Speciation in vanadium bioinorganic systems. 3. A potentiometric and 51V, 13C and 1H NMR study of the aqueous H+-vanadate(V)-L-prolyl-L-alanine/L-alanyl-glycine systems

Abstract

The speciation as well as some structural and kinetic properties of the complexes that form between vanadate and prolylalanine (PAH), and vanadate and alanylglycine (AGH) respectively, have been characterised using potentiometry and 51V, 13C and 1H NMR spectroscopy. Formation constants were determined in 0.600 M Na(Cl) at 25°C. Data from the combined EMF and 51V NMR study were evaluated with the computer program LAKE and cover the pH range 3.3≤pH≤9.8 for the PAH system, and 2.5≤pH≤9.2 for the AGH system. The pKa values for prolylalanine were determined as 8.79 and 3.27, and for alanylglycine as 8.11 and 3.15. The ternary system H+-H2VO4--PAH contains one -1 charged complex, VPAH-, having log β0,1,1=2.44±0.02. The corresponding AGH system also contains one -1 charged complex, VAGH-, with log β0,1,1= 1.715±0.009. The errors given are 3σ. The structural characteristics were further explored by 13C and 1H NMR spectroscopy. Both ligands were found to bind to vanadium through the α-N, the deprotonated peptide N and the carboxylate O, thus acting as a tridentate ligand. Equilibrium conditions are illustrated in distribution diagrams, and proposed structures of the complexes are given. Furthermore, some kinetic measurements at neutral pH were performed. The formation of the vanadium-prolylalanine complex is complete after 60 h, whereas the vanadium-alanylglycine system needs only 5 h to reach equilibrium. © Acta Chemica Scandinavica 1997.