Proton and phosphorus-31 NMR study of the dependence of diadenosine tetraphosphate conformation on metal ions.

Adenosine 5'-tetraphospho-5'-adenosine (Ap4A) plays a role in cellular metabolism in a wide variety of organisms. Because the divalent cations Mg2+ and Zn2+ are involved in the synthesis and function of Ap4A, the effect of divalent cations on the dinucleotide's conformation is of interest. 1H and 31P chemical shift experiments were carried out as a function of Mg2+ concentration and pH. We propose that Mg2+ stabilizes the unusual ring-stacked conformation of Ap4A at pH greater than 2 by interacting with the beta-phosphates. To further probe conformational effects, stable complexes of Ap4A with Co3+ were studied using 1H and 31P NMR. Co3+ forms two different bidentate complexes with Ap4A, independent of whether the other four octahedral coordination sites are occupied by ammonia or trimethylenediamine. NMR results suggest that in one complex the Co3+ is coordinated to two beta-phosphates and ring stacking is stabilized. In the other complex, Co3+ is coordinated to an alpha-phosphate and its neighboring beta-phosphate and ring stacking is destabilized. These results further support the hypothesis that Mg2+ stabilizes the ring-stacked conformation by interacting symmetrically with the two beta-phosphate groups.