Hydration and dewetting near graphite-CH(3) and graphite-COOH plates

The dynamics of water near the nanoscale hydrophobic (graphite-CH(3)) and hydrophilic (graphite-COOH) plates has been studied in detail with molecular dynamics simulations in this paper. It is shown that these designed surfaces (by growing a layer of methyl or carboxyl groups on top of graphite) can have a significant impact on the neighboring water dynamics, with the hydrophilic carboxyl surface having even more profound effects. The water hydrogen bond lifetime is much longer near both types of surfaces than that in the bulk, while on the other hand the water diffusion constant is much smaller than that in the bulk. The difference in the diffusion constant can be as large as a factor of 8 and the difference in the hydrogen bond lifetime can be as large as a factor of 2, depending on the distance from the surface. Furthermore, the water molecules in the first solvation shell of surface atoms show a strong bias in hydroxyl group orientation near the surface, confirming some of the previous findings. Finally, the possible water dewetting transition between two graphite-CH(3) plates and the effect of the strength of the solute-solvent attractions on the water drying transition are investigated. The relationship among the dewetting transition critical distance, van der Waals potential well depth, and water contact angle on the graphite-CH(3) surface is also analyzed on the basis of a simple macroscopic theory, which can be used to predict the dewetting transition critical distance.     

Structural and dynamic properties of water within the solvation layer around various conformations of the glycine-based polypeptide

Several conformations of the solvated glycine-based polypeptides were investigated using molecular dynamics simulations. Some properties of water in the neighboring space around these molecules were investigated. It was found that water forms a well-defined layer-the first solvation shell-around the peptide molecule, and thickness of this layer is independent of the peptide structure and is equal to approximately 0.28 nm. Within this layer, water molecules show marked orientations relative to a peptide surface. Using the two-particle contribution to entropy as a measure of structural ordering of water, we found that the first solvation shell contributes 95% or more to the total water ordering around the peptide molecule. In investigating the dynamic properties of water, diffusion coefficients and lifetime of the hydrogen bond, clear differences between solvation layer and the bulk water were observed. It was found that the translational diffusion coefficient, D(T), decreases by 30% or more compared to bulk water; also, the lifetime of the water-water hydrogen bond clearly increases. The rotational diffusion coefficient, however, decreases only slightly, no more than approximately 10%. These differences correspond to the slightly higher energy of the hydrogen bond, and to its slightly distorted geometry. Analyzing the translational dynamics of water in the vicinity of the peptide molecule, it was deduced that the structure of the first solvation shell becomes more rigid than the structure of the bulk water. Investigation of a "pure hydrophobic" form of the polypeptide shows that the structure and the properties of water within the solvation shell are predominantly determined by the hydrophobic effect. The specific interactions between water molecules and various charge groups of the peptide molecule modifies this effect only slightly.     

Hydrogen bond lifetime dynamics at the interface of a surfactant monolayer

The dynamics of water near the polar headgroups of surfactants in a monolayer adsorbed at the air/water interface is likely to play a decisive role in determining the physical behavior of such organized assemblies. We have carried out an atomistic molecular dynamics (MD) simulation of a monolayer of the anionic surfactant sodium bis(2-ethyl-1-hexyl) sulfosuccinate (aerosol-OT or AOT) adsorbed at the air/water interface. The simulation is performed at room temperature with a surface coverage of that at the critical micelle concentration (78 Angstrom(2)/molecule). Detailed analyses of the lifetime dynamics of surfactant-water (SW) and water-water (WW) hydrogen bonds at the interface have been carried out. The nonexponential hydrogen bond lifetime correlation functions have been analyzed by using the formalism of Luzar and Chandler, which allowed identification of the bound states at the interface and quantification of the dynamic equilibrium between bound and quasi-free water molecules, in terms of time-dependent relaxation rates. It is observed that the water molecules present in the first hydration layer form strong hydrogen bonds with the surfactant headgroups and hence have longer lifetimes. Importantly, it is found that the overall relaxation of the SW hydrogen bonds is faster for those water molecules which form two hydrogen bonds with the surfactant headgroups than those forming one such hydrogen bond. Equally interestingly, it is further noticed that water molecules beyond the first hydration layer form weaker hydrogen bonds than pure bulk water.     

Exploring DNA groove water dynamics through hydrogen bond lifetime and orientational relaxation

Dynamics of water molecules in the grooves of DNA are of great interest both for practical (functionality of DNA) and fundamental (as examples of confined systems) interest. Here the authors employ atomistic molecular dynamics simulations to understand varying water dynamics at the minor and the major grooves of a 38 base-pair long DNA duplex in water. In order to understand and quantify the diversity in the nature of hydrogen bond due to many hydrogen bond donors and acceptors present in the four bases, they have undertaken study of hydrogen bond lifetime (HBLT) correlation functions of all the specific hydrogen bonds between the base atoms and water molecules. They find that the HBLT correlation functions are in general multiexponential, with the average lifetime depending significantly on the specificity and may thus be biologically relevant. The average hydrogen bond lifetime is longer in the minor groove than that in the major groove by almost a factor of 2. Analysis further shows that water hydrogen bonds with phosphate oxygen have substantially shorter lifetimes than those with the groove atoms. They also compute two different orientational time correlation functions (OTCFs) of the water molecules present at the major and the minor grooves and attempt to correlate OTCF with HBLT correlation function. The OTCFs in the minor groove exhibit three time scales, with the time constant of the slowest component one to two orders of magnitude longer than what is observed for bulk water. A slow component is also present for the major groove water but with shorter time constant. Interestingly, correlation between reformations allowed HBLT correlation function [C(HB)(t)] and the OTCF markedly deviates from each other in the grooves, indicating enhanced rigidity of water molecules in the grooves.     

Hydrogen bonded structure and dynamics of liquid-vapor interface of water-ammonia mixture: an ab initio molecular dynamics study

We have carried out ab initio molecular dynamics simulations of a liquid-vapor interfacial system consisting of a mixture of water and ammonia molecules. We have made a detailed analysis of the structural and dynamical properties of the bulk and interfacial regions of the mixture. Among structural properties, we have looked at the inhomogeneous density profiles of water and ammonia molecules, hydrogen bond distributions, orientational profiles, and also vibrational frequency distributions of bulk and interfacial molecules. It is found that the interfacial molecules show preference for specific orientations so as to form water-ammonia hydrogen bonds at the interface with ammonia as the acceptor. The structure of the system is also investigated in terms of inter-atomic voids present in the system. Among the dynamical properties, we have calculated the diffusion, orientational relaxation, hydrogen bond dynamics, and vibrational spectral diffusion in bulk and interfacial regions. It is found that the diffusion and orientation relaxation of the interfacial molecules are faster than those of the bulk. However, the hydrogen bond lifetimes are longer at the interface which can be correlated with the time scales found from the decay of frequency time correlations.     

Hydrogen-bond dynamics in the air-water interface

Hydrogen-bond (H-bond) dynamics in the air-water interface is studied by molecular dynamics simulations. The analysis reveals that the dynamics of breaking and forming hydrogen bonds in the air-water interface is faster than that in bulk water for the polarizable water models. This is in contrast to the results found on a protein surface. We show that the difference stems from more rapid translational diffusion in the interface. When the effect of pair diffusion is eliminated, the hydrogen-bond dynamics in the interface is observed to be slower than that in the bulk. This occurs because the number of water molecules adjacent to a hydrogen-bonded pair and available to accept or donate a hydrogen bond is smaller in the interface than in the bulk. The comparison between polarizable water models and fixed-charge models highlights the potential importance of the polarization effect in the water-vapor interface.     

Hydrogen‐Bond Free Energy of Local Biological Water

Here, we propose an experimental methodology based on femtosecond‐resolved fluorescence spectroscopy to measure the hydrogen (H)‐bond free energy of water at protein surfaces under isothermal conditions. A demonstration was conducted by installing a non‐canonical isostere of tryptophan (7‐azatryptophan) at the surface of a coiled‐coil protein to exploit the photoinduced proton transfer of its chromophoric moiety, 7‐azaindole. The H‐bond free energy of this biological water was evaluated by comparing the rates of proton transfer, sensitive to the hydration environment, at the protein surface and in bulk water, and it was found to be higher than that of bulk water by 0.4 kcal mol^?1. The free‐energy difference is dominated by the entropic cost in the H‐bond network among water molecules at the hydrophilic and charged protein surface. Our study opens a door to accessing the energetics and dynamics of local biological water to give insight into its roles in protein structure and function.     

Energetics of protein backbone hydrogen bonds and their local electrostatic environment

MD simulation study of several peptides including a polyalanine,a helix(pdb:2I9M),and a leucine zipper were carried out to investigate hydrogen bond energetics using dynamic polarized protein-specific charge(DPPC)to account for the polarization effect in protein dynamics.Results show that the backbone hydrogen-bond strength is generally correlated with its specific local electrostatic environment,measured by the number of water molecules near the hydrogen bond in the first solvation shell.The correlation coefficient is found to be 0.89,0.78,and 0.80,respectively,for polyalanine,2I9M protein,and leucine zipper.In the polyalanine,the energies of the backbone hydrogen bonds are very similar to each other due to their similar local electrostatic environment.The current study helps demonstrate and support the understanding that hydrogen bonds are stronger in a hydrophobic surrounding than in a hydrophilic one.For comparison,the result from simulation using standard force field shows a much weaker correlation between hydrogen bond energy and local electrostatic environment due to the lack of polarization effect in the force field.     

A molecular dynamics simulation of the adsorption of water molecules surrounding an Au nanoparticle

This study uses molecular dynamics simulations performed in a parallel computing environment to investigate the adsorption of water molecules surrounding Au nanoparticles of various sizes. An observation of the oxygen and hydrogen atom distributions reveals that the adsorption of the water molecules creates two shell-like formations of water in close vicinity to the Au nanoparticle surface. These shell-like formations are found to be more pronounced around smaller Au nanoparticles. The rearrangement of water molecules in this region reduces the local hydrogen bond strength to below that which is observed in the bulk region. Finally, the simulation results indicate that the absolute value of the interaction energy between the water molecules and the Au nanoparticle is reduced when the water molecules surround a nanoparticle of larger diameter. This observation implies that a stronger adsorption effect exists between smaller Au nanoparticles and water molecules. Hence, the value of the adsorption constant increases for smaller Au nanoparticles.     

The hydrophobic effect: molecular dynamics simulations of water confined between extended hydrophobic and hydrophilic surfaces

Structural and dynamic properties of water confined between two parallel, extended, either hydrophobic or hydrophilic crystalline surfaces of n-alkane C(36)H(74) or n-alcohol C(35)H(71)OH, are studied by molecular dynamics simulations. Electron density profiles, directly compared with corresponding experimental data from x-ray reflectivity measurements, reveal a uniform weak de-wetting characteristic for the extended hydrophobic surface, while the hydrophilic surface is weakly wetted. These microscopic data are consistent with macroscopic contact angle measurements. Specific water orientation is present at both surfaces. The ordering is characteristically different between the surfaces and of longer range at the hydrophilic surface. Furthermore, the dynamic properties of water are different at the two surfaces and different from the bulk behavior. In particular, at the hydrophobic surface, time-correlation functions reveal that water molecules have characteristic diffusive behavior and orientational ordering due to the lack of hydrogen bonding interactions with the surface. These observations suggest that the altered dynamical properties of water in contact with extended hydrophobic surfaces together with a partial drying of the surfaces are more indicative of the hydrophobic effect than structural ordering, which we suggest to be independent of surface topology.     

Hydrogen bond dynamics and water structure in glucose-water solutions by depolarized Rayleigh scattering and low-frequency Raman spectroscopy

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.     

Anomalous dielectric relaxation of water molecules at the surface of an aqueous micelle

Dielectric relaxation of aqueous solutions of micelles, proteins, and many complex systems shows an anomalous dispersion at frequencies intermediate between those corresponding to the rotational motion of bulk water and that of the organized assembly or macromolecule. The precise origin of this anomalous dispersion is not well-understood. In this work we employ large scale atomistic molecular dynamics simulations to investigate the dielectric relaxation (DR) of water molecules in an aqueous micellar solution of cesium pentadecafluorooctanoate. The simulations clearly show the presence of a slow component in the moment-moment time correlation function [PhiMW(t)] of water molecules, with a time constant of about 40 ps, in contrast to only 9 ps for bulk water. Interestingly, the orientational time correlation function [Cmu(t)] of individual water molecules at the surface exhibits a component with a time constant of about 19 ps. We show that these two time constants can be related by the well-known micro-macrorelations of statistical mechanics. In addition, the reorientation of surface water molecules exhibits a very slow component that decays with a time constant of about 500 ps. An analysis of hydrogen bond lifetime and of the rotational relaxation in the coordinate frame fixed on the micellar body seems to suggest that the 500 ps component owes its origin to the existence of an extended hydrogen bond network of water molecules at the surface. However, this ultraslow component is not found in the total moment-moment time correlation function of water molecules in the solution. The slow DR of hydration water is found to be well correlated with the slow solvation dynamics of cesium ions at the water-micelle interface.     

Single particle and pair dynamics in water-formic acid mixtures containing ionic and neutral solutes: nonideality in dynamical properties

A series of molecular dynamics simulations of water-formic acid mixtures containing either an ionic solute or a neutral hydrophobic solute has been performed to study the extent of nonideality in the dynamics of these solutes for varying composition of the mixtures. The diffusion coefficients of the charged solutes, both cationic and anionic, are found to show nonideal behavior with variation of composition, and similar nonideality is also observed for the diffusion and orientational relaxation of solvent molecules in these mixtures. The diffusion coefficient of a neutral hydrophobic solute, however, decreases monotonically with increase in water concentration. We have also investigated some of the pair dynamical properties such as water-water and water-formic acid hydrogen bond relaxation and residence dynamics of water molecules in water and formic acid hydration shells. The lifetimes of water-water hydrogen bonds are found to be longer than those between formic acid carbonyl oxygen-water hydrogen bonds, whereas the lifetimes of formic acid hydroxyl hydrogen-water hydrogen bonds are longer than those of water-water hydrogen bonds. In general, the hydrogen bond lifetimes for both water-water and water-formic acid hydrogen bonds are found to decrease with increase in water concentration. Residence times of water molecules also show the same trend with increase in formic acid concentration. Interestingly, these pair dynamical properties show a monotonic dependence on composition without any maximum or minimum and behave almost ideally with respect to changes in the composition of the mixtures. The present calculations are performed with fixed-charge nonpolarizable models of the solvent and solute molecules without taking into account many-body polarization effects in an explicit manner.     

Communication: On the locality of hydrogen bond networks at hydrophobic interfaces

The formation of structured hydrogen bond networks in the solvation shells immediate to hydrophobic solutes is crucial for a large number of water mediated processes. A long lasting debate in this context regards the mutual influence of the hydrophobic solute into the bulk water and the role of the hydrogen bond network of the bulk in supporting the solvation structure around a hydrophobic molecule. In this context we present a molecular dynamics study of the solvation of various hydrophobic molecules where the effect of different regions around the solvent can be analyzed by employing an adaptive resolution method, which can systematically separate local and nonlocal factors in the structure of water around a hydrophobic molecule. A number of hydrophobic solutes of different sizes and two different model potential interactions between the water and the solute are investigated.     

The effect of pressure on the hydration structure around hydrophobic solute: a molecular dynamics simulation study

Molecular dynamics simulations are performed to study the effects of pressure on the hydrophobic interactions between neopentane molecules immersed in water. Simulations are carried out for five different pressure values ranging from 1 atm to 8000 atm. From potential of mean force calculations, we find that with enhancement of pressure, there is decrease in the well depth of contact minimum (CM) and the relative stability of solvent separated minimum over CM increases. Lower clustering of neopentane at high pressure is also observed in association constant and cluster-structure analysis. Selected site-site radial distribution functions suggest efficient packing of water molecules around neopentane molecules at elevated pressure. The orientational profile calculations of water molecules show that the orientation of water molecules in the vicinity of solute molecule is anisotropic and this distribution becomes flatter as we move away from the solute. Increasing pressure slightly changes the water distribution. Our hydrogen bond properties and dynamics calculations reveal pressure-induced formation of more and more number of water molecules with five and four hydrogen bond at the expense of breaking of two and three hydrogen bonded water molecules. We also find lowering of water-water continuous hydrogen bond lifetime on application of pressure. Implication of these results for relative dispersion of hydrophobic molecules at high pressure are discussed.     

Interfacial water on hydrophobic surfaces recognized by ions and molecules

Recent spectrophotometric and molecular dynamics simulation studies have shown that the physicochemical properties and structures of water in the vicinity of hydrophobic surfaces differ from those of the bulk water. However, the interfacial water acting as a separation medium on hydrophobic surfaces has never been detected and quantified experimentally. In this study, we show that small inorganic ions and organic molecules differentiate the interfacial water formed on the surfaces of octadecyl-bonded (C(18)) silica particles from the bulk water and the chemical separation of these solutes in aqueous media with hydrophobic materials can be interpreted with a consistent mechanism, partition between the bulk water phase and the interfacial water formed on the hydrophobic surface. Thermal transition behaviour of the interfacial water incorporated in the nanopores of the C(18) silica materials and the solubility parameter of the water calculated from the distribution coefficients of organic compounds have indicated that the interfacial water may have a structure of disrupted hydrogen bonding. The thickness of the interfacial water or the limit of distance from the hydrophobic surface at which molecules and ions can sense the surface was estimated to be 1.25 ± 0.13 nm from the volume of the interfacial water obtained by a liquid chromatographic method and the surface area, suggesting that the hydrophobic effect may extend beyond the first solvation shell of water molecules directly surrounding the surfaces.     

Viscosity of the aqueous liquid/vapor interfacial region: 2D electrochemical measurements with a piperidine nitroxy radical probe

Surface partitioning of 2,2,6,6-tetramethyl-1-piperidynyloxy radical (Tempo) to the air/water interface follows a Langmuir isotherm. The partition constant was obtained by the surface tension measurements in the concentration range of 1.0 x 10(-4)-2.4 x 10(-3) M yielding K = 640 +/- 99 M(-1). The lateral mobility of Tempo at the air/water interface was measured electrochemically in the surface concentration range of 2.0 x 10(-11)-1.4 x 10(-10) mol/cm2, corresponding to ca. 7.3-50% full monolayer coverage. The measurements employed cyclic voltammetry with line microelectrodes touching the air/water interface. The Tempo lateral diffusion constant of (1.5 +/- 0.7) x 10(-4) cm2/s is independent of surface concentration below 4.0 x 10(-11) mol/cm2. The extent of Tempo water interactions was assessed by the electronic structure calculations. These calculations showed that, at most, two water molecules can hydrogen bond with the oxygen atom of the nitroxyl group of Tempo, and that a single water molecule forms a hydrogen bond that is ca. 30% stronger than the H2O-H2O hydrogen bond. These calculations led to a postulate that Tempo diffuses along the interface largely unimmersed, and that it is coupled to the interfacial water via hydrogen bonding with H2O. In view of this postulate, the viscosity of the aqueous liquid/vapor interfacial region obtained by interpreting the Tempo diffusion constant in the low concentration region is as much as 4 times smaller than that of bulk liquid water.     

Effect of nanostructure on the properties of water at the water-hydrophobic interface: a molecular dynamics simulation

The local structure of water near hydrophobic surfaces of different surface topographies has been analyzed by molecular dynamics simulation. An alkane crystal has been taken as the parent model for a hydrophobic surface. Surface structures were created by placing pits into it, which were half a nanometer deep and several nanometers wide. Around all structures, the water has a lower density, less orientational ordering, fewer water-water hydrogen bonds, and fewer surface contacts than for a flat unstructured surface. This indicates that the structured surfaces are more hydrophobic than the flat surface. Of the structures investigated, pits with a diameter of approximately 2.5 nm were effective in increasing the hydrophobic character of the surface.     

How protein surfaces induce anomalous dynamics of hydration water

Water around biomolecules slows down with respect to pure water, and both rotation and translation exhibit anomalous time dependence in the hydration shell. The origin of such behavior remains elusive. We use molecular dynamics simulations of water dynamics around several designed protein models to establish the connection between the appearance of the anomalous dynamics and water-protein interactions. For the first time we quantify the separate effect of protein topological and energetic disorder on the hydration water dynamics. When a static protein structure is simulated, we show that both types of disorder contribute to slow down water diffusion, and that allowing for protein motion, increasing the spatial dimensionality of the interface, reduces the anomalous character of hydration water. The rotation of water is, instead, altered by the energetic disorder only; indeed, when electrostatic interactions between the protein and water are switched off, water reorients even faster than in the bulk. The dynamics of water is also related to the collective structure--à voir the hydrogen bond (H-bond) network--formed by the solvent enclosing the protein surface. We show that, as expected for a full hydrated protein, when the protein surface offers pinning sites (charged or polar sites), the superficial water-water H-bond network percolates throughout the whole surface, hindering the water diffusion, whereas it does not when the protein surface lacks electrostatic interactions with water and the water diffusion is enhanced.     

Structure and dynamics of water confined in silica nanopores

We report the results of molecular simulation of water in silica nanopores at full hydration and room temperature. The model systems are approximately cylindrical pores in amorphous silica, with diameters ranging from 20 to 40 ?. The filled pores are prepared using grand canonical Monte Carlo simulation and molecular dynamics simulation is used to calculate the water structure and dynamics. We found that water forms two distinct molecular layers at the interface and exhibits uniform, but somewhat lower than bulk liquid, density in the core region. The hydrogen bond density profile follows similar trends, with lower than bulk density in the core and enhancements at the interface, due to hydrogen bonds between water and surface non-bridging oxygens and OH groups. Our studies of water dynamics included translational mean squared displacements, orientational time correlations, survival probabilities in interfacial shells, and hydrogen bond population relaxation. We found that the radial-axial anisotropy in translational motion largely follows the predictions of a model of free diffusion in a cylinder. However, both translational and rotational water mobilities are strongly dependent on the proximity to the interface, with pronounced slowdown in layers near the interface. Within these layers, the effects of interface curvature are relatively modest, with only a small increase in mobility in going from the 20 to 40 ? diameter pore. Hydrogen bond population relaxation is nearly bulk-like in the core, but considerably slower in the interfacial region.