title: Ab Initio Liquid Water Dynamics in Aqueous TMAO Solution.
abstract: Ab initio molecular dynamics (AIMD) simulations in trimethylamine N-oxide (TMAO)-D2O solution are employed to elucidate the effects of TMAO on the reorientational dynamics of D2O molecules. By decomposing the O-D groups of the D2O molecules into specific subensembles, we reveal that water reorientational dynamics are retarded considerably in the vicinity of the hydrophilic TMAO oxygen (O(TMAO)) atom, due to the O-D···O(TMAO) hydrogen-bond. We find that this reorientational motion is governed by two distinct mechanisms: The O-D group rotates (1) after breaking the O-D···O(TMAO) hydrogen-bond, or (2) together with the TMAO molecule while keeping this hydrogen-bond intact. While the orientational slow-down is prominent in the AIMD simulation, simulations based on force field models exhibit much faster dynamics. The simulated angle-resolved radial distribution functions illustrate that the O-D···O(TMAO) hydrogen-bond has a strong directionality through the sp(3) orbital configuration in the AIMD simulation, and this directionality is not properly accounted for in the force field simulation. These results imply that care must be taken when modeling negatively charged oxygen atoms as single point charges; force field models may not adequately describe the hydration configuration and dynamics.
authors.affiliation: †Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Kota
authors.initials: K
authors.lastname: Usui
authors.affiliation: †Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Hunger
authors.affiliation: ‡Johannes Gutenberg University Mainz, Staudingerweg 7, 55099 Mainz, Germany.
authors.forename: Marialore
authors.initials: M
authors.lastname: Sulpizi
authors.affiliation: §Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan.
authors.forename: Tatsuhiko
authors.initials: T
authors.lastname: Ohto
authors.affiliation: †Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Mischa
authors.initials: M
authors.lastname: Bonn
authors.affiliation: †Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.
authors.forename: Yuki
authors.initials: Y
authors.lastname: Nagata