OXOVANADIUM(IV) COMPLEXES OF THE DIPEPTIDES GLYCYL-L-ASPARTIC ACID, L-ASPARTYLGLYCINE AND RELATED LIGANDS - A SPECTROSCOPIC AND POTENTIOMETRIC STUDY

The equilibria in the systems VO2+ + L (L = Gly-L-Asp, L-Asp-Gly, N-ac etyl-L-aspartic acid or succinic acid) have been studied at 25 degrees C and 0.2 mol dm(3) K(CI) medium by a combination of potentiometric a nd spectroscopic methods (ESR, circular dichroism and visible absorpti on). Formation constants were calculated from pH-metric data with tota l oxovanadium(Iv) concentrations of(0.6-4) x 10(-3) mol dm(-3) and lig and-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-or dination 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-ace tyl-L-aspartic and succinic acids is similar. The results of potentiom etric and spectroscopic methods are self consistent. Isomeric structur es are discussed for each stoichiometry proposed and the results compa red with those for L-aspartic acid and dipeptides with non-coordinatin g side chains.