Thermodynamics of Ion Association. Part XXIII. Copper, Zinc, and Cadmium Complexes of Some Dipeptides

Calorimetric and potentiometric studies have been made at 25 degrees C of the formation of zinc and cadmium complexes of diglycine and of the copper complexes of glycylsarcosine, glycineamide, glycylproline, glycyl-L-alanine, alpha-alanylglycine, and glycyl-gamma-aminobutyric acid. In addition, a potentiometric investigation has been made of the formation of copper L-alanyl-L-alanine complexes. The data have been used to calculate Delta G, Delta H, and Delta S values for all the equilibria involved and, in the case of copper, these data strongly support a common structure for all the mono complexes at lower pH in which the copper is bound to the terminal-NH(2) and the oxygen of the peptide group. At higher pH, thermodynamic functions have been determined for the dissociation of the peptide hydrogen atom which is labilized by the copper atom.