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

Mixed-ligand complexes of the type [M(ATP)(L)]3–[ATP = adenosine 5′-triphosphate, L =L-histidinate (hisO–)] and [M(ATP)(L)]2–[L = histamine (hm)], with M = Cu2+ or Zn2+, have been studied by potentiometric and calorimetric titrations at 25 °C and I= 0.1 mol dm–3. Detailed analysis of the thermodynamic parameters concerning the simple complexes of L-histidine and histamine has been carried out and has allowed us to determine details of the bonding of the main species present in aqueous solution. The two unprotonated mixed complexes show equal ΔG[minus sign in circle] values both in the case of zinc(II) and copper(II) systems but different ΔH[minus sign in circle] and ΔG[minus sign in circle] contributions. The differences in enthalpy and entropy changes reveal the presence of a ligand–ligand interaction between the purine moiety of ATP and the imidazole ring of histamine in the [Zn(ATP)(hm)]2– complex. Further support for this is gained by comparing the differences between the ΔH[minus sign in circle] and ΔS[minus sign in circle] values for the formation of [Cu(ATP)(hm)]2– and [Zn(ATP)(hm)]2– and those for [Cu(ATP)(hisO)]3– and [Zn(ATP)(hisO)]3–.