Small Number Of Water Molecules Research Articles

Hydrophobic Hydration in Water-tert-Butyl Alcohol Solutions by Extended Depolarized Light Scattering.

Molecular dynamics and structural properties of water-tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.

Effect of stepwise microhydration on the guanidinium···π interaction

The characteristics of the interaction of microhydrated guanidinium cation with the aromatic moieties present in the aromatic amino acids side chains have been studied by means of computational methods. The most stable minima found for non-hydrated complexes correspond in all cases to structures with guanidinium oriented toward the ring and interacting by means of N-H···π hydrogen bonds. The interaction becomes stronger when going from benzene (-14 kcal mol⁻¹) to phenol (-17 kcal mol⁻¹) to indole (-21 kcal mol⁻¹). These complexes are held together mainly by electrostatics, but with important contributions from induction and dispersion. The presence of a small number of water molecules significantly affects the characteristics of the complexes. Hydrogen bonds formed by water with the cation, another water molecule, or the aromatic units become more and more similar in intensity as water molecules are included in the complex, leading to a great variety of minima with similar stability but showing very different structural patterns. The behavior is similar with the three aromatic units, the differences in stability mainly being a consequence of the different strength of the cation···π contact.

Conformational preferences of taurine in the gas phase and in water

All feasible local minima of the neutral taurine (2-aminoethanesulfonic acid) in the gas phase and its zwitterionic form in water are explored using a density functional (M06-2X) that provides an accurate treatment of medium-range correlation energy and the recently improved solvation model SMD with the larger basis set (cc-pVTZ). In the gas phase, the conformer gtGg (II1) with the gauche− conformation around the NCH2CH2S sequence and the hydrogen bond between the sulfonic OH and the amine NH2 is found to be most preferred, which can be ascribed to the favored hyperconjugative n→σ* interaction between the lone electron pair of the amine nitrogen and the antibonding OH orbital of the sulfonic moiety. The calculated rotational constants of the conformer gtGg are consistent with the values obtained from LA-MB-FTMW experiments. In addition, the feasible structures of taurine±·(H2O)n complexes with a small number of water molecules (n=0, 1, 2, and 4) were investigated in the implicit SMD aqueous medium. The calculated results of taurine± with four water molecules support that taurine exists as a mixture of gauche and trans conformers in aqueous solution. Its gauche population is calculated to be 71%, which is in good agreement with the observed value of 67% from 1H NMR experiments in D2O. It is known that the increased population of the trans conformer for the taurine±·(H2O)4 complex is ascribed to the preferred solvation by the interactions between the solute and the discrete water molecules in the complex and by the polarization of the continuum water due to the complex.

Water’s Structure around Hydrophobic Solutes and the Iceberg Model

The structure of water in the hydration shells of small hydrophobic solutes was investigated through molecular dynamics. The results show that a subset of water molecules in the first hydration shell of a nonpolar solute have a significantly enhanced tetrahedrality and a slightly larger number of hydrogen bonds, relative to the molecules in water at room temperature, consistent with the experimentally observed negative excess entropy and increased heat capacity of hydrophobic solutions at room temperature. This ordering results from the rearrangement of a small number of water molecules near the nonpolar solutes that occupy one to two vertices of the enhanced water tetrahedra. Although this structuring is not nearly like that often associated with a literal interpretation of the term "iceberg" in the Frank and Evans iceberg model, it does support a moderate interpretation of this model. Thus, the tetrahedral orientational order of this ensemble of water molecules is comparable to that of liquid water at ~10 °C, although not accompanied by the small contraction of the O-O distance observed in cold water. Further, we show that the structural changes of water in the vicinity of small nonpolar solutes cannot be inferred from the water radial distribution functions, explaining why this increased ordering is not observed through neutron diffraction experiments. The present results restore a molecular view where the slower translational and reorientational dynamics of water near hydrophobic groups has a structural equivalent resembling water at low temperatures.

Poly(sulfonated phenylene)-block-poly(arylene ether sulfone) copolymer for polymer electrolyte fuel cell application

Novel poly(2-(3-sulfo)benzoyl-1,4-phenylene)-block-poly(arylene ether sulfone) copolymers (PSP-b-PAESs) were successfully synthesized by Ni(0)-catalyzed copolymerization of 2,5-dichloro-3′-sulfo-benzophenone (DCSB) and chlorobenzophenone-endcapped oligo(arylene ether sulfone). Their physical property, morphology and polymer electrolyte fuel cell (PEFC) performance were investigated compared to those of the DCSB-based random copolymers and Nafion112. PSP-b-PAES with a measured ion exchange capacity (IEC) of 1.82 meq g−1, of which the hydrophilic/hydrophobic block lengths were evaluated as 17/8.4, showed the relatively small number of water molecules sorbed per sulfonic acid group (λ = 15) in water and the anisotropic membrane swelling with 2.4 times larger through-plane swelling than in-plane one, whereas it showed the almost isotropic proton conductivity. The PSP-b-PAES exhibited a microphase-separated structure composed of hydrophobic and hydrophilic domains, whereas the random copolymers exhibited a homogenous morphology. The PSP-b-PAES had the larger proton conductivity than the random copolymer with an IEC of 2.01 meq g−1, especially under the low relative humidities. Even at a low humidification of 17% RH at 90 °C and 0.2 MPa, the PSP-b-PAES exhibited the high PEFC performance; namely, cell voltage of 0.69 V at load current density of 0.5 A cm−2 and maximum output of 0.73 W cm−2, which were much higher than those at 30% RH for the random copolymer (0.65 V and 0.51 W cm−2) and Nafion112 (0.70 V and 0.61 W cm−2). The PSP-b-PAES showed the fairly high durability of 750 h under PEFC operation at 90 °C in spite of the relatively low molecular weight. PSP-b-PAESs have the high potential as polymer electrolyte membrane for PEFC applications.

An atomistic level description of guest molecule effect on the formation of hydrate crystal nuclei by ab initio calculations

In the present study, we report the results of a systematic investigation of cage-like water structures using the first-principles calculations. These results show that, in the case of methane hydrate, the following nucleation mechanism can be revealed. The formation of small water cavities filled with methane is the first step of the formation of methane hydrate. It is not necessary to occupy all dodecahedral cages by guest molecules. After that small cavities start to form the H-bonding network with surrounding water molecules and a small number of water molecules is enough for the formation of a stable hydrogen-bonding network. The structural information contained in such nuclei is conserved in the forming crystal. Moreover, the presence of a methane molecule between small cages is also important to prevent the adhesion of cavities. It found that the ozone molecule can also stabilize the small cage since the value of the interaction energy between the ozone guest and the water host framework is very close to that obtained for the methane case. However, ozone affects the structure of large cavities and hence, the second guest is necessary to stabilize the hydrate structure.

A DFT Study of the Interaction between Microhydrated Anions and Naphthalendiimides

The characteristics of the interaction of anions with naphthalendiimides, the basic structural motif of a newly synthesized anion channel based on anion···π interactions, are studied by computational methods. Stable complexes are formed with bromide, chloride, fluoride or hydroxide anions, which exhibit strong anion···π interactions in the gas phase. Following the sequence of the polarizing power of the anions, hydroxide and fluoride complexes are the most strongly interacting. The presence of a small number of water molecules strongly affects the anion···π interactions, especially for hydroxide and fluoride complexes, so the differences in interaction strength among the anions drop significantly. The calculations suggest that a small number of water molecules can be crucial to reducing dehydration cost and contributing to stabilizing interactions with the naphthalendiimide units.

Acidification of Reverse Micellar Nanodroplets by Atmospheric Pressure CO2

Water absorption of atmospheric carbon dioxide lowers the solution pH due to carbonic acid formation. Bulk water acidification by CO(2) is well documented, but significantly less is known about its effect on water in confined spaces. Considering its prominence as a greenhouse gas, the importance of aerosols in acid rain, and CO(2)-buffering in cellular systems, surprisingly little information exists about the absorption of CO(2) by nanosized water droplets. The fundamental interactions of CO(2) with water, particularly in nanosized structures, may influence a wide range of processes in our technological society. Here results from experiments investigating the uptake of gaseous CO(2) by water pools in reverse micelles are presented. Despite the small number of water molecules in each droplet, changes in vanadium probes within the water pools, measured using vanadium-51 NMR spectroscopy, indicate a significant drop in pH after CO(2) introduction. Collectively, the pH-dependent vanadium probes show CO(2) dissolves in the nanowater droplets, causing the reverse micelle acidity to increase.

Aggregation-Induced Chemical Reactions: Acid Dissociation in Growing Water Clusters

Understanding chemical reactivity at ultracold conditions, thus enabling molecular syntheses via interstellar and atmospheric processes, is a key issue in cryochemistry. In particular, acid dissociation and proton transfer reactions are ubiquitous in aqueous microsolvation environments. Here, the full dissociation of a HCl molecule upon stepwise solvation by a small number of water molecules at low temperatures, as relevant to helium nanodroplet isolation (HENDI) spectroscopy, is analyzed in mechanistic detail. It is found that upon successive aggregation of HCl with H(2)O molecules, a series of cyclic heteromolecular structures, up to and including HCl(H(2)O)(3), are initially obtained before a precursor state for dissociation, HCl(H(2)O)(3)···H(2)O, is observed upon addition of a fourth water molecule. The latter partially aggregated structure can be viewed as an "activated species", which readily leads to dissociation of HCl and to the formation of a solvent-shared ion pair, H(3)O(+)(H(2)O)(3)Cl(-). Overall, the process is mostly downhill in potential energy, and, in addition, small remaining barriers are overcome by using kinetic energy released as a result of forming hydrogen bonds due to aggregation. The associated barrier is not ruled by thermal equilibrium but is generated by athermal non-equilibrium dynamics. These "aggregation-induced chemical reactions" are expected to be of broad relevance to chemistry at ultralow temperature much beyond HENDI spectroscopy.

Effect of Mg2+ versus Ca2+ on the behavior of Annexin A5 in a membrane-bound state

Annexin A5 (AnxA5) binds to negatively charged phospholipid membranes in a Ca(2+) dependent manner. Several studies already demonstrate that Mg(2+) ions cannot induce the binding. In this paper, quartz crystal microbalance with dissipation monitoring (QCM-D), Brewster angle microscopy (BAM), polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) and molecular dynamics (MD) were performed to elucidate the high specificity of Ca(2+) versus Mg(2+) on AnxA5 binding to membrane models. In the presence of Ca(2+), AnxA5 showed a strong interaction with lipids, the protein is adsorbed mainly in α-helix under the DMPS monolayer, with an orientation of the α-helices axes slightly tilted with respect to the normal of the phospholipid monolayer as revealed by PMIRRAS. The Ca(2+) ions interact strongly with the phosphate group of the phospholipid monolayer. In the presence of Mg(2+), instead of Ca(2+), no interaction of AnxA5 with lipids was detected. Molecular dynamics simulations allow us to explain the high specificity of calcium. Ca(2+) ions are well exposed and surrounded by labile water molecules at the surface of the protein, which then favour their binding to the phosphate group of the membrane, explaining their specificity. To the contrary, Mg(2+) ions are embedded in the protein structure, with a smaller number of water molecules strongly bound. We conclude that the embedded Mg(2+) ions inside the AnxA5 structure are not able to link the protein to the phosphate group of the phospholipids for this reason.

The surface tension of TIP4P/2005 water model using the Ewald sums for the dispersion interactions

The liquid-vapor phase equilibria and surface tension of the TIP4P/2005 water model is obtained by using the Ewald summation method to determine the long range Lennard-Jones and electrostatic interactions. The method is implemented in a straightforward manner into standard simulation programs. The computational cost of using Ewald sums in dispersion interactions of water is estimated in direct simulation of interfaces. The results of this work at 300 K show a dramatic change in surface tension with an oscillatory behavior for surface areas smaller than 5x5sigma(2), where sigma is the Lennard-Jones oxygen diameter. The amplitude of such oscillations substantially decreases with temperature. Finite size effects are less important on coexisting densities. Phase equilibria and interfacial properties can be determined using a small number of water molecules; their fluctuations are around the same size of simulation error at all temperatures, even in systems where the interfaces are separated a few molecular diameters only. The difference in surface tension of this work compared to the results of other authors is not significant (on the contrary, there is a good agreement). What should be stressed is the different and more consistent approach to obtain the surface tension using the Ewald sums for dispersion interactions. There are two relevant aspects at the interface: An adsorption of water molecules is observed at small surface areas and its thickness systematically increases with system size.

Shape memory polymer based on chemically cross-linked poly(vinyl alcohol) containing a small number of water molecules

In this paper, shape memory polymers based on poly(vinyl alcohol) (SM-PVA) cross-linked with different contents of glutaraldehyde were prepared. Because PVA is a hydrophilic polymer, all samples prepared always have a small number of water molecules exposed to air, and the water molecules are helpful for shape memory characteristics. The influences of water contents on the prepared materials were investigated. The properties of SM-PVA were investigated by dynamic–mechanical analysis, differential scanning calorimetry, and thermogravimetric analysis. Shape memory behavior of SM-PVA, depending on the switching of chain segments, occurred at around T g. Thermo-mechanical cycle test was performed to investigate shape memory properties, including the percentage shape recovery, shape recovery ratio, and percentage shape fixity. The studied materials show promising shape memory and cyclic behavior indicative of potential applications of SM-PVA as shape memory materials.

Enhanced Acidity of Zn2+ in the Presence of Small Numbers of Water Molecules

Experiments performed on gas phase [Zn(H2O)N]2+ complexes, for N in the range 4-7, show the ions readily undergo unimolecular (metastable) decay with respect to proton release via the reaction [Zn(H2O)N]2+--> Zn(2+)OH(-)(H2O)M + H3O(+)(H2O)N-M-2. To account for these products, it is proposed that the larger complexes have a stable [Zn(H2O)4]2+ core to which additional molecules are retained in an outer shell through hydrogen bonding. At N = 7, this arrangement would make it possible for proton release to be associated with a chain of up to four water molecules, which equates with ideas proposed for the activity of Zn2+ in metalloenzymes.

A microsolvation approach to the prediction of the relative enthalpies and free energies of hydration for ammonium ions

Hartree–Fock (HF) and second-order Moller–Plesset (MP2) calculations were used to investigate the structures and thermochemistry of methylammonium–water clusters (Me4-m NH + (H2O) n , m=1–4, n=1–4). Water molecules were treated ab initio and with effective fragment potentials (EFP). In addition to a thorough phase-space search, the importance of basis set, electron correlation, and thermodynamic effects was systematically examined. Cluster structures resulted from hydrogen bond formation between the ammonium group and water molecules; upon saturation of the hydrogen bonding sites of the ammonium group, water molecules entered the second hydration shell. With only four water molecules, the experimental relative enthalpies of hydration were well reproduced at the HF level, while the MP2 relative free energies were in best agreement with experiment. Absolute energies of hydration were calculated using an empirical correction. These results strongly suggest that a HF-based microsolvation approach employing a small number of water molecules can be used to compute relative enthalpies of hydration.

Minimalist explicit solvation models for surface loops in proteins.

We have performed molecular dynamics simulations of protein surface loops solvated by explicit water, where a prime focus of the study is the small numbers (e.g., ~100) of explicit water molecules employed. The models include only part of the protein (typically 500 - 1000 atoms), and the water molecules are restricted to a region surrounding the loop. In this study, the number of water molecules (N(w)) is systematically varied, and convergence with large N(w) is monitored to reveal N(w)(min), the minimum number required for the loop to exhibit realistic (fully hydrated) behavior. We have also studied protein surface coverage, as well as diffusion and residence times for water molecules as a function of N(w). A number of other modeling parameters are also tested. These include the number of environmental protein atoms explicitly considered in the model, as well as two ways to constrain the water molecules to the vicinity of the loop (where we find one of these methods to perform better when N(w) is small). The results (for RMSD and its fluctuations for four loops) are further compared to much larger, fully solvated systems (using ~10,000 water molecules under periodic boundary conditions and Ewald electrostatics), and to results for the GBSA implicit solvation model. We find that the loop backbone can stabilize with a surprisingly small number of water molecules (as low as 5 molecules per amino acid residue). The side chains of the loop require somewhat larger N(w), where the atomic fluctuations become too small if N(w) is further reduced. Thus, in general, we find adequate hydration to occur at roughly 12 water molecules per residue. This is an important result, because at this hydration level, computational times are comparable to those required for GBSA. Therefore these "minimalist explicit models" can provide a viable and potentially more accurate alternative. The importance of protein loop modeling is discussed in the context of these, and other, loop models, along with other challenges including the relevance of appropriate free energy simulation methodology for assessment of conformational stability.

Interaction of Water with Single-Walled Carbon Nanotubes: Reaction and Adsorption

The interaction of water vapor with carbon nanotubes at room temperature has been investigated using Fourier transform (FT) IR spectroscopy and density functional theory (DFT) calculations. FTIR data indicate that water molecules adsorb on single-walled carbon nanotubes at room temperature. Comparison to previous studies suggests that the water forms hydrogen-bonded structures inside the nanotubes. Analysis of the FTIR data demonstrates that a small number of water molecules react with the nanotubes, forming C-O bonds, whereas a majority of the water molecules adsorb intact. The DFT calculations show that cleavage of an O-H bond upon adsorption to form adsorbed -H and -OH groups is energetically favorable at defect sites on nanotubes.

Hydrophilic Interactions between Organic and Water Molecules as Models for Monolayers at the Gas/Water Interface

Model clusters of surfactant prototypes with small number of water molecules are calculated at different levels of theory. All approaches used yield correct trends in the variation of the dipole moment upon tail elongation or polar headgroup variation. Models including one, two, or more water molecules are optimized. The most stable structures are those with maximum number of atoms involved in hydrogen bonding. The normal components of the dipole moment prove to be less sensitive to the nature (aliphatic or aromatic) of the hydrophobic tail, in accord with findings from the phenomenological models. Values of the dipole moment approaching the experimental estimates required inclusion of sufficient aqueous environment (>20 water molecules per hydrophilic head) and of lateral intersurfactant interactions into the model.

Structural correlations of water molecules in a concentrated alcohol solution

Hydrogen/deuterium isotopic substitution neutron scattering techniques andempirical potential structure refinement have been used to investigate thestructure in a 0.86 mole fraction solution of tertiary butanol in water. Theresulting model structure suggests that in this mixture, the relatively smallquantity of water molecules tend to associate in small hydrogen bonded clusters oftwo or three molecules, solvated by the alcohol hydroxyl groups. An interestingobservation on investigating this novel local water environment is that thetetrahedral hydrogen bonding motif of water still strongly influences thelocation of both the water and alcohol first neighbours. The combinationof the small number of water molecules in the mixture and the bulkymolecular structure of the alcohol appears to result in the incompletesatisfaction of the water molecule’s full hydrogen bonding capabilities.

Computational studies of aliphatic amine basicity.

Computational studies have been used to examine the structural and energetic effects of adding small numbers of water molecules to ammonia, methylamine, dimethylamine, and trimethylamine, and their respective ammoniums ions using the effective fragment potential method. Distinct structural effects with only a few fragment water molecules are revealed. The complexity of structures increases with the number of water fragments with the water fragments forming complex networks. Structural and energetic effects are used to probe the so-called anomalous basicity effect of ammonia and the methylamines on going from the gas phase to aqueous solution.

On the Debye-Waller factor of hexagonal ice: a computer simulation study.

We investigate by molecular dynamics (MD) simulations the temperature dependence of the Debye-Waller (DW) factor of hexagonal ice with 25 different proton-disordered configurations. Each initial configuration is composed of 288 water molecules with no net dipole moment. The intermolecular interaction of water is described by TIP4P potential. Each production run of the simulation is 15 ns or longer. We observe a change in slope of the DW factor around 200 K, which cannot be explained within the framework of either classical or quantum harmonic approximation. Configurations generated by MD simulations are subjected to the steepest descent energy minimization. Analysis of the local energy minimum structures reveals that water molecules above 200 K jump to other lattice sites via some local energy minimum structures which contain some water molecules sitting on the locations other than the lattice sites. As time evolves, these defect molecules move back and forth to the lattice sites yielding defect-free structures. Those motions are responsible for the unusual increase in the DW factor at high temperatures. In making a transition from an energy-minimum structure to another one, a small number of water molecules are involved in a highly cooperative fashion. The larger DW factor at higher temperature arises from jump-like motions of water molecules among these locally stable configurations which may or may not be a family of the proton-disordered ice forms satisfying the "ice rule".