Studies on transition-metal–peptide complexes. Part 1. Equilibrium and thermochemical study of the copper( II ) complexes of glycylglycine, glycyl- DL -α-alanine, DL -α-alanylglycine, and DL -α-alanyl- DL -α-alanine

pH Titrimetry and calorimetry have been used to determine the stoicheiometries, stability constants, and enthalpies and entropies of formation of the complexes formed in the systems of copper(II) with glycylglycine, glycyl-DL-α-alanine, DL-α-alanylglycine, and DL-α-alanyl-DL-α-alanine, at ionic strength I= 0.2 mol dm–3 KCl and at 25 °C. There is no possibility of the titrimetric indication of the formation of the complexes [CuL2] and [CuL2H–2]2–(L = dipeptide anion H2N–CHR–CO–NH–CHR′–CO2–) in these equilibrium systems, even when their maximum concentration is 10–15%. The systematic changes within the series have been interpreted by simultaneous consideration of the electron-donating and steric-hindrance effects of the methyl groups. From the values of the formation constants and enthalpy changes, it is concluded that equatorial–axial co-ordination of the second peptide ligand occurs in the complex [CuL2H–1]–, while in the binuclear complex [Cu2L2H–3]– an [OH]– ligand bridges the two [CuLH–1] units equatorially.