Session B26: Chemical Physics of Hydrogen Bonding II

Water at silica/liquid water interfaces investigated by DFT-MD simulations

This talk is dedicated to probing the microscopic structural organization of water at silica/liquid water interfaces including electrolytes by first principles DFT-based molecular dynamics simulations (DFT-MD). We will present our very recent DFT-MD simulations of electrolytic (KCl, NaCl, NaI) silica/liquid water interfaces in order to unravel the intertwined structural properties of water and electrolytes at the crystalline quartz/liquid water and amorphous silica/liquid water interfaces. DFT-MD simulations provide direct knowledge of the structural organization of water and the H-Bond network formed between the water molecules within the different water layers above the silica surface. One can furthermore extract vibrational signatures of the water molecules within the interfacial layers from the DFT-MD simulations, especially non-linear SFG (Sum Frequency generation) signatures that are active at solid/liquid interfaces. The strength of the simulated spectra is that a detailed analysis of the signatures in terms of the water/water H-Bond networks formed within the interfacial water layers and in terms of the water/silica or water/electrolytes H-Bond networks can be given. Comparisons of SFG spectra between quartz/water/electrolytes and amorphous silica/water/electrolytes interfaces allow us to definitely conclude on how the structural arrangements of liquid water at these electrolytic interfaces modulate the final spectroscopic signatures.   

Modulators of heterogeneous protein surface water dynamics

The hydration water that solvates proteins is a major factor in driving or enabling biological events, including protein-protein and protein-ligand interactions. We investigate the role of the protein surface in modulating the hydration water fluctuations on both the picosecond and nanosecond timescale with an emerging experimental NMR technique known as Overhauser Dynamic Nuclear Polarization (ODNP). We carry out site-specific ODNP measurements of the hydration water fluctuations along the surface of Chemotaxis Y (CheY), and correlate the measured fluctuations to hydropathic and topological properties of the CheY surface as derived from molecular dynamics (MD) simulation. Furthermore, we compare hydration water fluctuations measured on the CheY surface to that of other globular proteins, as well as intrinsically disordered proteins, peptides, and liposome surfaces to systematically test characteristic effects of the biomolecular surface on the hydration water dynamics. Our results suggest that the labile (ps) hydration water fluctuations are modulated by the chemical nature of the surface, while the bound (ns) water fluctuations are present on surfaces that feature a rough topology and chemical heterogeneity such as the surface of a folded and structured protein.\\ \\In collaboration with: Ryan Barnes, Dept of Chemistry and Biochemistry, University of California Santa Barbara   

Enhanced Tetrahedral Order in Hydrophobic Hydration-Shells

The influence of oily molecules on water structure has long been a subject of speculation. Early thermodynamic evidence was interpreted as indicating the formation of "icebergs" around oily molecules, while later simulations and neutron scattering studies found no evidence of such structures. More recently, Raman multivariate curve resolution (Raman-MCR) studies of the OH stretch band of water in hydrophobic hydration-shells have found evidence of both hydrogen bond strengthening and the formation of broken hydrogen bonds (dangling OH groups). Here we use Raman-MCR to show that the enhanced tetrahedral order in cold liquid water, as well as in solid clathrate-hydrates, gives rise to the emergence of a peak near 200 cm$^{-1}$ whose intensity is correlated with the OH stretch shoulder near 3200 cm$^{-1}$. Moreover, we observe the same two correlated bands in the hydration-shells of oily molecules, thus providing clear experimental evidence of enhanced tetrahedral order in hydrophobic hydration-shells.   

Phosphate vibrations as reporters of DNA hydration

The asymmetric phosphate stretch vibrational frequency is extraordinarily sensitive to its local solvent environment. Using density functional theory calculations on the model compound dimethyl phosphate, the asymmetric phosphate stretch vibrational frequency was found to shift linearly with the magnitude of an electric field along the symmetry axis of the PO$_2$ moiety (i.e. the asymmetric phosphate stretch is an excellent linear vibrational Stark effect probe). With this linear relationship established, asymmetric phosphate stretch vibrational frequencies were computed during the course of a molecular dynamics simulation of fully hydrated DNA. Moreover, contributions to shifts in the frequencies from subpopulations of water molecules (e.g. backbone, minor groove, major groove, etc.) were calculated to reveal how phosphate vibrations report the onset of DNA hydration in experiments that vary the relative humidity of non-condensing (dry) DNA samples.   

First-Principles Molecular Dynamics Simulations of NaCl in Water: Performance of Advanced Exchange-Correlation Approximations in Density Functional Theory

Our ability to correctly model the association of oppositely charged ions in water is fundamental in physical chemistry and essential to various technological and biological applications of molecular dynamics (MD) simulations. MD simulations using classical force fields often show strong clustering of NaCl in the aqueous ionic solutions as a consequence of a deep contact pair minimum in the potential of mean force (PMF) curve. First-Principles Molecular Dynamics (FPMD) based on Density functional theory (DFT) with the popular PBE exchange-correlation approximation, on the other hand, show a different result with a shallow contact pair minimum in the PMF. We employed two of most promising exchange-correlation approximations, $\omega $B97xv by Mardiorossian and Head-Gordon (1) and SCAN by Sun, Ruzsinszky and Perdew (2), to examine the PMF using FPMD simulations. $\omega $B97xv is highly empirically and optimized in the space of range-separated hybrid functional with a dispersion correction while SCAN is the most recent meta-GGA functional that is constructed by satisfying various known conditions in well-defined physical limits. We will discuss our findings for PMF, charge transfer, water dipoles, etc. (1) Mardirossian, Narbe, and Martin Head-Gordon. \textit{Physical Chemistry Chemical Physics} 16.21 (2014): 9904-9924. (2) Sun, Jianwei, Adrienn Ruzsinszky, and John P. Perdew. \textit{Physical review letters} 115.3 (2015): 036402.   

Computational Investigation of Surface Freezing in a Molecular Model of Water

Ice formation plays a pivotal role in atmospheric processes. Yet, its microscopic mechanism is difficult to determine from experiments. One of the biggest unresolved questions about ice formation in the atmosphere is whether a vapor-liquid interface enhances or suppresses freezing at its vicinity, a conundrum regarded as of the ten biggest unresolved questions about ice and snow [1]. Despite earlier suggestion that ice formation must be enhanced at a free interface [2], the experimental evidence for and against it are inconclusive. In this work, we address this question computationally and use a path sampling technique known as forward flux sampling to study the kinetics and mechanism of ice nucleation in freestanding nanofilms of supercooled water modeled using the TIP4P/Ice force field, one of the best molecular models of water. We observe that nucleation is almost seven orders of magnitude faster than the bulk in the film geometry. Yet, the nucleation process is homogeneous in nature, and starts not at the free surface, but within an interior region of the film that favors the formation of double diamond cages, and therefore the cubic polymorph of ice. [1] Bartels-Rauch, Nature, 494, 27 (2013). [2] Tabazadeh, \emph{et al}, PNAS, 99, 15873 (2002).   

Real space imaging of buckled tetramer and flat hexamer water cluster on Au(111) surface: determine structures of water clusters on solid surface

The configuration of hydrogen bond network in water clusters is important but there still exist debates. For example, a flat configuration of water hexamer has been observed by scanning tunneling microscope (STM), whereas a buckled configuration is favored in density functional theory (DFT) simulation. One of the difficulties in solving such puzzle is due to the lack of high-resolution images and controlled cluster reconstruction, especially for water clusters consists of no more than 6 water molecules. Here we present low-temperature STM images of water clusters with unprecedented spatial resolution which clearly reveal precise configurations of isolated water clusters on Au(111) surface. While water tetramers present a buckled configuration, water hexamers all features a flat configuration, in which the apparent height of each water molecule is the same as the height of the two low-lying molecules in the buckled tetramer. Transformations from a flat hexamer to buckled tetramer and pentamer are achieved by molecule manipulations, which illustrates the competition between hydrogen bond interactions and van der Waals interactions. With combined DFT calculations, the features of hydrogen bond network and van der Waals interactions in such a system are further determined.   

Molecular structure and polarization of water interfaces: physical picture from the hydrogen bonding of constrained geometry

In this talk, we highlight the fundamental relationship between hydrogen bonding and aqueous interfacial molecular polarization. We describe this relationship, and how it is mediated by the density profile of water interface, in terms of a mean field model of interfacial hydrogen bonding. Specifically the model can predict the orientational distribution of interfacial water molecules based on the anisotropic local density and given hydrogen bond geometry. We demonstrate that the fluctuation in bond geometry at the interface is fairly different from that in the bulk environment and it is closely related with the mean polarization and its mean fluctuation observed at the interface.