Adenosine/guanosine-3 ', 5 '-bis-phosphates as biocompatible and selective Zn2+-ion chelators. Characterization and comparison with adenosine/guanosine-5 '-di-phosphate

Although involved in various physiological functions, nucleoside bis-phosphate analogues and their metal-ion complexes have been scarcely studied. Hence, here, we explored the solution conformation of 2'-deoxyadenosine- and 2'-deoxyguanosine-3', 5'-bisphosphates, 3 and 4, d(pNp), as well as their Zn2+/Mg2+ binding sites and binding-modes (i.e. inner- vs. outer-sphere coordination), acidity constants, stability constants of their Zn2+/Mg2+ complexes, and their species distribution. Analogues 3 and 4, in solution, adopted a predominant Southern ribose conformer (ca. 84%), gg conformation around C4'-C5' and C5'-O5' bonds, and glycosidic angle in the anti-region (213-270 degrees). H-1- and P-31-NMR experiments indicated that Zn2+/Mg2+ ions coordinated to P5' and P3' groups of 3 and 4 but not to N7 nitrogen atom. Analogues 3 and 4 formed ca. 100-fold more stable complexes with Zn2+ vs. Mg2+-ions. Complexes of 3 and 4 with Mg2+ at physiological pH were formed in minute amounts (11% and 8%, respectively) vs. Zn2+ complexes (46% and 44%). Stability constants of Zn2+/Mg2+ complexes of analogues 3 and 4 (log K-ML(M) = 4.65-4.75/2.63-2.79, respectively) were similar to those of the corresponding complexes of ADP and GDP (log K-ML(M) = 4.72-5.10/2.95-3.16, respectively). Based on the above findings, we hypothesized that the unexpectedly low log K values of Zn2+-d(pNp) as compared to Zn2+-NDP complexes, are possibly due to formation of outer-sphere coordination in the Zn2+-d(pNp) complex vs. inner- sphere in the NDP-Zn2+ complex, in addition to loss of chelation to N7 nitrogen atom in Zn2+-d(pNp). Indeed, explicit solvent molecular dynamics simulations of 1 and 3 for 100 ns supported this hypothesis.