Molecular dynamics study of the structure and dynamics of the hydration shell of alkaline and alkaline-earth metal cations

Molecular dynamics simulations of hydrated alkaline and alkaline-earth metal cations at room temperature (T = 300 K) were carried out using the CHARMM22 force field, Dynamic and static properties of systems containing one ion and 123 or 525 water molecules were investigated by analysis of trajectories of 1 ns duration and compared to experimental and theoretical results. In addition, the size and the direction of the elementary motions of both the ions and the water molecules were investigated on the scale of the integration time step of 1 fs. Comparison between systems of different size revealed that for the larger system the diffusion coefficient and the number of hydrogen bonds were increased. Radial pair distribution functions and coordination numbers are in good agreement with X-ray and neutron scattering data. The diffusion coefficient D of bulk TIP3P water in a system with 528 water molecules was by one-fourth higher than the experimental value. Minor differences of-approximately 10% between experimental and simulated diffusion coefficients were found for Li+, Na+, K+, and Mg2+. On the other hand, D was underestimated by the simulation for Ca2+ and Sr2+ by as much as 30%. On the average, 2.9 hydrogen bonds per bulk water molecule were found. The observed order of residence times for the monovalent ions, tau(Li+) > tau(Na+) > tau(K+), is in good agreement with the literature. Although tau was expected to increase with decreasing mass of the ion, the exchange of water molecules from the solvation shell of Mg2+ occurred much faster than for Ca2+.