SPECIATION IN VANADIUM BIOINORGANIC SYSTEMS .3. A POTENTIOMETRIC AND V-51, C-13 AND H-1-NMR STUDY OF THE AQUEOUS H-VANADATE(V)-L-PROLYL-L-ALANINE()L-ALANYL-GLYCINE SYSTEMS/

The speciation as well as some structural and kinetic properties of th e complexes that form between vanadate and prolylalanine(PAH), and van adate and alanylglycine(AGH) respectively, have been characterised usi ng potentiometry and V-51, C-13 and H-1 NMR spectroscopy. Formation co nstants were determined in 0.600 M Na(Cl) at 25 degrees C. Data from t he combined EMF and V-51 NMR study were evaluated with the computer pr ogram LAKE and cover the pH range 3.3 less than or equal to pH less th an or equal to 9.8 for the PAH system, and 2.5 less than or equal to p H less than or equal to 9.2 for the AGH system. The pK(a) values for p rolylalanine were determined as 8.79 and 3.27, and for alanylglycine a s 8.11 and 3.15. The ternary system H+-H2VO4--PAH contains one -1 char ged complex, VPAH(-), having log beta(0,1,1)=2.44+/-0.02. The correspo nding AGH system also contains one -1 charged complex, VAGH(-), with l og beta(0,1,1)=1.715+/-0.009. The errors given are 3 sigma. The struct ural characteristics were further explored by C-13 and H-1 NMR spectro scopy. Both ligands were found to bind to vanadium through the alpha-N , the deprotonated peptide N and the carboxylate O, thus acting as a t ridentate ligand. Equilibrium conditions are illustrated in distributi on diagrams, and proposed structures of the complexes are given. Furth ermore, some kinetic measurements at neutral pH were performed. The fo rmation of the vanadium-prolylalanine complex is complete after 60 h, whereas the vanadium-alanylglycine system needs only 5 h to reach equi librium.