Ab initio molecular dynamics of Na⁺ and Mg²⁺ countercations at the backbone of RNA in water solution

The interactions between sodium or magnesium ions and phosphate groups of the RNA backbone represented as dinucleotide fragments in water solution have been studied using ab initio Born-Oppenheimer molecular dynamics. All systems have been simulated at 300 and 320 K. Sodium ions have mobility higher than that of the magnesium ions and readily change their position with respect to the phosphate groups, from directly bonded to completely solvated state, with a rough estimate of the lifetime of bonded Na(+) of about 20-30 ps. The coordination number of the sodium ions frequently changes in irregular intervals ranging from several femtoseconds to about 10 ps with the most frequently encountered coordination number five, followed by six. The magnesium ion is stable both as directly bonded to an oxygen atom from the phosphate group and completely solvated by water. In both states the Mg(2+) ion has exactly six oxygen atoms in the first coordination shell; moreover, during the whole simulation of more than 100 ps no exchange of ligand in the first coordination shells has been observed. Solvation of the terminal phosphate oxygen atoms by water molecules forming hydrogen bonds in different locations of the ions is also discussed. The stability of the system containing sodium ions essentially does not depend on the position of the ions with respect to the phosphate groups.