Speciation of ternary complexes of Ca(II), Zn(II) and Mn(II) with L-proline and L-valine in acetonitrile-water mixtures

Chemical speciation of ternary complexes of Ca(II), Zn(II) and Mn(II) with L-proline and L-valine was studied in various concentrations (0-60% v/v) of acetonitrile-water mixtures maintaining an ionic strength of 0.16 mol L-1 at 303.0 K. Alkalimetric titrations were carried out with different relative concentrations (M: L: X = 1:2.5:2.5, 1:2.5:5.0, 1:5.0:2.5) of metal (M) to L-proline (L) to L-valine (X). Stability constants of ternary complexes were calculated and various models were refined with MINIQUAD75. The best-fit chemical models were selected based on statistical parameters and residual analysis. The trend of the variation in the stability constants with changing dielectric constant (D) of the medium was explained based on the electrostatic interactions of the ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. Distribution diagrams with pH at different compositions of aqua-organic mixtures and structures of plausible ternary complexes were also presented.