Sodium and Magnesium Ion Location at the Backbone and at the Nucleobase of RNA: Ab Initio Molecular Dynamics in Water Solution.

The interactions between Na + or Mg 2+ ions with different parts of single-stranded RNA molecules, namely, the oxygen atoms from the phosphate groups or the guanine base, in water solution have been studied using first-principles molecular dynamics. Sodium ions were found to be much more mobile than Mg 2+ ions and readily underwent transitions between a state directly bonded to RNA oxygen atoms and a completely solvated state. The inner solvation shell of Na + ions fluctuated stochastically at a femtosecond timescale coordinating on average 5 oxygen atoms for bonded Na + ions and 5.5 oxygen atoms for solvated Na + ions. In contrast, the inner solvation shell of Mg 2+ ions was stable in both RNA-bonded and completely solvated states. In both cases, Mg 2+ ions coordinated 6 oxygen atoms from the inner solvation shell. Consistent with their stable solvation shells, Mg 2+ ions were more effective than Na + ions in stabilizing the RNA backbone conformation. The exclusion zones between the first and second solvation shells, solvation shell widths, and angles for binding to carbonyl oxygen of guanine for solvated Na + or Mg 2+ ions exhibited a number of quantitative differences when compared with RNA crystallographic data. The presented results support the distinct capacity of Mg 2+ ions to support the RNA structure not only in the crystal phase but also in the dynamic water environment both on the side of the phosphate moiety and on the side of the nucleobase.