Thermodynamics of Metal Complexes with Ligand-Ligand Interaction. Simple and Mixed Complexes of Copper(II) and Zinc(II) with Adenosine 5'-Triphosphate and L-Tryptophan or L-Alanine.

Thermodynamic parameters for the formation of simple copper(II) and zinc(II) complexes with adenosine 5′-triphosphate (ATP), L-tryptophanate (trpO), and L-alaninate (alaO) were determined by means of potentiometric and calorimetric measurements. Detailed analysis of the potentiometric data allowed reliable values of stability constants to be obtained. For the first time, the thermodynamic functions for the [M 2(ATP)] species are also reported. On the basis of ΔH ⊖ and ΔS ⊖ changes, the structure, in aqueous solution, of the main species present in the M-ATP systems are discussed. The ΔG ⊖, ΔH ⊖, and ΔS ⊖ values for the [M(ATP)L] 3- mixed-ligand complex (L = amino-acid anion) were also determined. The larger positive ΔH ⊖ and the less positive ΔS ⊖ accompanying formation of [M(ATP)(trpO)] 3- compared with [M(ATP)(alaO)] 3- are considered to indicate the presence of ligand-ligand interaction in the former complex. Further support for this is gained by comparing the differences between the ΔH ⊖ and ΔS ⊖ values for formation of [Cu(ATP)(trpO)] 3- and [Zn(ATP)(trpO)] 3- and those for [Cu(ATP)(alaO)] 3- and [Zn(ATP)(alaO)] 3-. The presence of the stacking interaction is also discussed in relation to the geometric requirements of the metal ions