Dependence of Water Exchange Kinetics on the Size and Charge of Metal Ion

Abstract

While structural and dynamical properties of water on a labile metal ion such as Na+ and Ca2+ are straightforwardly evaluated using molecular dynamics (MD) simulations, direct evaluation of water exchange kinetics on a relatively more inert metal ion, Mg2+, is computationally demanding. Here, we propose a universal relationship which allows us to determine the water exchange time on a metal ion with varying charges and hydration radii. The proposed relationship highlights the ultra-sensitivity of water lifetime to the ion size, as exemplified between two divalent ions, Ca2+ (∼100 ps) and Mg2+ (∼1.5 μs). Importantly, the current MD force fields underestimate the hydration radius of Mg2+ by ∼2.5 %, which results in ∼ 400 fold overestimation of water exchange time. Accurate determination of ion size is critical for correctly modeling the water dynamics around multivalent ions.