Zn(II) enhances nucleotide binding and dephosphorylation in the presence of a poly(ethylene imine) dendrimer

Abstract

The interaction of a second-generation poly(ethylene imine) dendrimer (L) with the nucleotides AMP, ADP and ATP was analysed by means of potentiometric titrations (0.1M Me4NCl, 298.1K), in the absence and in the presence of Zn2+, to perform speciation of the systems and determination of complex stability constants. Protonated forms of L interact with anionic forms of the nucleotides giving rise to stable anion complexes. In the presence of Zn2+, ternary complexes (ion-pair complexes) become the main species in solution, the presence of each partner enhancing the binding of the other one (positive cooperativity effect). The stability constants determined by the same method and under the same experimental conditions for the formation of PO43−, P2O74− and P3O105− complexes with L showed a strict similarity between the binding of AMP, ADP and ATP and their inorganic counterparts, indicating that the main interaction of nucleotides with protonated ligand forms takes places through their inorganic tails.Dephosphorylation of ATP to form ADP and phosphate was monitored by means of 31P NMR spectra at pH 3 and 9 in the presence of L and of its Zn2+ complexes. The rate of ATP dephosphorylation is enhanced by about 2 times at pH 3 and 5 times at pH 9 in the presence of L and by about 4 times at pH 3 and 6 times at pH 9 in the presence of its metal complex, relative to the uncatalysed process, the dephosphorylation reactions being much faster in the acidic media where successive cleavage of ADP to give AMP and phosphate was also observed. Depending on the pH, the observed dephosphorylation enhancements have been interpreted in terms of formation of phosphoramidate intermediates, formation of ATP–Zn2+ coordinative bonds and positive allosteric effects due to metal ion coordination.