Structural Properties Of Water Research Articles

Water as a Structural Marker in Gelatin Hydrogels with Different Cross-Linking Nature

We have studied the molecular properties of water in physically and chemically cross-linked gelatin hydrogels by FTIR-spectroscopy, NMR relaxation, and diffusivity and broadband dielectric spectroscopy, which are sensitive to dynamical properties of water, being a structural marker of polymer network. All experiments demonstrated definite reinforcement of the hydrogel net structure and an increase in the amount of hydrate water. FTIR experiments have shown that the chemical cross-linking of gelatin molecules initiates an increase in the collagen-like triple helices “strength”, as a result of infused restriction on protein molecular mobility. The “strengthening” of protein chains hinders the mobility of protein fragments, introducing complex modifications into the structural properties of water which are remained practically unchanged up to up to 30–40 °C.

Relationships between Molecular Structural Order Parameters and Equilibrium Water Dynamics in Aqueous Mixtures

Water's unique thermophysical properties and how it mediates aqueous interactions between solutes have long been interpreted in terms of its collective molecular structure. The seminal work of Errington and Debenedetti [Nature 2001, 409, 318-321] revealed a striking hierarchy of relationships among the thermodynamic, dynamic, and structural properties of water, motivating many efforts to understand (1) what measures of water structure are connected to different experimentally accessible macroscopic responses and (2) how many such structural metrics are adequate to describe the collective structural behavior of water. Diffusivity constitutes a particularly interesting experimentally accessible equilibrium property to investigate such relationships because advanced NMR techniques allow the measurement of bulk and local water dynamics in nanometer proximity to molecules and interfaces, suggesting the enticing possibility of measuring local diffusivities that report on water structure. Here, we apply statistical learning methods to discover persistent structure-dynamic correlations across a variety of simulated aqueous mixtures, from alcohol-water to polypeptoid-water systems. We investigate a variety of molecular water structure metrics and find that an unsupervised statistical learning algorithm (namely, sequential feature selection) identifies only two or three independent structural metrics that are sufficient to predict water self-diffusivity accurately. Surprisingly, the translational diffusivity of water across all mixed systems studied here is strongly correlated with a measure of tetrahedral order given by water's triplet angle distribution. We also identify a separate small number of structural metrics that well predict an important thermodynamic property, the excess chemical potential of an idealized methane-sized hydrophobe in water. Ultimately, we offer a Bayesian method of inferring water structure by using only structure-dynamics linear regression models with experimental Overhauser dynamic nuclear polarization (ODNP) measurements of water self-diffusivity. This study thus quantifies the relationships among several distinct structural order parameters in water and, through statistical learning, reveals the potential to leverage molecular structure to predict fundamental thermophysical properties. In turn, these findings suggest a framework for solving the inverse problem of inferring water's molecular structure using experimental measurements such as ODNP studies that probe local water properties.

Molecular Dynamics Characterization of Radiosensitizing Coated Gold Nanoparticles in Aqueous Environment.

Functionalized metal nanoparticles (NPs) have been proposed as promising radiosensitizing agents for more efficient radiotherapy treatment using photons and ion beams. Radiosensitizing properties of NPs may depend on many different parameters (such as size, composition, and density) of the metal core, the organic coatings, and the molecular environment. A systematic exploration of each of these parameters on the atomistic level remains a formidable and costly experimental task, but it can be addressed by means of advanced computational modeling. This paper describes a detailed computational procedure for construction and atomistic-level characterization of radiosensitizing metal NPs in explicit molecular media. The procedure is general and is extensible to many different combinations of the core, coating, and environment. As an illustrative and experimentally relevant case study, we consider nanometer-sized gold NPs coated with thiol-poly(ethylene glycol)-amine molecules of different length and surface density and solvated in water at ambient conditions. The radial distribution of different atoms in the coatings as well as distribution and structural properties of water around the coated NPs are analyzed and linked to radiosensitizing properties of the NPs. It is revealed that the structure of the coating layer on the solvated NPs depends strongly on the surface density of ligands. At surface densities below ∼3 molecules/nm2 the coating represents a mixture of different conformation states, whereas elongated "brush"-like structures are formed at higher densities of ligands. The water content in denser coatings is significantly lower at distances from 1 nm up to 3 nm from the gold surface depending on the length of ligand molecules. Such dense and thick coatings may suppress the production of hydroxyl radicals by low-energy electrons emitted from the metal NPs and thus diminish their radiosensitizing properties. The presented computational framework provides precise information for a quantitative atomistic-level description of the structural properties of coated metal NPs in biologically relevant environments and so may form a basis for future developments to achieve a more realistic description of irradiation-driven chemistry effects in the vicinity of coated metal NPs.

Extrapolation and interpolation strategies for efficiently estimating structural observables as a function of temperature and density.

Thermodynamic extrapolation has previously been used to predict arbitrary structural observables in molecular simulations at temperatures (or relative chemical potentials in open-system mixtures) different from those at which the simulation was performed. This greatly reduces the computational cost in mapping out phase and structural transitions. In this work, we explore the limitations and accuracy of thermodynamic extrapolation applied to water, where qualitative shifts from anomalous to simple-fluid-like behavior are manifested through shifts in the liquid structure that occur as a function of both temperature and density. We present formulas for extrapolating in volume for canonical ensembles and demonstrate that linear extrapolations of water's structural properties are only accurate over a limited density range. On the other hand, linear extrapolation in temperature can be accurate across the entire liquid state. We contrast these extrapolations with classical perturbation theory techniques, which are more conservative and slowly converging. Indeed, we show that such behavior is expected by demonstrating exact relationships between extrapolation of free energies and well-known techniques to predict free energy differences. An ideal gas in an external field is also studied to more clearly explain these results for a toy system with fully analytical solutions. We also present a recursive interpolation strategy for predicting arbitrary structural properties of molecular fluids over a predefined range of state conditions, demonstrating its success in mapping qualitative shifts in water structure with density.

Molecular dynamic study on structural and dynamic properties of water, counter-ions and polyethylene glycols in Na-montmorillonite interlayers

Understanding the interfacial behavior of PEGs and water-PEG interaction in montmorillonite (MMT) interlayer is of great importance to the design of high-performance drilling fluids. In this work, we apply molecular dynamic (MD) simulations to study the structural properties of three low molecular weight PEGs (PEG2, PEG4 and PEG8) in MMT interlayers. We found that PEG2 incline to be parallel to the MMT surfaces, while PEG4 molecules tend to form a crown-like structure, hosting Na+ ions within. PEG8 molecules likely adopt helical or coil configurations caging Na+ inside. Thanks to the wrapping and caging effect, hydration ability of Na+ ion is significantly reduced. Moreover, at a higher concentration with PEG4 and PEG8, more Na+ ions are trapped in the middle of the pore due to the PEG caging. As PEG concentration increases, some water molecules are displaced from the pore surface. As a result, the hydrogen bonding number between water and pore surface decreases. Furthermore, the diffusion coefficients of water and Na+ ions are suppressed in the presence of PEGs. Our study provides important insights into the structural properties of water, counter-ions, and PEG molecules in MMT interlayers and the optimization of high-performance drilling fluids.

Structural and comparative study of water confined in a mesoporous bioglass by X-ray total scattering

ABSTRACT The structural properties of water confined in two silica matrices characterised by well-controlled organised porosity with a narrow pore size distribution: i) a new mesoporous bioactive glass (MBG 92S6) and ii) SiO2 MCM-41 were studied using laboratory total X-ray scattering coupled to molecular pair distribution function (PDF). The PDF analysis shows that the hydrophilic/hydrophobic character of the water-bioglass interface affects the structural properties of the confined water in the same way as water confined in different mesoporous matrices presenting an intermediate hydrophilic and hydrophobic interface regardless of their pore size and distribution. We also compare the effect of the confinement inside MBG 92S6 and different mesoporous hydrophilic and hydrophobic silica matrices. We show that the pore surface properties have a stronger influence on the structural organisation of the confined water than the pore size distribution.

Structure of ice confined in carbon and silica nanopores

In this work, water confined in silica and carbon nanopores has been examined. The purpose of this study is to describe the melting behaviour and structure of ice confined in silica nanopores, KIT-6 and ordered carbon nanopores, CMK-3, having pore diameters of 5.9 and 5.2 nm, respectively. To determine the melting temperature of ice inside the nanopores, we performed differential scanning calorimetry measurements of the systems studied. We found that the melting temperature of confined ice is reduced relative to the bulk melting point and this shift is 16 K for water confined in KIT-6 and 21 K for water confined in CMK-3. The structural properties of water at the interfaces were analysed by using the neutron diffraction method (ND). The ND measurements for all the systems studied, showed the features of both hexagonal ice, $$I_\mathrm{h}$$, and cubic ice, $$I_\mathrm{c}$$. However, we show that the ice confined in nanopores does not have a structure corresponding to the typical hexagonal form or the metastable cubic form. The ice confined in nanopores has a structure made up of cubic sequences interlaced with hexagonal sequences, which produce the stacking disordered ice (ice $$I_\mathrm{sd})$$.

Functional Hydration Behavior: Interrelation between Hydration and Molecular Properties at Lipid Membrane Interfaces

Water is an abundant commodity and has various important functions. It stabilizes the structure of biological macromolecules, controls biochemical activities, and regulates interfacial/intermolecular interactions. Common aspects of interfacial water can be obtained by overviewing fundamental functions and properties at different temporal and spatial scales. It is important to understand the hydrogen bonding and structural properties of water and to evaluate the individual molecular species having different hydration properties. Water molecules form hydrogen bonds with biomolecules and contribute to the adjustment of their properties, such as surface charge, hydrophilicity, and structural flexibility. In this review, the fundamental properties of water molecules and the methods used for the analyses of water dynamics are summarized. In particular, the interrelation between the hydration properties, determined by molecules, and the properties of molecules, determined by their hydration properties, are discussed using the lipid membrane as an example. Accordingly, interesting water functions are introduced that provide beneficial information in the fields of biochemistry, medicine, and food chemistry.

Assimilating Radial Distribution Functions To Build Water Models with Improved Structural Properties.

The structural properties of three- and four-site water models are improved by extending the ForceBalance parametrization code to include a new methodology allowing for the targeting of any radial distribution function (RDF) during the parametrization of a force field. The mean squared difference (MSD) between the experimental and simulated RDFs contributes to an objective function, allowing for the systematic optimization of force field parameters to reach closer overall agreement with experiment. RDF fitting is applied to develop modified versions of the TIP3P and TIP4P/2005 water models in which the Lennard-Jones potential is replaced by a Buckingham potential. The optimized TIP3P-Buckingham and TIP4P-Buckingham potentials feature 93 and 98% lower MSDs in the OO RDF compared to the TIP3P and TIP4P/2005 models respectively, with marked decreases in the height of the first peak. Additionally, these Buckingham models predict the entropy of water more accurately, reducing the error in the entropy of TIP3P from 11 to 3% and the error in the entropy of TIP4P/2005 from 11 to 2%. These new Buckingham models have improved predictive power for many nonfitted properties particularly in the case of TIP3P. Our work directly demonstrates how the Buckingham potential can improve the description of water's structural properties beyond the Lennard-Jones potential. Moreover, adding a Buckingham potential is a favorable alternative to adding interaction sites in terms of computational speed on modern GPU hardware.

Exclusion zone water is associated with material that exhibits proton diffusion but not birefringent properties

The structural properties of water have been under investigation for decades but remain a subject of debate. A recent controversy has arisen with evidence suggesting that the region of water located at the interface with hydrophilic surfaces is highly structured and extends up to 500 μm. The supporting evidence for this claim is the physical exclusion of particulates resulting in an Exclusion Zone, or “EZ” and birefringence as seen through a polarized light microscope. Several studies have been undertaken to investigate alternate causes for the Exclusion Zone and these suggest that a flow of ions from the hydrophilic material is an alternate explanation. This study investigates both claims with particular emphasis on the presence of birefringence in the EZ. The findings indicate that any observed birefringence in EZ water is not due to a crystalline structuring of water but is rather an optical reflection phenomenon from the material at the water-surface interface.

Molecular interactions insights underlying temperature-dependent structure of water molecules on TiO2 nanostructured film: A computational study using reactive and non-reactive force fields

Understanding the interaction of water molecules with TiO2 surface is vitally important in practical purposes. Hence, this study is focused on studying the temperature-dependent effect of water on the 5 nm-thick TiO2 film by molecular dynamics (MD) approach. A non-reactive force field proposed by Matsui–Akaogi is applied to simulate the bulk and surface properties of TiO2 anatase, while a developed ReaxFF reactive force field is used to predict the molecular and dissociative adsorption configurations of water molecules on the anatase (001) and (101) surfaces. The density profiles of the mentioned system provided molecular insights into the multilayer-adsorbed water on the TiO2 surfaces at different temperatures, T. The structural properties of water, TiO2 (anatase) and their interface were studied through the radial distribution functions of the related atomic pairs over the range 273 ≤ T ≤ 373 K. At T ≤ 313 K, strong hydrogen bonds (HBs) are found in the near-surface water molecules because of the strong liquid-surface integrations. At T ≥ 353 K, strong HBs are formed in the near-surface water molecules, while weak HBs are created on the upper layers of water because of liquid-gas interface. The reactive MD simulations showed dissociative adsorption of water molecules on the (101) surface are higher than that on the (001). This property is in good agreement with the literature data.

Hydrogen Bond Network of Water around Protein Investigated with Terahertz and Infrared Spectroscopy

The dynamical and structural properties of water at protein interfaces were characterized on the basis of the broadband complex dielectric constant (0.25 to 400 THz) of albumin aqueous solutions. Our analysis of the dielectric responses between 0.25 and 12 THz first revealed hydration water with retarded reorientational dynamics extending ∼8.5 Å (corresponding to three to four layers) out from the albumin surface. Second, the number of nonhydrogen-bonded water was decreased in the presence of the albumin solute, indicating protein inhibits the fragmentation of the water hydrogen-bond network. Finally, water molecules at the albumin interface were found to form a distorted hydrogen-bond structure due to topological and energetic disorder of the protein surface. In addition, the intramolecular O-H stretching vibration of water (∼100 THz), which is sensitive to hydrogen-bond environment, pointed to a trend that hydration water has a larger population of strongly hydrogen-bonded water molecules compared with that of bulk water. From these experimental results, we concluded that the “strengthened” water hydrogen bonds at the protein interface dynamically slow down the reorientational motion of water and form the less-defective hydrogen-bond network by inhibiting the fragmentation of water-water hydrogen bonds. Nevertheless, such a strengthened water hydrogen-bond network is composed of heterogeneous hydrogen-bond distances and angles, and thus characterized as structurally “distorted.”

Seawater Pervaporation through Zeolitic Imidazolate Framework Membranes: Atomistic Simulation Study.

An atomistic simulation study is reported for seawater pervaporation through five zeolitic imidazolate framework (ZIF) membranes including ZIF-8, -93, -95, -97, and -100. Salt rejection in the five ZIFs is predicted to be 100%. With the largest aperture, ZIF-100 possesses the highest water permeability of 5 × 10(-4) kg m/(m(2) h bar), which is substantially higher compared to commercial reverse osmosis membranes, as well as zeolite and graphene oxide pervaporation membranes. In ZIF-8, -93, -95, and -97 with similar aperture size, water flux is governed by framework hydrophobicity/hydrophilicity; in hydrophobic ZIF-8 and -95, water flux is higher than in hydrophilic ZIF-93 and -97. Furthermore, water molecules in ZIF-93 move slowly and remain in the membrane for a long time but undergo to-and-fro motion in ZIF-100. The lifetime of hydrogen bonds in ZIF-93 is found to be longer than in ZIF-100. This simulation study quantitatively elucidates the dynamic and structural properties of water in ZIF membranes, identifies the key governing factors (aperture size and framework hydrophobicity/hydrophilicity), and suggests that ZIF-100 is an intriguing membrane for seawater pervaporation.

Hybrid Atomistic and Coarse-Grained Molecular Dynamics Simulations of Polyethylene Glycol (PEG) in Explicit Water.

In-silico design of polymeric biomaterials requires molecular dynamics (MD) simulations that retain essential atomistic/molecular details (e.g., explicit water around the biofunctional macromolecule) while simultaneously achieving large length and time scales pertinent to macroscale function. Such large-scale atomistically detailed macromolecular MD simulations with explicit solvent representation are computationally expensive. One way to overcome this limitation is to use an adaptive resolution scheme (AdResS) in which the explicit solvent molecules dynamically adopt either atomistic or coarse-grained resolution depending on their location (e.g., near or far from the macromolecule) in the system. In this study we present the feasibility and the limitations of AdResS methodology for studying polyethylene glycol (PEG) in adaptive resolution water, for varying PEG length and architecture. We first validate the AdResS methodology for such systems, by comparing PEG and solvent structure with that from all-atom simulations. We elucidate the role of the atomistic zone size and the need for calculating thermodynamic force correction within this AdResS approach to correctly reproduce the structure of PEG and water. Lastly, by varying the PEG length and architecture, we study the hydration of PEG, and the effect of PEG architectures on the structural properties of water. Changing the architecture of PEG from linear to multiarm star, we observe reduction in the solvent accessible surface area of the PEG, and an increase in the order of water molecules in the hydration shells.

Influence of layer charge, hydration state and cation nature on the collective dynamics of interlayer water in synthetic swelling clay minerals

The dynamical and structural properties of water in clay interlayers can be considered as those of a bidimensional confined fluid with significant anisotropy as revealed by measurements and simulations of water diffusion at the molecular scale. The present paper investigates water collective motions by using coherent inelastic neutron scattering (INS) on deuterated samples of synthetic saponite samples with low (0.7 per unit cell) and high (1.4 per unit cell) charge, Na+ and Ca2+ interlayer cations, various hydration states and at different temperatures. In all cases, two excitations are needed to fit the inelastic signals. The low-energy excitation of the collective dynamics of interlayer water evidences a clear anisotropy and displays a behavior significantly different from that of bulk water, this difference being stronger in the direction perpendicular to the clay layer. In the parallel direction, experimental results and molecular dynamics simulations show that the features of the low-energy excitation and in particular, the energy corresponding to the nondispersive component are linked to the structuration and rigidity of the interlayer water network. Indeed, this position displays an upward energy shift with increasing confinement and decreasing temperature. In contrast, data obtained in the perpendicular direction, display a dispersive behavior for Q values ≥0.5Å−1 in contrast with what is observed for bulk water and in the parallel direction. Such a behavior must somehow be linked to a correlation between the motions of water molecules located in adjacent interlayer space that is likely transmitted through the solid clay layer.

Insights into the microscopic behaviour of nanoconfined water: host structure and thermal effects

The hydration of nanoporous materials is relevant to many fundamental and industrial applications. In this context, zeolites are usually used, but metal–organic frameworks constitute an emerging class of useful materials. The singular properties of water ascribed to the molecular association lead to a variety of behaviors in the confining systems that are not well understood. There is little of both experimental and computational information available, and most of which are limited to room temperature. This work addresses the adsorption and structural properties of water in a series of porous materials for a fixed topology, in particular LTA, and at various temperatures in the range of 298–573 K. The targeted structures were the all-silica zeolite ITQ-29, the aluminosilicate form with charge-balancing sodium and calcium cations LTA-5A, and the Zn-based zeolitic imidazolate framework. The adsorption process was computed using Monte Carlo simulations in the grand canonical ensemble and comprehensively rationalized at the molecular level on the basis of energetic factors and radial distribution functions. The structure of confined water for the different hydration levels was characterized in detail by using a specific criterion of hydrogen bond formation. The influence of both host characteristics and temperature on the microscopic behaviour of adsorbed water was assessed. Overall, this work proves that water–water hydrogen bonding is enhanced by the hydrophobic character of the pore walls and the large confining spaces. The increase in temperature induces progressive destruction of hydrogen bonds, but the majority of water molecules remain associated even when saturation is not reached. Concerning the thermodynamics of water adsorption, it is mainly affected by the peculiarities of the pore structure.

Effects of Crowding, Osmolytes, Temperature and Pressure on the Interaction Potential of Dense Protein Solutions

We studied the effect of pressure on the structure and intermolecular interactions of dense lysozyme solutions in various cosolvent mixtures and upon addition of various Hofmeister anions using small-angle X-ray scattering in combination with liquid-state theoretical approaches [1-3]. Supplementary thermodynamic information was obtained by employing calorimetric techniques, densitometry and ultrasound velocimetry. We show that the particular structural properties of water and specific ion effects play a crucial major role in protein stabilisation, notably under high hydrostatic pressure conditions. Also the effect of confinement on the solvational properties and intermolecular interaction of proteins was studied, including the effects of self-crowding and macromolecular crowders on the temperature-pressure stability diagram of proteins [4]. We also discuss the effect of pressure on the second virial coefficient and how pressure can be used to control and fine-tune protein crystallization. Moreover, we present results on the phase behavior of dense lysozyme solutions in the liquid-liquid phase separation region. A re-entrant liquid-liquid phase separation region has been discovered at elevated pressures, which originates in the pressure dependence of the solvent-mediated protein-protein interactions [3].[1] M. A. Schroer, J. Markgraf, D. C. F. Wieland, C. J. Sahle, J. Moller, M. Paulus, M. Tolan, R. Winter, Phys. Rev. Lett. 106 (2011) 178102[2] M. A. Schroer, Y. Zhai, D. C. F. Wieland, C. J. Sahle, J. Nase, M. Paulus, M. Tolan, R. Winter, Angew. Chem. Int. Ed. 50 (2011) 11413[3] J. Moller, S. Grobelny, J. Schulze, S. Bieder, A. Steffen, M. Erlkamp, M. Paulus, M. Tolan, R. Winter, Phys. Rev. Lett. 112 (2014) 028101[4] M. Erlkamp, S. Grobelny, R. Winter, Phys. Chem. Chem. Phys. 16 (2014) 5965

Dynamics of a network of hydrogen bonds upon water electrocrystallization

Computer simulation is employed to study the dynamics of a network of hydrogen bonds and the structural properties of water placed between graphene layers. The presence of the graphene walls has been shown to substantially affect the water phase diagram. Glass transition processes are observed in the system, and liquid water completely passes to an amorphous state. Moreover, it has been established that the imposition of an external electric field with strength E ≥ 0.5 V/A on the system subjected to increased pressure results in structural ordering of water. It has been found that water located between graphene layers is transformed into Ic cubic ice. The electrocrystallization of water has been shown to substantially change the dynamics of the network of hydrogen bonds.

Mesoscale simulation of water

A coarse-grained model for water is developed which maps five water molecules onto one bead. The coarse-grained potential is derived by iteratively matching the radial distribution function of water in the coarse-grained and target models. It is shown that the coarse-grained model has the optimal balance between computational efficiency and accuracy in structural properties. The model has been used to calculate a number of static and dynamic properties, including the density, isobaric thermal expansivity, isothermal compressibility, surface tension, and diffusion coefficient of water. The effect of coarsening on these properties has been discussed. It is shown that while the present coarse-grained model well describes the structural properties of water, expectedly it has a much faster dynamics than the corresponding atomistic models.

Properties of water in the region between a tubulin dimer and a single motor head of kinesin

A kinesin is a molecular motor that can perform movement on a microtubule track in a stepping-like manner. This motion is connected with processes of association and dissociation of kinesin and tubulin. Water is an important participant in these kinds of molecular interactions. This is why we have decided to investigate the dynamical and structural properties of water in the region between the kinesin catalytic domain and the tubulin dimer. Using the molecular dynamics method, we found that these properties are different from the ones of bulk water. The changes in structure and dynamics are visible for water beyond the first solvation layers, even for the longest analyzed distance between proteins equal to 2.0 nm. However, these changes are not always enhanced compared to the situation when only one protein surface is present. One factor that distinguishes the investigated situation from the one with a single protein is the presence of an additional electric field originating from the second protein. The tendency of vectors of dipole moments of water molecules between the proteins to follow the vectors of electric field generated by the proteins causes a distortion of the water-water hydrogen bond network. It has been shown that this distortion affects the properties of water in this region: it induces structural changes in solvation water, and leads to increased water density and increased stiffness of the water structure.