General information
The main
emphasis of the group’s research activities focuses on biological and
medicinal aspects of vanadium and, to some extent, also molybdenum and
tungsten. Vanadium is a biologically relevant metal: It is in the
active centre of three groups of enzymes, viz. vanadate-dependent
haloperoxidases, vanadium-nitrogenases and vanadium-containing nitrate
reductase. In addition, vanadium is accumulated by certain life forms
such as sea squirts (Ascidiaceae) and Amanita mushrooms. More
generally, vanadium appears to be involved in the regulation of
phosphate-metabolising enzymes; many vanadium coordination compounds
exhibit an anti-diabetic potency. Vanadium is also widely used to
catalyse oxidation, reduction and polymerisation reactions; soluble
“vanadiumoxides” (polyoxovanadates) are a more recent development in
this field.
Our research is directed towards all of these aspects of vanadium chemistry, stressing biological and medicinal aspects.
Biological
Our
studies of the biological chemistry of vanadium centres around
coordination compounds as models of the active sites in
haloperoxidases, nitrogenases and nitrate reductases, and direct
research into the enzyme-substrate interaction. Figs. 1 and 2 show
reactions which are catalysed by haloperoxidases, and
structural/functional models of the active centre.
Fig. 1. A model (right) for the hydroperoxo intermediate in the catalytic cycle inherent of vanadate-dependent haloperoxidases.
Fig. 2. The enantio-selective oxidation of
sulfides is catalysed by vanadium compounds which model the active
centre (in green) of vanadate-dependent haloperoxidases.
Solution speciation studies of vanadate-ligand and vanadate-peroxide-ligand systems, carried out on the basis of multinuclear NMR and potentiometry (co-operation with L. Pettersson, Umeå ) or EPR and potentiometry (cooperation with T. Kiss, Szeged) complement investigations of the solid state structures, and help to elucidate the structure-function synergism.
Medicinal
The insulin-mimetic/enhancing behaviour of vanadium compounds,
such as their ability to trigger glucose uptake by glucose-metabolising
cells, is investigated in the frame of a Europe-wide COST programme
(COST D21-009-01) and in cooperation with the Pharmaceutical University
in Kyoto (H. Sakurai). We synthesise vanadium compounds with features
(Fig. 3) which minimise toxicity, optimise stability and absorption,
and mimic/enhance the in vitro anti-diabetic effects of
insulin.
Fig. 3. A new family of effective insulin-mimetic vanadium compounds: Bis(picolinato)vanadium(IV) complexes. Xcan be OR (R= alkyl, galactosyl, inositolyl) or NHR (an amino acid residue)
Catalysis and Materials
The
functionalisation, "shaping" and stabilisation of polyoxometalate
clusters by embedment into macrocycles (such as the cryptand
[212]-stabilised decavanadate in Fig. 4) allows for the design of
“soluble oxides” as homogenous oxidation catalysts.
Fig. 4. Decavanadate [H2V10O28]4- stabilised (through electrostatic and hydrogen bonds) by two cryptand cations [C212H2]2+. Blue: V.
Double-helical chains are formed as thiotungstates are reacted with silver ions and thiocyanate; Fig. 5.
Fig. 5. Double-stranded W-Ag-S chains. Colour code: W (magenta), Ag (black), S (red), N (green), O (blue), C (cyan).
In cooperation with the group of Prof. Achim Müller, Bielefeld, and
Dr. E. Haupt, Hamburg, we investigated the uptake/release of cations
(such as Li+ and Na+) by "artificial cells" based on porous polyoxomolybdates. The method employed is 7Li- and 23Na-NMR.
