Oxovanadium(IV) complexes of the dipeptides glycyl-L-aspartic acid, L-aspartylglycine and related ligands; a spectroscopic and potentiometric study

Abstract

The equilibria in the systems VO²⁺ + L (L = Gly-L-Asp, L-Asp-Gly, N-acetyl-L-aspartic acid or succinic acid) have been studied at 25 °C and 0.2 mol dm³ K(Cl) medium by a combination of potentiometric and spectroscopic methods (ESR, circular dichroism and visible absorption). Formation constants were calculated from pH-metric data with total oxovanadium(IV) concentrations of (0.6–4) × 10^–3 mol dm^–3 and ligand-to-metal (L∶M) ratios of 2–8 (AspGly) or 4–15∶1 (other systems). The position of the Asp residue in the peptide chain affects the co-ordination mode of the ligands: while in the GlyAsp system bis complexes start to form at pH less than 2, for AspGly only 1∶1 complexes form, with relatively high CD signal. The co-ordination behaviour of N-acetyl-L-aspartic and succinic acids is similar. The results of potentiometric and spectroscopic methods are self consistent. Isomeric structures are discussed for each stoichiometry proposed and the results compared with those for L-aspartic acid and dipeptides with non-co-ordinating side chains.