Interaction of polyamine macrocycles with Zn(II) and ATP in aqueous solution. Binary and ternary systems. A potentiometric, NMR and fluorescence emission study

ATP binding to ligands L1 and L3 and to their Zn(II) complexes has been examined by means of potentiometric and 1H and 31P NMR measurements in aqueous solution. Their coordination features have been compared to those of ligand L2 and its Zn(II) complex. In all the three cases, the Zn(II) complexes proved to be better receptors than free ligands, due to the synergetic action of metal ion and ammonium functions in ATP binding. Among the three complexes, Zn(II) complex with L1 shows the highest equilibrium constant, which can be ascribed to the fact that, being coordinated by the dipyridine nitrogens outside the macrocyclic cavity, it is less saturated by ligand donors. The 31P NMR investigation showed that the nucleotide interacts via the Pγ and Pβ phosphate groups with both free ligands and complexes, while the 1H spectra revealed that the binding is reinforced by the presence of π–π interactions. Photophysical studies showed that the fluorescence emission intensity of the Zn(II) complexes is enhanced upon interaction with ATP.