Effect of Mixed Water–Acetone Solvent on the Stability of Copper(II) Glycylglycinate Complexes

Abstract

Potentiometric titration at a temperature of 298 K and ionic strength of solutions of 0.1 (NaClO4) is used to determine the stability constants of normal and deprotonated mono- and bis-glycylglycinate copper(II) complexes in a water–acetone solvent of variable composition. The peptide group dissociation constants of the glycylglycinate copper(II) complex in water–acetone mixtures are calculated using the obtained data. It is found that the stability of all complexes of copper(II) with glycylglycinate ion increases along with the concentration of acetone in the solution. The contributions from the resolvation of reagents in a water–acetone solvent to the change in the Gibbs energy of the formation of copper(II) glycylglycinate are estimated. It is established that the growth of the stability constant of the glycylglycinate copper(II) complex in an aqueous-acetone solution is determined by the weakening of ligand solvation. The results are compared to similar data for the complexation reactions of the glycylglycinate ion with copper(II) and nickel(II) ions in aqueous solutions of ethanol and dimethylsulfoxide.