Protonated macrocyclic Zn(II) complexes as polyfunctional receptors for ATP

Zn(II) coordination by the phenanthroline-containing macrocycle 2,6,10,14-tetraaza[15](2,9)cyclo(1,10)phenanthrolinophane (L4) has been studied by means of potentiometric measurements in aqueous solution. Its coordination properties have been compared with those of other phenanthroline- or dipyridine-containing open-chain (L1, L2) or cyclic (L3) ligands. ATP binding to the Zn(II) complexes with L1-L4 has been examined by means of potentiometric and 1H and 31P NMR measurements in aqueous solution. In the ATP adducts with the [ZnL]2+ complexes, the nucleotide interacts with the metal via the terminal P(gamma) phosphate group; the equilibrium constants for the addition of ATP to the complexes depend on the number and arrangement of the nitrogen donors coordinated to the metal ion. Protonation of the [ZnL]2+ complexes gives [ZnHxL](x+ 2)+ species, which contain two binding sites for the phosphate chain of ATP; while the P(gamma) phosphate group gives a coordination bond with the metal, the P(beta) one interacts via P-O- ... H-N+ salt bridges with the ammonium functions of the complex. In consequence, protonated complexes are better ATP receptors than the simple [ZnL]2+ species and even than the protonated forms [HxL]x+ of the ligands, due to the synergetic action of the metal ion and of ammonium functions in ATP binding.