Studies of Mg2+/Ca2+ complexes of naturally occurring dinucleotides: potentiometric titrations, NMR, and molecular dynamics

Dinucleotides (Np (n) N'; N and N' are A, U, G, or C, n = 2-7) are naturally occurring physiologically active compounds. Despite the interest in dinucleotides, the composition of their complexes with metal ions as well as their conformations and species distribution in living systems are understudied. Therefore, we investigated a series of Mg2+ and Ca2+ complexes of Np (n) N's. Potentiometric titrations indicated that a longer dinucleotide polyphosphate (N is A or G, n = 3-5) linker yields more stable complexes (e.g., log K of 2.70, 3.27, and 3.73 for Ap (n) A-Mg2+, n = 3, 4, 5, respectively). The base (A or G) or ion (Mg2+ or Ca2+) has a minor effect on values. In a physiological medium, the longer Ap (n) As (n = 4, 5) are predicted to occur mostly as the Mg2+/Ca2+ complexes. P-31 NMR monitored titrations of Np (n) N's with Mg2+/Ca2+ ions showed that the middle phosphates of the dinucleotides coordinate with Mg2+/Ca2+. Multidimensional potential of mean force (PMF) molecular dynamics (MD) simulations suggest that Ap(2)A and Ap(4)A coordinate Mg2+ and Ca2+ ions in both inner-sphere and outer-sphere modes. The PMF MD simulations additionally provide a detailed picture of the possible coordination sites, as well as the cation binding process. Moreover, both NMR and MD simulations showed that the conformation of the nucleoside moieties in Np (n) N'aEuro"Mg2+/Ca2+ complexes remains the same as that of free mononucleotides. Dinucleotides form complexes with Mg2+/Ca2+ ions which coordinate to the middle phosphate groups in the polyphosphate chain. Mg2+/Ca2+ coordination slightly increases intramolecular pi-stacking interactions in the complexes, whereas the conformation of nucleoside moieties remains almost the same as in the corresponding Na+ salts. Considering physiological Mg2+/Ca2+ concentrations, the Ap(4)A-Mg2+/Ap(5)A-Mg2+ and Ap(4)A-Ca2+/Ap(5)A-Ca2+ species are expected to be dominant (approximately 70-90 %) in living systems versus the free dinucleotide.