IONIC-STRENGTH DEPENDENCE OF FORMATION-CONSTANTS .15. POTENTIOMETRIC AND SPECTROSCOPIC STUDY OF COPPER(II) COMPLEXES WITH L-PROLYL-L-LEUCYL-GLYCINAMIDE IN AQUEOUS-SOLUTION

Complex formation in the system copper(II) - L-prolyl-L-leucylglycinamide (PLG, L) - nitrate has been studied in aqueous solution as a function of ionic strength (0.03 less-than-or-equal-to/less-than-or-equal-to 1.0 mol dm-3), in KNO3, at 25-degrees-C. The type of species in solution shows successive deprotonation of peptide groups of PLG (when complexing copper(II)) up to deprotonation of the C-terminal amido group. First the stability constant values have been determined without allowing for weak interactions and subsequently also considering ion pair formation such as (CuNO3)+ and (LHNO3)-degrees. The stability of species (LHNO3)-degrees is very similar to that found for the ion-pair formed by many other amines with monocharged anions and confirms the reliability of our model for the dependence of stability constants on ionic strength. Visible electronic and circular dichroism spectra were also obtained for the different complexes.