very stable complexes can be obtained by interaction of VO2+ with the antitumoral antibiotic bleomycin [149], [150],

Conclusions and perspectives

The VO2+ systems so far investigated have undoubtedly contributed to a better understanding of different aspects of the complex biochemical behavior of vanadium and have notably extended our knowledge on the coordination chemistry of this oxocation, as a direct consequence of the introduction of new ligand types.

Our own studies in this field have contributed to attain a deeper insight into some basic aspects related to vanadium metabolism in the higher forms of life. They have also brought new

Acknowledgements

It is a great pleasure to acknowledge the contributions of the collaborators and colleagues whose names appear in the references. Work from this laboratory herein reported has been supported by the Consejo Nacional de Investigaciones Cientı́ficas y Técnicas de la República Argentina (CONICET), the Comisión de Investigaciones Cientı́ficas de la Provincia de Buenos Aires and the Agencia Nacional de Promoción Cientı́fica y Tecnológica (PICT Nr. 119). The author is a member of the Research Career

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