Water Rotational Dynamics Research Articles

Shape of AOT Reverse Micelles: The Mesoscopic Assembly Is More Than the Sum of the Parts.

AOT reverse micelles are a common and convenient model system for studying the effects of nanoconfinement on aqueous solutions. The reverse micelle shape is important to understanding how the constituent components come together to form the coherent whole and the unique properties observed there. The shape of reverse micelles impacts the amount of interface present and the distance of the solute from the interface and is therefore vital to understanding interfacial properties and the behavior of solutes in the polar core. In this work, we use previously introduced measures of shape, the coordinate-pair eccentricity (CPE) and convexity, and apply them to a series of simulations of AOT reverse micelles. We simulate the most commonly used force field for AOT reverse micelles, the CHARMM force field, but we also adapt the OPLS force field for use with AOT, the first work to do so, in addition to using both 3- and 4-site water models. Altogether, these simulations are designed to examine the impact of the force field on the shape of the reverse micelles in detail. We also study the time autocorrelation of shape, the water rotational anisotropy decay, and how the CPE changes between the water pool and AOT tail groups. We find that although the force field changes the shape noticeably, AOT reverse micelles are always amorphous particles. The shape of the micelles changes on the order of 10 ns. The water rotational dynamics observed match the experiment and demonstrate slower dynamics relative to bulk water, suggesting a two-population model that fits a core/shell hypothesis. Taken together, our results indicate that it is likely not possible to create a perfect force field that can reproduce every aspect of the AOT reverse micelle accurately. However, the magnitude of the differences between simulations appears relatively small, suggesting that any reasonably derived force field should provide an acceptable model for most work on AOT reverse micelles.

Determination of the first hydration layer from the perspective of local electric field reorientation

In this work, we delve into the molecular mechanism governing water rotational dynamics through the lens of electrostatic interactions, employing molecular dynamics (MD) simulation. By monitoring the momentary electric field exerted on individual water molecules, we observe an alignment between the relaxation time of water rotation and the relaxation time of the exerted field reorientation for each water molecule in the studied sugar and sugar alcohol solutions. Furthermore, the reorientation time constant of the local field drops dramatically within 3.1 Å from the solute surface and approaches sub-picosecond beyond that, which is a novel perspective for identifying the hydration layer.

Effects of Hydrogen Peroxide on the Hydrogen Bonding Structure and Dynamics of Water and Its Influence on the Aqueous Solvation of the Insulin Monomer.

The hydrogen bond structure and dynamics of water and hydrogen peroxide (H2O2) in their binary mixtures have been studied at 298 K by classical molecular dynamics simulations. Twelve different concentrations of aqueous-H2O2 solutions are considered for this study. We have analyzed the interactions between water and H2O2 by site-site pair correlation functions and observed that the probability of formation of OW···HP hydrogen bonds are higher compared to OP···HW. The second solvation shell of water is strongly affected by increasing H2O2 concentrations (XP > 0.50), which signifies the destruction of the tetrahedral network structure of water. The translational and rotational dynamics of water and H2O2 do not significantly change up to 25% of H2O2 in aqueous mixtures. The hydrogen bond lifetime of water-water, water-H2O2, and H2O2-H2O2 in the aqueous-H2O2 solutions shows a very minimal change with increasing H2O2 concentrations. In addition to this, we also investigated the effect of H2O2 on the insulin monomer and observed that higher concentrations of H2O2 (XP = 0.10) change the secondary structure. The influence of H2O2 is more on chain-B than that on chain-A in the insulin monomer. The H2O2 occupancy at the protein surface is higher for negatively charged (GLU) and polar (ASN and THR) amino acid residues compared with that for positively charged and neutral residues in the solutions.

What do far-infrared spectra of solitary water in "water-in-solvent" systems reveal about water's solvation and dynamics?

Classical molecular dynamics simulations of water in ionic and dipolar solvents were used to interpret the far-infrared (FIR) rotation/libration spectra of "solitary water" in terms of water's rotational dynamics and interactions with solvents. Seven solvents represented by nonpolarizable all-atom force fields and a series of idealized variable-charge solvents were used to span the range of solvent polarities (hydrogen bonding) studied experimentally. Simulated spectra capture the solvent dependence observed, as well as the relationship between the frequencies of water libration (νL) and OH stretching bands (νOH). In more strongly interacting solvents, simulated νL are ∼20% higher than those of experiment. In all solvents, the simulated spectra are composites of rotational motions about the two axes perpendicular to water's dipole moment, and the different frequencies of these two motions are responsible for the breadth of the libration band and the bimodal shape observed in halide ionic liquids. Simulations overestimate the separation of these two components in most solvents. The character of water rotational motions changes markedly with solvent polarity, from quasi-free rotation in nonpolar and weakly polar solvents to highly constrained libration in strongly hydrogen bonding environments. The changeover to librational motions dominating the spectrum occurs between solvents such as benzene (νL ∼ 250cm-1) and acetonitrile (νL ∼ 400cm-1). For solvents in the latter category, the mean frequency of the experimental FIR band provides a direct measure of mean-squared torques and, therefore, force constants associated with interactions constraining water's librational motion.

Dynamic Electric Field Alignment Determines the Water Rotational Motion around Protein.

Water rotational dynamics in biomolecular solution is crucial to evaluating and controlling biomolecule stability. In this molecular dynamics simulation (MD) study on lysozyme solutions, we present how the exerted internal electric field determines water rotational dynamics. We find that the relaxation time of water rotation is equivalent to that of the reorientation of the exerted overall electric field for every single water molecule, regardless of its translation mode. Namely, water molecular rotation synchronizes with the exerted field reorientation. We also map the reorientation process of the electric field at fixed points relative to protein in the solution, which displays the local hydration dynamics commensurate with the reported time-dependent fluorescence Stokes shift (TDFSS) measurements. Comparing the spatial distribution of local field reorientation relaxation time with that of rotational relaxation time, we further suggest that water rotation dynamics are subject to the reorientation of the local overall field within the hydration layer. While outside the hydration layer, the relaxation time of the local electric field reorientation is short enough (subpicosecond) to assume the δ function, showing the electric force with randomly changing orientation is applied to each water molecule.

Stochastic Analysis of Molecular Dynamics Reveals the Rotation Dynamics Distribution of Water around Lysozyme.

Water dynamics is essential to biochemical processes by mediating all such reactions, including biomolecular degeneration in solutions. To disentangle the molecular-scale distribution of water dynamics around a solute biomolecule, we investigated here the rotational dynamics of water around lysozyme by combining molecular dynamics (MD) simulations and broadband dielectric spectroscopy (BDS). A statistical analysis using the relaxation times and trajectories of every single water molecule was proposed, and the two-dimensional probability distribution of water at a distance from the lysozyme surface with a rotational relaxation time was given. For the observed lysozyme solutions of 34-284 mg/mL, we discovered that the dielectric relaxation time obtained from this distribution agrees well with the measured γ relaxation time, which suggests that rotational self-correlation of water molecules underlies the gigahertz domain of the dielectric spectra. Regardless of protein concentration, water rotational relaxation time versus the distance from the lysozyme surface revealed that the water rotation is severely retarded within 3 Å from the lysozyme surface and is nearly comparable to pure water when farther than 10 Å. The dimension of the first hydration layer was subsequently identified in terms of the relationship between the acceleration of water rotation and the distance from the protein surface.

Effect of Osmolytes on Water Mobility Correlates with Their Stabilizing Effect on Proteins.

There is a long, ongoing debate on how small molecules (osmolytes) affect the stability of proteins. The present study found that change in collective rotational dynamics of water in osmolyte solutions likely has a dominant effect on protein denaturation. According to THz spectroscopy analysis, osmolytes that stabilize proteins are accompanied by bound hydration water with slow dynamics, while the collective rotational dynamics of water is accelerated in the case of denaturant osmolytes. Among 15 osmolytes studied here, there is a good correlation between the change in mobility in terms of water rotational dynamics and the denaturation temperature of ribonuclease A. The changes in water dynamics due to osmolytes can be regarded as a pseudo-temperature-change, which agrees well with the change in protein denaturation temperature. These results indicate that the molecular dynamics of water around the protein is a key factor for protein denaturation.

Molecular understanding of aqueous electrolyte properties and dielectric effect in a CDI system

Capacitive deionization (CDI) provides performance of ion removal via electrodes with high porosity and confined nanopores. Hitherto, the main emphasis has been on the enhancement of removal efficiency based on a lab-scale setup, neglecting anomalous dynamic and dielectric properties in confinement. This work was completed to reveal structural and dynamic properties of hydrated ions in a 3-nm nanopore of a CDI electrode using molecular dynamics simulation based on the nonpolarizable (SPC/E) and polarizable (SWM4-DP/-NDP) force fields. It was shown that all the force fields caused marginal differences in hydration structure but greater differences in ionic dynamics. Furthermore, dielectric constants obtained from all the force fields were lower in confinement than in bulk, although they were slightly higher in the polarizable models. Angular distributions of water bonds and dipole vectors revealed this decrease was caused by the restriction of water rotational dynamics and hence a depression of dipolar fluctuations. Hydrogen bond lifetimes indicated that higher availability of hydrogen bonding sites also facilitated increasing the dielectric constant as it allowed a more frequent “switch” of the fast hydrogen bond. Both the angular distribution and the hydrogen bonding contributed to the slightly higher dielectric constant in the polarizable model than in the nonpolarizable model. Our local sensitivity analysis of a ion transport model revealed that the dielectric constant greatly determined ion transport in the electrode, although the diffusion coefficients of typical ions had limited influences. This work provides fundamental theories and great implications for the theoretical analysis of ion electrosorption in a CDI electrode and enhances its application in water treatment.

Water as a Lévy Rotor.

A probability density function describing the angular evolution of a fixed-length atom-atom vector as a Lévy rotor is derived containing just two dynamical parameters: the Lévy parameter α and a rotational time constant τ. A Lévy parameter α<2 signals anomalous (non-Brownian) motion. Molecular dynamics simulation of water at 298K validates the probability density function for the intramolecular ^{1}H─^{1}H dynamics. The rotational dynamics of water is found to be approximately Brownian at subpicosecond time intervals, becomes increasingly anomalous at longer time intervals due to hydrogen-bond breaking and reforming, before becoming indistinguishable from Brownian dynamics beyond about 25ps. The Lévy rotor model is used to estimate the intramolecular contribution to the longitudinal nuclear-magnetic-resonance (NMR) relaxation rate R_{1,intra}. It is found that R_{1,intra} contributes 65%±7% to the overall relaxation rate of water at room temperature.

Structure and rotational dynamics of water around hydrogen peroxide

The density functional theory-based molecular dynamics simulations were induced to study various properties of water around different solvation pockets around hydrogen peroxide(H2O2). Our study aims to understand water behavior in the vicinity of H2O2 molecule through analyzing structure, hydrogen bond statistics, orientational and non-diffusive jump profile. A water-like peroxide molecule can form four hydrogen bonds with nearby water molecules: two as a proton donor and two as a proton acceptor, which must disturb the local structure and dynamics of the surrounding water. The presence of two different peak maxima in radial distribution function (RDF) allows us to resolve the solvation environment into three solvation sites: solvation shell I, solvation shell II, and bulk. The probability distribution of the average distance of central water molecule from 4 nearest water molecules shows a shift in the peak for solvation shell I water molecules towards a ∼0.3Å higher value compared to two other solvation shells. This further affects tetrahedral order parameters as water molecules shift from the regular tetrahedron. For the first solvation layer, the water molecules lose their tetrahedrality to a significant extent as qtet probability distribution peaks at 0.5 and broaden in the higher values of qtet without any shoulder. For the second solvation layer, we find the qtet peak at 0.87 with a minor shoulder appearing at qtet = 0.55, a sign of slight loss of tetrahedrality in the unstructured water compared to undisturbed water. Loss of tetrahedrality hints at the change of orientation of H2O molecules in the vicinity of H2O2 and affects the water-water hydrogen bond (HB) formation. A noteworthy reduction in the HB donor/acceptor account and per water HB count shows interference of H2O2. The water molecules prefer H2O2 compared to neighboring water molecules for HB formation. The orientation autocorrelation function provides the rotational pace of water molecules in the different solvation layers. The reorientation time constant difference in the solvation shells I and II fluctuate by mere 0.20 ps, showcasing the trivial nature of hydrogen peroxide and water HBs. Using the HB descriptors, we also report the non-diffusive jump profile involving vicinal water molecules of H2O2. The jump angle profile of solvation shell water molecules is in line with bulk water.

Rotational Dynamics of Water at the Phospholipid Bilayer Depending on the Head Groups Studied by Molecular Dynamics Simulations

Hydration states are a crucial factor that affect the self-assembly and properties of soft materials and biomolecules. Although previous experiments have revealed that the hydration state strongly depends on the chemical structure of lipid molecules, the mechanisms at the molecular level remain unknown. Classical and density-functional tight-binding (DFTB) molecular dynamics (MD) simulations are employed to determine the mechanisms underlying dissimilar water dynamics between lipid membranes with phosphatidylcholine (PC) and phosphatidylethanolamine (PE) head groups. The classical MD simulation shows that rotational relaxations of water are faster on the PE lipid than on the PC lipid, which is consistent with a previous experimental study using terahertz spectroscopy. Furthermore, DFTB-MD simulation of N(CH3)4+ and NH4+ ions, which correspond to the different head groups in PC and PE, respectively, shows qualitative agreement with the classical MD simulation. Remarkably, the PE lipids and the NH4+ ions break hydrogen bonds between water molecules in the secondary hydration shell. In contrast, the PC lipids and the N(CH3)4+ ions bind water molecules weakly in both the primary and secondary hydration shells and increase hydrogen bonds between water. Our atomistic simulations show that these changes in the hydrogen-bond network of water molecules cause the different rotational relaxation of water molecules between the two lipids.

Heterogeneity of water structure and dynamics at the protein-water interface.

In this molecular dynamics simulation study, we analyze the local structural and dynamic properties of water hydrating the protein ubiquitin on a spatial grid with 1 Å resolution. This allows for insights into the spatial distribution of water number densities, molecular orientations, translations, and rotations as a function of distance from the protein surface. Water molecule orientations follow a heterogeneous distribution with preferred local orientations of water dipoles and O-H bond vectors up to 10-15 Å distances from the protein, while local variations of the water number density converge to homogeneous bulk-like values within less than 8 Å. Interestingly, we find that the long-ranged orientational structure of water does not impact either the translational or rotational dynamics of water. Instead, heterogeneous distributions of local dynamical parameters and averaged dynamical retardation factors are only found close to the protein surface and follow a distance dependence comparable to heterogeneities in the local water number density. This study shows that the formation of nanodomains of preferred water orientations far from the protein does not significantly impact dynamical processes probed as a non-local average in most experiments.

Cosolvent effect on the dynamics of water in aqueous binary mixtures

ABSTRACTWater rotational dynamics in the mixtures of water and amphiphilic molecules, such as acetone and dimethyl sulfoxide (DMSO), measured by femtosecond infrared, often vary non-monotonically as the amphiphilic molecule's molar fraction changes from 0 to 1. Recent study has attributed the non-ideal water rotation with concentration in DMSO–water mixtures to different microscopic hydrophilic–hydrophobic segregation structure in water-rich and water-poor mixtures. Interestingly, the acetone molecule has very similar molecular structure to DMSO, but the extremum of the water rotational time in the DMSO-water mixtures significantly shifts to lower concentration and the rotation of water is much faster than those in acetone-water mixtures. The simulation results here shows that the non-ideal rotational dynamics of water in both mixtures are due to the frame rotation during the interval of hydrogen bond (HB) switchings. A turnover of the frame rotation with concentration takes place as the structure transition of mixture from the hydrogen bond percolation structure to the hydrophobic percolation structure. The weak acetone–water hydrogen bond strengthens the hydrophobic aggregation and accelerates the relaxation of the hydrogen bond, so that the structure transition takes places at lower concentration and the rotation of water is faster in acetone–water mixture than in DMSO–water mixture. A generally microscopic picture on the mixing effect on the water dynamics in binary aqueous mixtures is presented here.

The opposite effects of sodium and potassium cations on water dynamics.

Water rotational dynamics in NaSCN and KSCN solutions at a series of concentrations are investigated using femtosecond infrared spectroscopy and theory. Femtosecond infrared measurements, consistent with previous NMR observations, detect that sodium slows down while potassium accelerates the water O-H bond rotation. Results of reported neutron scattering measurements, on the other hand, suggested that these two cations have similar structure-breaking effects on water, and therefore should both accelerate water rotation through the presumably dominating large-amplitude angular jump component. To explain this discrepancy, theoretical studies with both classical and ab initio models were carried out, which indicate that both ions indeed accelerate the large-amplitude angular jump rotation of the water molecules, while the observed cation specific effect originates from the non-negligible opposite impact of the sodium and potassium cations on the diffusive rotation of water molecules.

Translational and rotational dynamics of water contained in aged Portland cement pastes studied by quasi-elastic neutron scattering

Cement is a widely used construction material in the world. The quality and durability of aged cement pastes have a strong relationship with the water contained in it. The translational and rotational dynamics of water in ordinary Portland cement (OPC) pastes cured for 7, 14 and 30days were studied by analyzing Quasi-elastic Neutron Scattering (QENS) data. The effect of a new super-plasticizer (SP) additive was also studied by comparing the samples with and without the additive. By fitting the QENS spectra with the Jump-diffusion and Rotation-diffusion Model (JRM), six important parameters including the bound water index (BWI), the self-diffusion coefficient, Dt, the average residence time, τ0, the rotational diffusion constant, Dr, the rotational residence time, τr, and the mean squared displacement (MSD), 〈u2〉, were obtained. From these parameters, we can quantitatively follow the evolution of the bound water fraction (BWI). We can clearly see the different time ranges for the translational and rotational dynamics of water contained in the OPC pastes by τ0 and τr. From the MSD values compared with those of molecular dynamics simulation, we can distinguish between immobile water (mainly bound water) and mobile water, which includes confined water and ultraconfined water. Furthermore, by the fitted parameters’ values and their change of slopes with increasing setting time for cement pastes with and without additive SP, it becomes clear that the effect of additive SP is to make the mobile water more confined and induce a more uniform the aging process during the evolution of the OPC pastes.

Water Behavior in MCM-41 As a Function of Pore Filling and Temperature Studied by NMR and Molecular Dynamics Simulations

The behavior of water confined in MCM-41 mesopores has been investigated by means of 1H NMR methods as a function of pore filling and temperature. The translational and rotational dynamics of water have been explored in the frame of molecular dynamics simulations providing a good understanding of the solid-state NMR results for three samples with different pore fillings: 10% (MCM-A), 45% (MCM-B), and completely filled (MCM-C). In MCM-B and MCM-C samples one can distinguish the presence of core and surface water, while all water molecules in MCM-A are attached to the surface and do not retain any translational degree of freedom. The activation energies for translational motion (MCM-B and MCM-C) obtained from NMR and MD simulations are in good agreement. For MCM-B and MCM-C samples we relate the decrease in the NMR intensity signal with decreasing temperature mainly with the loss of translational mobility of water molecules, while in the case of MCM-A the decrease is related to the freezing of rotational de...

Effects of co-solutes on the hydrogen bonding structure and dynamics in aqueous N-methylacetamide solution: a molecular dynamics simulations study

The effects of trimethylamine-N-oxide (TMAO), urea and tetramethyl urea (TMU) on the hydrogen bonding structure and dynamics of aqueous solution of N-methylacetamide (NMA) are investigated by classical molecular dynamics simulations. The modification of the water's hydrogen bonding structure and interactions is calculated in presence of these co-solutes. It is observed that the number of four-hydrogen-bonded water molecules in the solution decreases significantly in the presence of TMAO rather than urea and TMU. The lifetime and structural relaxation time of water–water and NMA–water hydrogen bonds show a strong increase with the addition of TMAO and TMU in the solution, whereas the change is nominal in case of urea solution. It is also found that the translational and rotational dynamics of water and NMA slowdown with increasing the concentration of these osmolytes. The slower dynamics of water and NMA is more pronounced in case of TMAO and TMU solution, as these co-solutes strengthen the average hydrogen bond energies between water–water and NMA–water, whereas urea has a little effect on the hydrogen bonding structure and dynamics of aqueous NMA solution. The calculated self-diffusion coefficient values for water and these co-solutes are in similar pattern with experimental observations.

Hydration of Sodium Alginate in Aqueous Solution

Alginates are naturally occurring biocompatible polysaccharides. They have a broad range of applications, mainly in connection to their ability to control the rheology of aqueous solutions. Specifically, addition of a small amount of alginate (10% wt) leads to a ∼100-fold increase in viscosity. Here we explore whether that pronounced retardation of the long-range correlations is accompanied by molecular-level changes of the water structure. We employ viscometry, dielectric spectroscopy (DS) and femtosecond infrared (fs-IR) pump–probe spectroscopy to study water dynamics in sodium alginate solutions. Remarkably, despite the large rheological effects of alginates in solution, the rotational dynamics of water are remarkably similar to those observed in bulk water. Only a small subensemble of water molecules is slowed down significantly, amounting to 6 ± 2 water molecules per saccharide unit. Furthermore, DS measurements reveal an additional ∼5 water molecules to be slowed down by the counterion (Na+). Our results reveal that the effect of alginate on the dynamics of water is restricted to the first hydration shell. This indicates that the large viscosity increase is determined by the polysaccharide network, with large water pools present between the polysaccharide chains.

Analysis of the Hydration Process and Rotational Dynamics of Water in a Nafion Membrane Studied by 1H NMR Spectroscopy

(1)H NMR spectroscopy is employed to reveal the hydration process of a Nafion membrane by measuring both the chemical shift and the spin-lattice relaxation time. In a former study, the hydration process was suggested to comprise two steps: the molecular adsorption of water on the sulfonic acid groups and wetting with liquid water. The present study has revealed the first step can further be divided into two steps. By introducing a new experimental technique, the quantitatively reliable NMR measurements of protons ((1)H) of water involved in the polymer membrane are realized. In addition, a new analytical procedure is developed using a reciprocal concentration on a saturation-adsorption model, and the hydration is clearly revealed to have three individual steps. Both the chemical shift and the relaxation time plots against the reciprocal concentration exhibit three linear parts with apparently different slopes. Of great interest is that the initial hydration is divided into two stages: the first hydration is a very strong adsorption of water probably on the hydroxyl group of the sulfonic acid group, and the second one is a relatively weak adsorption on another site of the sulfonic acid group. The third hydration is readily assigned to excess bulk (liquid-like) water as expected. These adsorption processes are readily correlated with the rotational motion of water by converting the spin-lattice relaxation time to the rotational correlation time.

Structure and Dynamics of Urea/Water Mixtures Investigated by Vibrational Spectroscopy and Molecular Dynamics Simulation

Urea/water is an archetypical "biological" mixture and is especially well-known for its relevance to protein thermodynamics as urea acts as a protein denaturant at high concentration. This behavior has given rise to an extended debate concerning urea's influence on water structure. On the basis of a variety of methods and of definitions of the water structure, urea has been variously described as a structure-breaker, a structure-maker, or as remarkably neutral toward water. Because of its sensitivity to microscopic structure and dynamics, vibrational spectroscopy can help resolve these debates. We report experimental and theoretical spectroscopic results for the OD stretch of HOD/H2O/urea mixtures (linear IR, 2DIR, and pump-probe anisotropy decay) and for the CO stretch of urea-D4/D2O mixtures (linear IR only). Theoretical results are obtained using existing approaches for water and a modification of a frequency map developed for acetamide. All absorption spectra are remarkably insensitive to urea concentration, consistent with the idea that urea only very weakly perturbs the water structure. Both this work and experiments by Rezus and Bakker, however, show that water's rotational dynamics are slowed down by urea. Analysis of the simulations casts doubt on the suggestion that urea immobilizes particular doubly hydrogen bonded water molecules.