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A4: Wasserrelaxationsdynamik auf Grenzflächen

Das Ziel dieses Projektes ist es, eine neue Multiskalen-Methodik zu entwickeln, die ab-initio-Dynamik und Kraftfeld-basierte Molekulardynamik kombiniert, um die Vibrations-Relaxationsdynamik in kondensierten Systemen und insbesondere in Wasser an Grenz- und Oberflächen zu verstehen. Die Herausforderung ist hier die Berechnung der Schwingungs-Response-Funktionen und die direkte Simulation der Energie-Relaxations-Prozesse auf der Zeitskala von Nanosekunden (nur so ist ein direkter Vergleich mit Experimenten möglich), in Systemen (Wasser und Elektrolyt-Lösungen), deren Eigenschaften kritisch durch ihre elektronische Struktur bestimmt sind.

A new force field including charge directionality for TMAO in aqueous solution
Kota Usui, Yuki Nagata, Johannes Hunger, Mischa Bonn and Marialore Sulpizi
J. Chem. Phys. 145, 064103 (2016);

We propose a new force field for trimethylamine N-oxide (TMAO), which is designed to reproduce the long-lived and highly directional hydrogen bond between the TMAO oxygen (OTMAO) atom and surrounding water molecules. Based on the data obtained by ab initio molecular dynamics simulations, we introduce three dummy sites around OTMAO to mimic the OTMAO lone pairs and we migrate the negative charge on the OTMAO to the dummy sites. The force field model developed here improves both structural and dynamical properties of aqueous TMAO solutions. Moreover, it reproduces the experimentally observed dependence of viscosity upon increasing TMAO concentration quantitatively. The simple procedure of the force field construction makes it easy to implement in molecular dynamics simulation packages and makes it compatible with the existing biomolecular force fields. This paves the path for further investigation of protein-TMAO interaction in aqueous solutions.

Molecular Dynamics Simulations of SFG Librational Modes Spectra of Water at the Water–Air Interface
Rémi Khatib, Taisuke Hasegawa, Marialore Sulpizi, Ellen H. G. Backus, Mischa Bonn, and Yuki Nagata
J. Phys. Chem. C 120 (33), 18665–18673 (2016);

At the water–air interface, the hydrogen-bond network of water molecules is interrupted, and accordingly, the structure and dynamics of the interfacial water molecules are altered considerably compared with the bulk. Such interfacial water molecules have been studied by surface-specific vibrational sum-frequency generation (SFG) spectroscopy probing high-frequency O–H stretch and H–O–H bending modes. In contrast, the low-frequency librational mode has been much less studied with SFG. Because this mode is sensitive to the hydrogen-bond connectivity, understanding the librational mode of the interfacial water is crucial for unveiling a microscopic view of the interfacial water. Here, we compute the SFG librational mode spectra at the water–air interface by using molecular dynamics simulation. We show that the modeling of the polarizability has a drastic effect on the simulated librational mode spectra, whereas the spectra are less sensitive to the force field models and the modeling of the dipole moment. The simulated librational spectra display a peak centered at ∼700 cm–1, which is close to the infrared peak frequency of the liquid water librational mode of 670 cm–1. This indicates that the librational mode of the interfacial water at the water–air interface closely resembles that of bulk liquid water.

Molecular Mechanism of Water Evaporation
Yuki Nagata, Kota Usui, Mischa Bonn
Phys. Rev. Lett. 115 (23), 236102 (2015);

The surface roughness, but not the water molecular orientation varies with temperature at the water–air interface
Yuki Nagata, Taisuke Hasegawa, Ellen H. G. Backus, Kota Usui, Seiji Yoshimune, Tatsuhiko Ohto, Mischa Bonn
Phys. Chem. Chem. Phys. 17 (36), 23559-23564 (2015);

Ultrafast Vibrational Dynamics of Water Disentangled by Reverse Nonequilibrium Ab Initio Molecular Dynamics Simulations
Yuki Nagata, Seiji Yoshimune, Cho-Shuen Hsieh, Johannes Hunger, Mischa Bonn
Physical Review X 5 (2), 021002 (2015);

Toward ab initio molecular dynamics modeling for sum-frequency generation spectra; an efficient algorithm based on surface-specific velocity-velocity correlation function
Tatsuhiko Ohto, Kota Usui, Taisuke Hasegawa, Mischa Bonn, Yuki Nagata
The Journal of Chemical Physics 143 (12), 124702 (2015);

Ab Initio Liquid Water Dynamics in Aqueous TMAO Solution
Kota Usui, Johannes Hunger, Marialore Sulpizi, Tatsuhiko Ohto, Mischa Bonn, and Yuki Nagata
J. Phys. Chem. B 119 (33), 10597–10606 (2015);
doi: 10.1021/acs.jpcb.5b02579

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 (OTMAO) atom, due to the O–D···OTMAO 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···OTMAO 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···OTMAO hydrogen-bond has a strong directionality through the sp3 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.

Lipid Carbonyl Groups Terminate the Hydrogen Bond Network of Membrane-Bound Water
Tatsuhiko Ohto, Ellen H. G. Backus, Cho-Shuen Hsieh, Marialore Sulpizi, Mischa Bonn, and Yuki Nagata
J. Phys. Chem. Lett., 6 (22), 4499–4503 (2015);

We present a combined experimental sum-frequency generation (SFG) spectroscopy and ab initio molecular dynamics simulations study to clarify the structure and orientation of water at zwitterionic phosphatidylcholine (PC) lipid and amine N-oxide (AO) surfactant monolayers. Simulated O–H stretch SFG spectra of water show good agreement with the experimental data. The SFG response at the PC interface exhibits positive peaks, whereas both negative and positive bands are present for the similar zwitterionic AO interface. The positive peaks at the water/PC interface are attributed to water interacting with the lipid carbonyl groups, which act as efficient hydrogen bond acceptors. This allows the water hydrogen bond network to reach, with its (up-oriented) O–H groups, into the headgroup of the lipid, a mechanism not available for water underneath the AO surfactant. This highlights the role of the lipid carbonyl group in the interfacial water structure at the membrane interface, namely, stabilizing the water hydrogen bond network.


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