Accurate Determination of Isotope Effects on the Dynamics of H-Bond Breaking and Making in Liquid Water

Isotopic substitution of light hydrogen atoms with heavier deuterium atoms in liquid water renders the resulting liquid, heavy water (D2O), poisonous to most organisms when it replaces a critical fraction of water in living organisms. The mechanisms through which heavy water disrupts biological function are challenging to disentangle experimentally. Isotopic substitution has long been known to affect the H-bond dynamics of liquid water, but experiments have yet to quantify the extent of the differences in the time scales of H-bond breaking and making processes between H2O and D2O. In this work, we analyze H-bond dynamics through extensive coupled cluster-quality path integral simulations of H2O and D2O under ambient conditions that grant access to unambiguous molecular analyses. We find substantial isotope substitution effects on the rates of H-bond formation and breaking, and H-bond lifetimes, with dynamics in D2O similar to 25% slower than in H2O. The toxicity of D2O can thus be ascribed, at least in part, to the effect of slowed H-bond dynamics on biochemical reactions.