Rotational Motion Of Water Molecules Research Articles

Differentially heterogeneous hydration environment of the familial mutants of α-synuclein.

The behavior of hydration water around familial Parkinson's disease linked mutants of α-synuclein may be linked to the early-onset of Parkinson's disease. For the first time, this study compares the local structure and dynamics of hydration water around different segments of some of the natural mutants of α-synuclein, i.e., E46K, G51D, A30P, and A53E, with that of the wild-type protein through explicit water MD simulations. The results show that the C-terminal segments of the fast aggregating mutants such as E46K and A30P are less exposed to water, while those of the slow aggregating ones such as A53E and G51D are more exposed to water relative to that of the wild-type protein. In addition, the water molecules are found to be more ordered around the C-terminal segment of the A53E and G51D mutants as compared to the wild-type protein. This is due to an increase in the overall charge of α-syn upon A53E and G51D mutations. The translational and rotational motions of water molecules in the hydration shell of the C-terminal segment of A53E and G51D mutants are found to be faster relative to that of the wild-type protein. This study validates the differential hydration environment around the C-terminal segment for the causative and protective mutants of α-synuclein.

Superdiffusive Rotation of Interfacial Water on Noble Metal Surface.

Interfacial water on a metal surface acts as an active layer through the reorientation of water, thereby facilitating the energy transfer and chemical reaction across the metal surface in various physicochemical and industrial processes. However, how this active interfacial water collectively behaves on flat noble metal substrates remains largely unknown due to the experimental limitation in capturing librational vibrational motion of interfacial water and prohibitive computational costs at the first-principles level. Herein, by implementing a machine-learning approach to train neural network potentials, we enable performing advanced molecular dynamics simulations with ab initio accuracy at a nanosecond scale to map the distinct rotational motion of water molecules on a metal surface at room temperature. The vibrational density of states of the interfacial water with two-layer profiles reveals that the rotation and vibration of water within the strong adsorption layer on the metal surface behave as if the water molecules in the bulk ice, wherein the O-H stretching frequency is well consistent with the experimental results. Unexpectedly, the water molecules within the adjacent weak adsorption layer exhibit superdiffusive rotation, contrary to the conventional diffusive rotation of bulk water, while the vibrational motion maintains the characteristic of bulk water. The mechanism underlying this abnormal superdiffusive rotation is attributed to the translation-rotation decoupling of water, in which the translation is restrained by the strong hydrogen bonding within the bilayer interfacial water, whereas the rotation is accelerated freely by the asymmetric water environment. This superdiffusive rotation dynamics may elucidate the experimentally observed large fluctuation of the potential of zero charge on Pt and thereby the conventional Helmholtz layer model revised by including the contribution of interfacial water orientation. The surprising superdiffusive rotation of vicinal water next to noble metals will shed new light on the physicochemical processes and the activity of water molecules near metal electrodes or catalysts.

Explicit Projection of Stokes Shifts onto Solvent Motion in an Aqueous Liquid and Linear Response Theory.

We investigate the molecular origin of the fluorescence Stokes shift in an aqueous liquid. By examining the speed of energy change, the solvation response function is explicitly projected onto the translational and rotational motions of water molecules for both nonequilibrium relaxation and equilibrium fluctuations. Molecular dynamics simulations of a tryptophan solution show that these two processes have highly consistent dynamics, not only for the total response function but also for the decomposed components in terms of specific molecular movements. We found that the rotational mode governs the relaxation of the Stokes shift, whereas the translational mode contributes non-negligibly with slower dynamics. This consistency implies the similarity of the underlying translational and rotational movements of water molecules as the system is far away from and at equilibrium, supporting the validity of the linear response theory at the molecular level. The decomposition methodology is also applicable to a rigid solvent.

The translational, rotational, and phonon dynamics of water in ZrO2 /water nanofluid

The translational, rotational, and phonon dynamics of water in zirconium oxide (ZrO2) nanofluid were investigated by quasi-elastic neutron scattering, 17O NMR relaxation, and inelastic X-ray scattering, respectively. The particle size of ZrO2 is 3～100 nm, and the concentration of ZrO2 nanoparticles is 5, 10, 20, and 30 wt%. The translational and rotational motions of water molecules become retarded with increasing the concentration of ZrO2. This indicates that there is an attractive interaction between water molecules and the surface of the nanoparticles. The high-frequency sound velocity of the nanofluid obtained from phonon dynamics measurements is slightly larger than that of pure water although it is not much difference compared to the case of the translation and rotation. These findings indicate that water dynamics in the nanofluid is different from that of the bulk. It might be reasons for the enhancement of the thermal conductivity of the nanofluid.

Ultrafast Excited-State Proton Transfer of a Cationic Superphotoacid in a Nanoscopic Water Pool.

The excited-state proton transfer (ESPT) of a cationic superphotoacid, N-methyl-7-hydroxyquinolium, was studied within the water pool of an anionic aerosol-OT (AOT), bis(2-ethylhexyl) sulfosuccinate, reverse micelle (RM). Previously, we had found that the cationic photoacid residing at the anionic AOT interface was conducive to ESPT to the bound water having concentric heterogeneity on the time scale of hundreds of picoseconds to nanoseconds. In our present study, on the time scale of hundreds of femtoseconds to a few tens of picoseconds, the photoacid underwent an ultrafast ESPT influenced by mobile water constituting the core of the RM. The two subpopulations of the core water molecules that determine the ultrafast biphasic deprotonation of the photoacid on time scales differing by an order of magnitude were identified. The core water molecules solvating the counteranion of the photoacid showed a higher basicity than typical water clusters in bulk resulting in ESPT on a subpicosecond time scale. Bare water clusters sensed by the photoacid showed a slower ESPT, over several picoseconds, as typically limited by the rotational motion of water molecules for similar types of the photoacid.

Structure of Water and Silica in Ion Exchange Resins

Abstract Dynamic and static structures of water sorbed in ion exchange resins have been investigated by 17O NMR and X-ray diffraction. Rotational motion of water molecules sorbed in ion exchange resins is significantly slowed according mainly to the amount of cross-linkage of polymer which decides the size of micropore surrounded by hydrophobic polymer networks, and not crucially affected by the difference of hydration properties of ion exchange groups. Static structure of water sorbed in anion exchange resin is revealed to be similar to that of pure water, which indicates a clathrate like structure enclosing a hydrophobic tetramethylammonium group. On the other hand the structure of water in cation exchange resin is quite different from that of pure water, but resembles the structure found in aqueous sulfuric acid solution, which is strongly influenced by the highly acidic atmosphere. The structure of silica adsorbed in anion exchange resin has also been observed by X-ray diffraction. Obtained radial distribution function is quite similar to that of bulk silica, which requires the existence of Si-O-Si interaction in quantitative analysis. The present results evidence that silica adsorption in anion exchange resin is not simple ion exchange but combined processes of ion exchange and micro silica gel growth.

Ions' motion in water.

Over the decades, a great deal of attention has been focused on the solvation and transport properties of small rigid monatomic ions such as Na+, K+, Li+, Cl-, and Br- due to their importance in physical chemistry. Much less attention has been devoted to polyatomic ions although many polyatomic ions (such as nitrate, acetate, sulfate, and ammonium) are of great importance in biological and chemical processes. While the translational diffusion of smaller rigid ions shows the remarkable nonmonotonic dependence on inverse ion size (known as the "breakdown of Walden product"), the intermediate- to large-sized polyatomic ions (such as nitrate, acetate, and sulfate) exhibit different anomalies pointed out only recently. In this Perspective article, we provide an overview of how rotational diffusion and translational diffusion of these ions themselves are coupled to translational and rotational motions of water molecules. We discuss how diffusion of polyatomic ions is different from that of monatomic ions due to the rotational self-motion of the former that enhances diffusion in specific cases because of symmetry. While a continuum hydrodynamic model fails to describe the motion of polyatomic ions, we discuss how a mode-coupling theory approach can capture many aspects of this coupling between the solute ion and solvent water. We discuss how ionic mobility in water and other dipolar solvents are intimately connected to the dipolar solvation dynamics, in particular to its ultrafast component. We point out how the usual thinking on the relation between the diffusion and entropy needs to be modified in the case of ion diffusion.

Rotational-Diffusion Propagator of the Intramolecular Proton-Proton Vector in Liquid Water: A Molecular Dynamics Study.

The rotational motion of water molecules plays the dominant role in determining NMR spin-relaxation properties of liquid water and many biological tissues. The traditional theory of NMR spin relaxation predominantly uses the assumption that the reorientational dynamics of water molecules is described by a continuous-time rotational-diffusion random walk with a single rotational-diffusion coefficient. However, recent experimental and theoretical studies have demonstrated that water reorientation occurs by large, discrete angular jumps superimposed on a continuous-time rotational-diffusion process. We have investigated the rotational-diffusion propagator of the proton-proton (H-H) vector of water molecules in liquid water at 298 K using molecular dynamics (MD) simulations. Analysis of the MD-simulated reorientational trajectories reveals that reorientation of the intramolecular H-H vector occurs through a combination of the two mechanisms: rotational diffusion proper and discrete large-angle jumps. We demonstrate that, empirically, the rotational-diffusion propagator of the intramolecular H-H vector in liquid water can be described in terms of multiple rotational-diffusion coefficients. A model with two rotational-diffusion coefficients was found to provide a reasonable (albeit imperfect) fit of the MD-simulated propagator on the time scales relevant to NMR spin relaxation near room or physiological temperature (picoseconds to nanoseconds). We report the apparent values of the two rotational-diffusion coefficients determined from the propagator analysis at 298 K and discuss their physical meaning.

Molecular Dynamics of a Hydrated Collagen Peptide: Insights into Rotational Motion and Residence Times of Single-Water Bridges in Collagen.

Magnetic resonance transverse spin relaxation time constants (T2) of water protons in ordered collagenous tissues are dependent on the orientation of the tissue relative to the static magnetic field. This dependence is commonly referred to as the magic angle (MA) effect and has been attributed to the restricted rotational motion of icelike water bridges in the hydrated triple-helix collagen molecule. Understanding of the molecular mechanism of the MA effect is important for clinical and research applications of magnetic resonance spectroscopy and imaging to tissues, such as articular cartilage, tendons, and ligaments. In this work, we have used molecular dynamics simulations to investigate the subnanosecond time scale dynamics of single-water bridges in a model collagen peptide. We ascertain the residence times and the patterns of restricted rotational motion of water molecules. The key findings are strongly anisotropic rotation patterns of water molecules at bridge sites and a dynamic, rather than icelike, nature of the single-water bridges within the individual triple-helix collagen molecule.

Fast rotational motion of water molecules increases ordering of hydrophobes in solutions and may cause hydrophobic chains to collapse.

Using the molecular dynamics simulations with separate thermostats for translational and rotational degrees of freedom, we investigate the effects of water's rotational motion on the interaction among Lennard-Jones solutes. The situation with rotational temperature higher than the translational one (TR > TT) is mimicking the effects of microwaves on model solutions. Molecular dynamics simulations suggest that solutions of Lennard-Jones solutes become increasingly more structured with the rise in TR, while keeping the TT constant. This is evidenced by an increase of the first and the second peak of the solute-solute radial distribution function. In addition, the first peak moves toward slightly larger distances; the effect seems to be caused by the destabilization of water molecules in the first hydration shell around hydrophobic solutes. More evidence of strong effects of the rotationally excited water is provided by the simulations of short hydrophobic polymers, which upon an increase in TR assume more compact conformations. In these simulations, we see the re-distribution of water molecules, which escape from hydrophobic "pockets" to better solvate the solvent exposed monomers.

Diffusion of Hydration Water around Intrinsically Disordered Proteins.

Hydration water dynamics around globular proteins have attracted considerable attention in the past decades. This work investigates the hydration water dynamics around partially/fully intrinsically disordered proteins and compares it to that of the globular proteins via molecular dynamics simulations. The translational diffusion of the hydration water is examined by evaluating the mean-square displacement and the velocity autocorrelation function, while the rotational diffusion is probed through the dipole-dipole time correlation function. The results reveal that the translational and rotational motions of water molecules at the surface of intrinsically disordered proteins/regions are less restricted as compared to those around globular proteins/ordered regions, which is reflected in their higher diffusion coefficient and lower orientational relaxation time. The restricted mobility of hydration water in the vicinity of the protein leads to a sublinear diffusion in a heterogeneous interface. A positive correlation between the mean number of hydrogen bonds and the diffusion coefficient of hydration water implies higher mobility of water molecules at the surface of disordered proteins, which is due to their higher number of hydrogen bonds. Enhanced hydration water mobility around disordered proteins/regions is also related to their higher hydration capacity, low hydrophobicity, and increased internal protein motions. Thus, we generalize that the intrinsically disordered proteins/regions are associated with higher hydration water mobility as compared to globular protein/ordered regions, which may help to elucidate their varied functional specificity.

Power-law trapping of water molecules on the lipid-membrane surface induces water retardation

Cell membranes provide unique local environments for biological reactions, where the diffusion of biomolecules as well as water molecules plays critical roles. Translational and rotational motions of water molecules near membranes are known to be slower than those in bulk. Using all-atom molecular dynamics simulations of a membrane, we show that the temperature dependence of the water molecular motions on the membrane surface is different from that in bulk. Decreasing temperature enhances the water retardation on the membrane surface, and the lateral motions of water molecules are correlated with the vertical motions. We find that trapping times of water molecules onto membrane surfaces are distributed according to a power-law distribution and that the power-law exponents depend linearly on temperature, suggesting a random energy landscape picture. Moreover, we find that water molecules on the membrane surfaces exhibit subdiffusions in translational motions.

Dynamics of Water Inside the Secy Translocon Complex

In bacteria and archea, the insertion of membrane proteins into the plasma membrane is governed by the SecY translocon complex. Understanding how the SecY translocon distinguishes a transmembrane (TM) segment from a secretory one is therefore of the utmost importance. The process of insertion of a TM segment into the membrane resembles a thermodynamic equilibrium process, which has allowed the determination of a “biological” hydrophobicity scale [1,2]. Recent measurements have shown that water-to-membrane partitioning is energetically different from translocon-to-membrane partitioning [3]. A possible explanation is the state of water within the translocon and consequently the strength of the hydrophobic effect. In order to shed light on translocon-to-membrane partitioning, we have investigated water dynamics inside the SecY. We performed molecular dynamics simulations using the crystal structure of SecYE from Pyrococcus furiosus [4] as the initial configuration. Approaching the central region of SecY the hydrogen-bond network between water molecules survives longer than that between bulk water molecules. The rotational motion of water molecules is slowed down and the translational dynamics is characterized by “anomalous” diffusion. These results might explain the difference between translocon-to-membrane partitioning and water-to-membrane partitioning. The features we observed are characteristic of water molecules located close to a macromolecule. Being affected by the complex shape and the physicochemical heterogeneity of the macromolecular surface, these water molecules manifest different properties from those of the bulk phase. A TM helix passing through the SecY protein-conducting channel will feel an environment different from that characterizing bulk water.[1] Hessa et al., Nature, 377, 433 (2005).[2] Hessa et al., Nature, 1026, 450 (2007).[3] K. O-jemalm et al., Proc. Natl. Acad. Sci. USA, E359, 109 (2011).[4] P. F. Egea and R. M. Stroud, Proc. Natl. Acad. Sci. USA, 17182, 107 (2010).

Transition Pathways of Hydrogen Bond Exchanging Reaction in Pure Water

Hydrogen bond (H-bond) kinetic of pure water was explored extensively by molecular dynamic simulations. The non-H-bond configuration should be considered as a transition state with a short lifetime scale of 0.1~0.2 ps. Two types of transition states were found, by which the H-bond exchanging reactions took place. Distortion of one H-bond stimulates the H-bond exchanging reaction with the water molecule in the first coordination shell by path 1. Elongation of one H-bond stimulates the H-bond exchanging reaction with the water molecule in the second coordination shell by path 2. The possibility of two paths in the process of H-bond exchange varies with the temperature. The H-bond exchanging events usually take place between the over-coordinated and under-coordinated H-bond acceptors, as well as a proper geometric configurations required. The H-bond exchanging reactions play a key role in the translational and rotational motions of water molecules.

Effects of ions on winter flounder antifreeze protein and water molecules near an ice/water interface

We have carried out molecular dynamics analysis on an aqueous solution of winter flounder antifreeze protein (AFP), sodium ions and chloride ions near ice surfaces to understand the effect of ions on water and protein. It is found from the statistical quantities that the translational motion and freely rotational motion of water molecules near the ions are restricted. The interaction between the AFP and water molecules, which are hydrated with the ions, is attenuated. When the ion concentration near a part of the protein and the ice surface is higher than the average, the part of the protein approaches the ice surface.

Dynamics of Hydration of Alkylsulfonate Anions in Aqueous Solutions

The 17O-NMR spin-lattice relaxation times (T1) of water molecules in aqueous solutions of n-alkylsulfonate (C1 to C6) and arylsulfonic anions were determined as a function of concentration at 298 K. Values of the dynamic hydration number, \((\mathrm{S}^{-}) = n_{\mathrm{h}}^{ -} (\tau_{\mathrm{c}}^{-} /\tau_{\mathrm{c}}^{0} - 1)\), were determined from the concentration dependence of T1. The ratios (\(\tau_{\mathrm{c}}^{ -}/\tau_{\mathrm{c}}^{0}\)) of the rotational correlation times (\(\tau_{\mathrm{c}}^{ -} \)) of the water molecules around each sulfonate anion in the aqueous solutions to the rotational correlation time of pure water (\(\tau_{\mathrm{c}}^{0}\)) were obtained from the nDHN(S−) and the hydration number (\(n_{\mathrm{h}}^{ -} \)) results, which was calculated from the water accessible surface area (ASA) of the solute molecule. The \(\tau_{\mathrm{c}}^{ -}/\tau_{\mathrm{c}}^{0}\) values for alkylsulfonate anions increase with increasing ASA in the homologous-series range of C1 to C4, but then become approximately constant. This result shows that the water structures of hydrophobic hydration near large size alkyl groups are less ordered. The rotational motions of water molecules around an aromatic group are faster than those around an n-alkyl group with the same ASA. That is, the number of water–water hydrogen bonds in the hydration water of aromatic groups is smaller in comparison with the hydration water of an n-alkyl group having the same ASA. Hydrophobic hydration is strongly disturbed by a sulfonate group, which acts as a water structure breaker. The disturbance effect decreases in the following order: \(\mbox{--} \mathrm{SO}_{3}^{-} > \mbox{--} \mathrm{NH}_{3}^{ +} > \mathrm{OH}> \mathrm{NH}_{2}\). The partial molar volumes and viscosity BV coefficients for alkylsulfonate anions are linearly dependent on their nDHN(S−) values.

Structure and dynamics of water on Li +-, Na +-, K +-, Cs +-, H 3O +-exchanged muscovite surfaces: A molecular dynamics study

The distribution and dynamics of water molecules and monovalent cations (Li +, Na +, K +, Cs +, and H 3O +) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution–muscovite interface, the co-adsorption of Cs + and Cl − ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li + ions near the muscovite surface. For Na +, K +, Cs +, and H 3O + ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li +, Na +, K +, and H 3O + ions adsorbed on the muscovite surfaces. The water molecules around Cs + ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs + ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li + ions and decreased with an increase in the ionic radius.

Comparison of spin–lattice relaxation measurements made in the presence of strong radiation damping

The scientific literature contains reports of external magnetic fields affecting the fundamental properties and structure of water, including a report of the rotational motion of water molecules being slowed due to increased hydrogen bonding resulting from magnetic treatment. To investigate such a change in molecular motion, 1H spin–lattice relaxation times of water were measured at increasing magnetic field strengths. Strong radiation damping was observed at each magnetic field strength when using inversion-recovery experiments. To measure the relaxation times in the presence of radiation damping, several experimental methods of saturation recovery to measure spin–lattice relaxation were applied to a 90% H 2O sample to obtain results as a function of field strength for proton frequencies from 300 to 800 MHz.

Molecular mechanism of pressure denaturation of proteins

We investigate the molecular mechanism of pressure denaturation of proteins using the angle-dependent integral equation theory combined with the multipole water model and the morphometric approach. We argue that the hydration entropy of a protein is the key quantity. It is verified that at an elevated pressure, a swelling structure--which has only moderately less compact than the native structure but has a much larger water-accessible surface area--turns more stable than the native structure in terms of the water entropy. The swelling structure is characterized by the penetration of water into the interior. The hydration entropy is decomposed into contributions from the translational and rotational restrictions for the molecular motions of water. Each contribution is further decomposed into the water-protein pair correlation component and the water-water-protein triplet and higher-order correlation components. The pair correlation component in the translational contribution is divided into two terms arising from the excluded volume and the water structure near the protein, respectively. It is found that pressure denaturation accompanies a loss of the translational and rotational entropies at the pair correlation level but a much larger gain of the translational entropy at the triplet and higher-order correlation levels. Although the translational and rotational motions of water molecules penetrating the protein interior and contacting the protein surface are constrained, the translational restriction for the water molecules well outside the protein is greatly reduced. The latter entropic gain dominates, leading to the denaturation.

Thermal Properties and Mixing State of Diol−Water Mixtures Studied by Calorimetry, Large-Angle X-Ray Scattering, and NMR Relaxation

Differential scanning calorimetry (DSC) has been performed on aqueous mixtures of three diols, which involve a linear carbon chain, HO-(CH 2) n -OH ( n = 3, 4, and 5), over the whole mole fraction range of diols. The DSC results have shown the alkyl chain parity for the freezing process of the aqueous mixtures: aqueous mixtures of 1,3-propanediol (PrD) and 1,5-pentanediol (PeD) are kept in the supercooled state or vitrified over a wide mole fraction range, while those of 1,4-butanediol (BuD) are easily crystallized. The structure of PrD-water mixtures has been elucidated by using the large-angle X-ray scattering (LAXS) technique. It has been suggested that the structural change of PrD-water mixtures occurs at PrD mole fractions of x PrD = 0.4 and 0.8: in the range of x PrD < or = 0.4 where the tetrahedral-like structure of water predominates, in the range of 0.4 < x PrD < 0.8 where both PrD and water structures coexist, and in the range of x PrD > or = 0.8 where the inherent structure of PrD is mainly formed. (17)O and (1)H NMR relaxation measurements have been made on aqueous mixtures of ethylene glycol (EG, n = 2), PrD, and BuD to clarify the dynamics of H 2 (17)O and diol molecules. The (17)O NMR relaxation rates have suggested that the rotational motion of water molecules is gradually retarded in the diol-water mixtures with increasing diol content and that the restriction of the motion is more remarkable in the order of EG < PrD < BuD. On the basis of all the results, together with comparison with those of methanol-water, ethanol-water, and 1-propanol-water mixtures previously reported, the mixing state of diol-water mixtures has been discussed at the molecular level.