Dynamics Of Interfacial Water Research Articles

Ion-size effect within the aqueous solution interface at the Pt(111) surface: molecular dynamics studies

All-atom classical force-field based molecular dynamics simulations have been employed to investigate the structure and dynamics of interfacial water in systems of pure water, 1 M LiOH and 1 M KOH aqueous solutions at an uncharged Pt(111) surface. Results indicate that the ordering of water molecules is affected as far as 9 Å from the Pt surface, corresponding to three layers of water molecules. Specific packing geometries of water in electrolyte solutions depend on the ionic radius, and both Li(+) and K(+) ions are found to adsorb directly onto the Pt surface. Significantly higher values of the water-dipole autocorrelation function in the adlayer are found for the system with Li(+) ions compared to the systems with K(+) ions or pure water. Also strongly reduced translational motion is observed in the case of Li(+), both in-plane and perpendicular to the surface. This result suggests a strong stabilizing role of Li(+) ions on water molecules. Decreased mobility of the water adlayer makes it difficult for other compounds in the aqueous solution to access the Pt surface. This implies that the reason for the reduced catalytic activity of Pt(111) surface in the presence of LiOH is due to the freezing effect Li(+) ions have on water.

Effect of Surface Charge on the Vibrational Dynamics of Interfacial Water

The effect of the structuring of interfacial water, induced by surface charge, on the ultrafast vibrational dynamics of the O-H stretch in the hydrogen bonded spectral region was studied at the H(2)O/fused silica interface. At high pH, where the electric field resulting from deprotonation of silanol groups polarizes several layers of water molecules, fast vibrational dynamics similar to the dynamics of bulk water is observed. At the neutral surface, where the structural ordering of interfacial water and the thickness of interfacial water are smaller than those at the charged surface, the vibrational lifetime of the O-H stretch becomes more than two times longer (T(1) approximately 570 fs). The longer vibrational lifetime is a result of reduced intermolecular coupling resulting from incomplete solvation of the interfacial water species.

Structure and dynamics of interfacial water in model lung surfactants

The last decade has seen a transformation in understanding of the role of membrane-bound interfacial water. Whereas until recently water was treated principally as a continuum (primarily screening charges of lipids and proteins), it has become apparent recently that consideration of water's molecular-level properties is critical in understanding a variety of biochemical and biophysical processes. Here we investigate the structure and dynamics of water in contact with a monolayer of artificial lung surfactant, composed of four types of lipids and one protein. We probe this water using frequency-domain sum-frequency generation (SFG) spectroscopy, and a newly developed time-domain, three-pulse technique, in which the vibrational relaxation of interfacial water molecules is followed in real time. We characterize interfacial water in three systems: a monolayer of the pure lipid that is the majority of the lung surfactant mixture, a monolayer of the four lipids constituting the mixture, and a monolayer of the four lipids and the protein. We find subtle differences in the water structure and dynamics that depend on the mixture density and composition. In particular, frequency-domain measurements suggest that in the lipid mixture and the lipid mixture + protein, the relatively bulky lipids (those that have either three or unsaturated hydrocarbon tails) tend to be squeezed out at higher pressure. Measurements using the time-domain, three-pulse technique make clear that structural relaxation of interfacial water is significantly slowed down upon adding small amounts of protein to the lipids. Both results are consistent with prior measurements using other techniques in which more fluid lipids were shown to be 'squeezed out' of lung surfactant at high compression and the role of protein in the mixture was demonstrated to be a catalyst for the formation of multilayers under compression that are subsequently reintegrated into the monolayer on expansion.

Hydrogen Bond Dynamics at the Water/Hydrocarbon Interface

The dynamics of hydrogen bond formation and breakage for water in the vicinity of water/hydrocarbon liquid interfaces is studied using molecular dynamics simulations. Several liquid alkanes are considered as the hydrocarbon phase in order to determine the effects of their chain length and extent of branching on the properties of the adjacent water phase. In addition to defining the interface location in terms of the laboratory-frame density profiles, the effects of interfacial fluctuations are considered by locating the interface in terms of the proximity of the molecules of the other phase. We find that the hydrogen bond dynamics of interfacial water is weakly influenced by the identity of the hydrocarbon phase and by capillary waves. In addition to calculating hydrogen bond time correlations, we examine how the hydrogen bond dynamics depend on local coordination and determine the extent of cooperativity in the population relaxation of the hydrogen bonds that a given molecule participates in. The contributions of translational diffusion and reorientation of molecular O-H bonds to the mechanism of hydrogen bond breakage and reformation are investigated. In previous work, we have shown that rotation of the principal axes of water is anisotropic at the interface and depends on the initial orientation of the molecule relative to the interface. Here, we extend this analysis to the reorientation of the O-H vector and to hydrogen bond time correlation. We find that hydrogen bond dynamics are also sensitive to the initial orientation of the molecules participating in the hydrogen bond.

Nitrous Oxide Vibrational Energy Relaxation Is a Probe of Interfacial Water in Lipid Bilayers

Ultrafast infrared spectroscopy of N 2O is shown to be a sensitive probe of hydrophobic and aqueous sites in lipid bilayers. Distinct rates of VER of the nu 3 antisymmetric stretching mode of N 2O can be distinguished for N 2O solvated in the acyl tail, interfacial water, and bulk water regions of hydrated dioleoylphosphatidylcholine (DOPC) bilayers. The lifetime of the interfacial N 2O population is hydration-dependent. This effect is attributed to changes in the density of intermolecular states resonant with the nu 3 band ( approximately 2230 cm (-1)) resulting from oriented interfacial water molecules near the lipid phosphate. Thus, the N 2O VER rate becomes a novel and experimentally convenient tool for reporting on the structure and dynamics of interfacial water in lipids and, potentially, in other biological systems.

Biomolecular simulations of membranes: Physical properties from different force fields

Phospholipid force fields are of ample importance for the simulation of artificial bilayers, membranes, and also for the simulation of integral membrane proteins. Here, we compare the two most applied atomic force fields for phospholipids, the all-atom CHARMM27 and the united atom Berger force field, with a newly developed all-atom generalized AMBER force field (GAFF) for dioleoylphosphatidylcholine molecules. Only the latter displays the experimentally observed difference in the order of the C2 atom between the two acyl chains. The interfacial water dynamics is smoothly increased between the lipid carbonyl region and the bulk water phase for all force fields; however, the water order and with it the electrostatic potential across the bilayer showed distinct differences between the force fields. Both Berger and GAFF underestimate the lipid self-diffusion. GAFF offers a consistent force field for the atomic scale simulation of biomembranes.

Beyond Bilayers: Interfacial Water Dynamics and Aggregation from First Principles

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Dynamics of Water Confined in the Interdomain Region of a Multidomain Protein

Molecular dynamics simulations are performed to study the dynamics of interfacial water confined in the interdomain region of a two-domain protein, BphC enzyme. The results show that near the protein surface the water diffusion constant is much smaller and the water-water hydrogen bond lifetime is much longer than that in bulk. The diffusion constant and hydrogen bond lifetime can vary by a factor of as much as 2 in going from the region near the hydrophobic domain surface to the bulk. Water molecules in the first solvation shell persist for a much longer time near local concave sites than near convex sites. Also, the water layer survival correlation time shows that on average water molecules near the extended hydrophilic surfaces have longer residence times than those near hydrophobic surfaces. These results indicate that local surface curvature and hydrophobicity have a significant influence on water dynamics.

Ultrafast electron crystallography of interfacial water.

We report direct determination of the structures and dynamics of interfacial water on a hydrophilic surface with atomic-scale resolution using ultrafast electron crystallography. On the nanometer scale, we observed the coexistence of ordered surface water and crystallite-like ice structures, evident in the superposition of Bragg spots and Debye-Scherrer rings. The structures were determined to be dominantly cubic, but each undergoes different dynamics after the ultrafast substrate temperature jump. From changes in local bond distances (OH.O and O.O) with time, we elucidated the structural changes in the far-from-equilibrium regime at short times and near-equilibration at long times.

Structure and Dynamics of Interfacial Water in an Lα Phase Lipid Bilayer from Molecular Dynamics Simulations

Based on molecular dynamics simulations, an analysis of structure and dynamics is performed on interfacial water at a liquid crystalline dipalmitoylphosphatidycholine/water system. Water properties relevant for understanding NMR relaxation are emphasized. The first and second rank orientational order parameters of the water O– H bonds were calculated, where the second rank order parameter is in agreement with experimental determined quadrupolar splittings. Also, two different interfacial water regions (bound water regions) are revealed with respect to different signs of the second rank order parameter. The water reorientation correlation function reveals a mixture of fast and slow decaying parts. The fast (ps) part of the correlation function is due to local anisotropic water reorientation whereas the much slower part is due to more complicated processes including lateral diffusion along the interface and chemical exchange between free and bound water molecules. The 100-ns-long molecular dynamics simulation at constant pressure (1 atm) and at a temperature of 50°C of 64 lipid molecules and 64 × 23 water molecules lack a slow water reorientation correlation component in the ns time scale. The 2H 2O powder spectrum of the dipalmitoylphosphatidycholine/water system is narrow and consequently, the NMR relaxation time T 2 is too short compared to experimental results.

Rotational dynamics of hydration water in dicalcium silicate by quasielastic neutron scattering.

Quasielastic neutron scattering (QENS) has been used to investigate the single-particle dynamics of interfacial water in dicalcium silicate (C2S)/water paste. Our previous neutron-scattering studies on interfacial water have focused attention on the translational dynamics of the center of mass of water molecules. In this paper, we have collected QENS data on a wider range of wave-vector transfer so that both translational and rotational motions of water molecules are detected. The data have been analyzed by models for translation and rotation we recently proposed for supercooled water. The evolution of the parameters describing the relaxational dynamics of water embedded in the C2S matrix is given at temperature T=303 K as a function of the curing time.

Analysis of interfacial water structure and dynamics in α-maltose solution by molecular dynamics simulation

The structure and dynamic behavior of the interfacial water around an α-maltose molecule are studied by molecular dynamics (MD) simulations. It is found that 8 hydrogen bonds (H-bonds) are formed between water and hydroxyl groups of each ring in maltose. The results of the radial distribution function of water around maltose are consistent with the hydrogen bonding analysis. The calculated diffusion coefficient of water around the different atom types in maltose shows that no general trend can be found, which may suggest that the solvent mobility is not influenced significantly by the atom types in the carbohydrate molecule.

Slow dynamics of interfacial water

The highlights of our recent high-resolution quasi-elastic incoherent neutron: scattering studies of the translational dynamics of water molecules contained in the micropores of Vycor glass and on the surface of a deuterated protein are summarized. The incoherent quasi-elastic spectra from the confined H2O are first analyzed by a confined diffusion model to obtain the elastic incoherent structure factor (EISF), the short-time self-diffusion constant (D), and the probability per unit time for a jump (1/τ0) as functions of the surface coverage and temperature. The effect of the confinement on the slowing down of the translational motion is discussed. An alternative interpretation of the slow dynamics based on a molecular mechanism of a cage effect and an approach to the kinetic glass transition is put forth.

Molecular dynamics simulation of interfacial water structure and dynamics in a parvalbumin solution

A 106-ps molecular dynamics simulation of the calcium-binding protein parvalbumin in water has been used as a basis for studying interfacial water. A general conclusion is that structure and dynamics of the interfacial water are only marginally affected by the presence of the protein. A few water molecules, which reside close to charge groups, are immobilized throughout the simulation. In the analysis the water molecules have been classified according to the distance to the nearest protein atom. Close to the protein the authors find a decrease in radial diffusion, while lateral diffusion is enhanced. In the inner water layers the dipole moment vector is preferentially oriented perpendicular to the radius vector of the protein. The reorientational correlation times have a minimum 4-5 /Angstrom/ from the protein, with values similar to those obtained from simulations of pure water.