Amino-acid and water molecules adsorbed on water clusters in a beam

Water clusters (H2On and (D2On (n相似文献   

Adsorption of water molecules in slit pores

We report molecular dynamics (MD) simulations on the adsorption of water in attractive and repulsive slit pores, where the slit and a bulk region are in contact with each other. Water structure, surface force and adsorption behavior are investigated as a function of the overall density in the bulk region. The gas–liquid transition in both types of pores occurs at similar densities of the bulk region.     

Position-dependent energy of molecules in nano-confined water

Real time decrease in the energy (or enthalpy) measured during confinement of controlled amounts of water in 2 nm radius pores of Vycor shows that exothermic transfer of bulk water to nanopores via the vapour-phase occurred in two stages. In the first stage, at saturation pressure, H2O molecules from the vapour rapidly accumulated in the nanopore channels near the Vycor surface. In the second, at vapour pressure below saturation, the accumulation rate abruptly decreased and water (slowly) diffused and redistributed in the nanopore channels until the vapour pressure equilibrium was attained. The energy decrease per H2O molecule was highest, 14.5 kJ mol(-1), at low amounts when the pore-wall was incompletely covered by H2O. This value approached zero at higher amounts when pores were gradually filled. The results show that the vibrational and configurational contributions to the energy of H2O molecules depend upon their position in the nanopore and these contributions approach their bulk water values at high water concentration, but do not attain those values for completely filled pores.     

Calorimetric and conductometric titrations of nanostructures of water molecules in iteratively filtered water

This work continue the study of the physico-chemical properties of samples of pure, twice distilled water, when subject to a procedure of iterative filtrations through Pyrex glass filters (Büchner funnels). After the filtrations, electrical conductivity and heat of mixing with NaOH and HCl solutions increase. The hypothesis is that the iterative filtration procedure, that involves a flux of energy and material in an open system, is able to induce the formation of “dissipative structures” or nanostructures of water molecules (WNS). Water exhibits an extraordinary auto-organization potentiality triggered by several kinds of perturbations, including mechanical ones. We measured the heats of mixing of acid or basic solutions with such iterated filtered waters (IFW) and their electrical conductivity, comparing with the analogous heats of mixing, electrical conductivity of the solvent. We found some relevant exothermic excess heats of mixing and higher conductivity than those of the untreated solvent. The heats of mixing and electrical conductivity of IFW show a good correlation, underlining a single cause for the behavior of the samples.     

Influence of water molecules on dimerization of silicic acid

By means of the quantum-chemical MO LCAO SCF method in the valence approximation of MNDO, it has been shown that effective interaction of two molecules of Si(OH)₄ is possible with the participation of a water molecule that plays the role of a bridge in forming a six-center complex.Institute of Surface Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 27, No. 4, pp. 485–487, July–August, 1991. Original article submitted February 14, 1991.     

Ligand-protein docking with water molecules

The presence of water molecules plays an important role in the accuracy of ligand-protein docking predictions. Comprehensive docking simulations have been performed on a large set of ligand-protein complexes whose crystal structures contain water molecules in their binding sites. Only those water molecules found in the immediate vicinity of both the ligand and the protein were considered. We have investigated whether prior optimization of the orientation of water molecules in either the presence or absence of the bound ligand has any effect on the accuracy of docking predictions. We have observed a statistically significant overall increase in accuracy when water molecules are included during docking simulations and have found this to be independent of the method of optimization of the orientation of water molecules. These results confirm the importance of including water molecules whenever possible in a ligand-protein docking simulation. Our findings also reveal that prior optimization of the orientation of water molecules, in the absence of any bound ligand, does not have a detrimental effect on the improved accuracy of ligand-protein docking. This is important, given the use of docking simulations to predict the binding modes of new ligands or drug molecules.     

Kinetics of self-diffusion of water molecules in natrolite

Solid-state ¹H NMR is applied to investigate the kinetics of diffusion of H₂O molecules in the fibrous zeolite natrolite [Na₂Al₂Si₃O₁₀]?2H₂O. It is found that the stepwise heating of the zeolite in air leads to the following pattern of molecular diffusion jumps: at first they increase in number and then decrease exponentially with time. Conducting such an experiment in an aqueous medium leads to the opposite effect. The results obtained confirm our previous suggestion about the importance of interstitial defects, or overhydrated local states, in molecular diffusion.     

Adsorption of water molecules on selected charged sodium-chloride clusters

The adsorption of water molecules (H(2)O) on sodium chloride cluster cations and anions was studied at 298 K over a mass range of 100-1200 amu using a custom-built laser desorption ionization reactor and mass spectrometer. Under the conditions used, the cations Na(3)Cl(2)(+) and Na(4)Cl(3)(+) bind up to three water molecules, whereas the larger cations, Na(5)Cl(4)(+) to Na(19)Cl(18)(+), formed hydrates with one or two only. The overall trend is a decrease in hydration with increasing cluster size, with an abrupt drop occurring at the closed-shell Na(14)Cl(13)(+). As compared to the cluster cations, the cluster anions showed almost no adsorption. Among smaller clusters, a weak adsorption of one water molecule was observed for the cluster anions Na(6)Cl(7)(-) and Na(7)Cl(8)(-). In the higher mass region, a substantial adsorption of one water molecule was observed for Na(14)Cl(15)(-). Density functional theory (DFT) computations were carried out for the adsorption of one molecule of H(2)O on the cations Na(n)Cl(n-1)(+), for n = 2-8, and the anions Na(n)Cl(n+1)(-), for n = 1-7. For each ion, the structure of the hydrate, the hydration energy, and the standard-state enthalpy, entropy, and Gibbs energy of hydration at 298 K were computed. In addition, it was useful to compute the distortion energy, defined as the electronic energy lost due to weakening of the Na-Cl bonds upon adsorption of H(2)O. The results show that strong adsorption of a H(2)O molecule occurs for the linear cations only at an end Na ion and for the nonlinear cations only at a corner Na ion bonded to two Cl ions. An unexpected result of the theoretical investigation for the anions is that certain low-energy isomers of Na(6)Cl(7)(-) and Na(7)Cl(8)(-) bind H(2)O strongly enough to produce the observed weak adsorption. The possible implications of these results for the initial hydration of extended NaCl surfaces are discussed.     

Interfacial water on hydrophobic surfaces recognized by ions and molecules

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

Internal water molecules and magnetic relaxation in agarose gels

Agarose gels have long been known to produce exceptionally large enhancements of the water 1H and 2H magnetic relaxation rates. The molecular basis for this effect has not been clearly established, despite its potential importance for a wide range of applications of agarose gels, including their use as biological tissue models in magnetic resonance imaging. To resolve this issue, we have measured the 2H magnetic relaxation dispersion profile from agarose gels over more than 4 frequency decades. We find a very large dispersion, which, at neutral pH, is produced entirely by internal water molecules, exchanging with bulk water on the time scale 10(-8)-10(-6) s. The most long-lived of these dominate the dispersion and give rise to a temperature maximum in the low-frequency relaxation rate. At acidic pH, there is also a low-frequency contribution from hydroxyl deuterons exchanging on a time scale of 10(-4) s. Our analysis of the dispersion profiles is based on a nonperturbative relaxation theory that remains valid outside the conventional motional-narrowing regime. The results of this analysis suggest that the internal water molecules responsible for the dispersion are located in the central cavity of the agarose double helix, as previously proposed on the basis of fiber diffraction data. The magnetic relaxation mechanism invoked here, where spin relaxation is induced directly by molecular exchange, also provides a molecular basis for understanding the water 1H relaxation behavior that governs the intrinsic magnetic resonance image contrast in biological tissue.     

Revisiting small clusters of water molecules

The geometries and relative energies of small clusters of water molecules, (H₂O)_n with 4 ⩽ n ⩽ 8, are reported. For each value of n we have considered the conformations corresponding to the lowest-energy minimum and those in nearby relative minima. Thus we report on six tetramers, four pentamers, six hexanlers, four heptamers, and eigth octamers. The geometrical conformations have been obtained using the Metropolis Monte Carlo method as a minimization technique, where the interaction energy is computed with the MCY potential plus three- and four-body corrections previously discussed. All the reported structures for a given cluster size are found to be close in energy. For the lowest conformation the geometry was optimized with ab initio SCF computations using energy gradients. Our results are compared with previous theoretical studies. We discuss the convergence of the interaction potential for liquid water when expressed in terms of a many-body series expansion.