INTRAMOLECULAR STACKING INTERACTION IN MIXED-LIGAND COMPLEXES CONTAINING ATP(4-) AND AROMATIC N-HETEROCYCLIC LIGANDS

The stability constants of the binary ML2+ and ternary M(ATP)L2- compl exes, where L=Iq (isoquinoline) or BIm (benzimidazole) and M=Zn2+ or C d2+, have been determined by potentiometric pH titration in aqueous so lution at I=0.1 mol/L (NaClO4), T=25 degrees C. The stability of the t ernary complexes characterized by Delta logK(M)=logK(M(ATP)L)(M(ATP))- logK(ML)(M) corresponding to the equilibrium M(ATP)(2-) + ML2+ = M(ATP )L2- + M2+ is higher than what would be expected on statistical ground s. The increase may be related to the stacking interaction between the aromatic ring of the ligands L and the purine moiety of ATP(4-). H-1 NMR studies of Zn2+/ATP(4-)/L confirm the presence of stacking in the ternary complexes. It is concluded that the strength of the intramolec ular stacking interaction is dependent on the structure of the aromati c ring of the ligand L and the formation of a metal ion bridge. Possib le implications are discussed briefly.