THERMODYNAMICS OF ION ASSOCIATION .17. COPPER COMPLEXES OF DIGLYCINE AND TRIGLYCINE

A sensitive differential calorimeter has been used to measure the enthalpy changes at 25° associated with the formation of copper diglycine and copper triglycine complexes and with the dissociations of the peptide hydrogen atoms at higher pH. The data have been combined with potentiometric measurements to yield ΔG, ΔH, and ΔS values at an ionic strength of 0.10 M for the various equilibria involved. At low pH, the values of the thermodynamic functions favor a bidentate structure for the species CuL+ similar to that in the solid state, in which the copper is bonded to the terminal -NH2 and the oxygen of the first peptide group. At higher pH, the thermodynamic data are discussed in terms of the important factors involved in the labilization of the peptide hydrogen atoms when the ligands are bound to the copper ion. © 1968, American Chemical Society. All rights reserved.