Population Of Water Molecules Research Articles

Heat capacity effects associated with urea and tetramethylurea hydration: insight from computer simulation

Molecular dynamic simulation of the hydrophilic urea and hydrophobic tetramethylurea aqueous solutions using the random network model of liquid water reveals that apolar Me groups induce an increase in the population of water molecules with linear and shorter H-bonds in their first hydration shell, whereas the carbonyl oxygen atom causes an opposite effect with elevation in the population of high angle/distance water molecules pairs. This behavior of water is the major reason for opposite changes in the heat capacity of hydration for apolar and polar species.

An equation for the isosbestic point wavelength in aqueous solutions of electrolytes

The existence of an isosbestic point in the OH bond vibration band of H2O molecules in solutions of strong electrolytes has been shown by many research groups but its physical meaning has been only recently read as the equilibrium point between two populations of water molecules where either water-ion or water-water interactions respectively prevail. Thus the isosbestic point provides useful information on the bulk displacement of water molecules in presence of solutes. Herein we show that the isosbestic point occurrence is correctly described by an equation describing the balance of the electrostatic forces. Furthermore, its wavelength is also related to the slope of the density vs. the solute concentration. These results support the view that the effect of the solute can be barely regulated within few hydration shells.

Examining water in model membranes by near infrared spectroscopy and multivariate analysis

By exploiting the sensitivity of the NIR spectrum, particularly the first overtone of water, to the number and strength of hydrogen bonds, the hydrogen bond network and water polymerization in membranes of DMPA (1,2-dimyristoyl-sn-glycero-3-phosphate) and DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) was investigated as a function of the temperature and the presence of this two phospholipids having the same tail but different polar head. Principal components analysis performed on the spectra was used to disclose subtle spectral changes that mirror the alteration of the vibrational energy of the water O-H bonds, as a measure of the H-bond network. Temperature showed a dominating effect on the H-bond network. Increasing temperatures diminished the number of strongly H-bonded water molecules and increased the number of weakly H-bonded waters. This main effect of temperature was missing after the subtraction of the pure water spectra from the lipid-containing ones. An intriguing secondary effect of temperature was also revealed. Phospholipids exhibited an effect qualitatively similar to that of the temperature. DMPA, and particularly DMPC, disrupted the H-bond network in the neighboring lipid-water interface, reducing water polymerization and strengthening the water O-H bonds. The type of the polar head affects the H-bonds more than duplicate the concentration of the lipid. A connection between head group structure and the effect on the H-bonds network, and the existence of two populations of water molecules are discussed.

Taurine as a water structure breaker and protein stabilizer

The enhancing effect on the water structure has been confirmed for most of the osmolytes exhibiting both stabilizing and destabilizing properties in regard to proteins. The presented work concerns osmolytes, which should be classified as “structure breaking” solutes: taurine and N,N,N-trimethyltaurine (TMT). Here, we combine FTIR spectroscopy, DSC calorimetry and DFT calculations to gain an insight into the interactions between osmolytes and two proteins: lysozyme and ubiquitin. Despite high structural similarity, both osmolytes exert different influence on protein stability: taurine is a stabilizer, TMT is a denaturant. We show also that taurine amino group interacts directly with the side chains of proteins, whereas TMT does not interact with proteins at all. Although two solutes weaken on average the structure of the surrounding water, their hydration spheres are different. Taurine is surrounded by two populations of water molecules: bonded with weak H-bonds around sulfonate group, and strongly bonded around amino group. The strong hydrogen-bonded network of water molecules around the amino group of taurine further improves properties of enhanced protein hydration sphere and stabilizes the native protein form. Direct interactions of this group with surface side chains provide a proper orientation of taurine and prevents the {text{SO}}_{3}^{ - } group from negative influence. The weakened {text{SO}}_{3}^{ - } hydration sphere of TMT breaks up the hydrogen-bonded network of water around the protein and destabilizes it. However, TMT at low concentration stabilize both proteins to a small extent. This effect can be attributed to an actual osmophobic effect which is overcome if the concentration increases.

Separating the pH-Dependent Behavior of Water in the Stern and Diffuse Layers with Varying Salt Concentration

Vibrational sum frequency generation (SFG) spectroscopy was utilized to distinguish different populations of water molecules within the electric double layer (EDL) at the silica/water interface. By systematically varying the electrolyte concentration, surface deprotonation, and SFG polarization combinations, we provide evidence of two regions of water molecules that have distinct pH-dependent behavior when the Stern layer is present (with onset between 10 and 100 mM NaCl). For example, water molecules near the surface in the Stern layer can be probed by the pss polarization combination, while other polarization combinations (ssp and ppp) predominantly probe water molecules further from the surface in the diffuse part of the electrical double layer. For the water molecules adjacent to the surface within the Stern layer, upon increasing the pH from the point-of-zero charge of silica (pH ∼2) to higher values (pH ∼12), we observe an increase in alignment consistent with a more negative surface with increasing...

46. Could it be possible to distinguish bending and crossing fibers in diffusion MRI data?

After diffusion tensor imaging (DTI) model (Basser et al., 1994), several approaches which are able to detect two or more crossing fibers in diffusion MRI (dMRI) data have been invented (e.g. Q-ball imaging, ball and stick model) (Tuch, 2004; Behrens et al., 2003). After that, some fiber bundles which had not been seen with DTI model were suddenly observed (e.g. in corpus callosum). Although it brought an improvement it seems that about 50% of detected fiber bundles are false positive results after tractography (Ciccarelli et al., 2008). One crucial problem is that tractography cannot decide if the bundles are crossing or bending because models are not estimating bending-tensor. For 2 crossing fibers, the tractography algorithm can trace from one point to three different places. For 2 bending fibers, there is only one possible way. We would like to introduce how the difference between dMRI data coming from crossing or bending fibers could be detected. Imagine a population of water molecules in the centre of crossing or bending and some applied gradient of diffusion measurement. For crossing fibers, the population of molecules can diffuse in all directions of fiber spreading, thus the phase of molecules can be affected by the whole gradient range. Contrary for one bending fiber, the population can diffuse only in directions of the fiber, thus the phase can be affected only by the narrower gradient range. It applies similarly for second bended fiber. From this point of view, phase distributions should differ for crossing and bending fibers respectively also resulting dMRI data should differ. For this statement testing, the dMRI data simulator which generates dMRI data based on Brownian motion of water molecules inside and outside axons per one voxel volume was created. Although there is several technical problems and aspects (e.g. periodic character of gradient space phase distribution) we are looking for sequence settings of dMRI measurement where the dMRI data would be statistically significantly different for crossing and bending fiber geometries. Acknowledgement Computational resources were provided by the MetaCentrum under the program LM2010005 and the CERIT-SC under the program Centre CERIT Scientific Cloud, part of the Operational Program Research and Development for Innovations, Reg. No. CZ.1.05/3.2.00/08.0144.

Proton transfer through hydrogen bonds in two-dimensional water layers: A theoretical study based on ab initio and quantum-classical simulations

The dynamics of proton transfer (PT) through hydrogen bonds in a two-dimensional water layer confined between two graphene sheets at room temperature are investigated through ab initio and quantum-classical simulations. The excess proton is found to be mostly solvated as an Eigen cation where the hydronium ion donates three hydrogen bonds to the neighboring water molecules. In the solvation shell of the hydronium ion, the three coordinated water molecules with two donor hydrogen bonds are found to be properly presolvated to accept a proton. Although no hydrogen bond needs to be broken for transfer of a proton to such presolvated water molecules from the hydronium ion, the PT rate is still found to be not as fast as it is for one-dimensional chains. Here, the PT is slowed down as the probability of finding a water with two donor hydrogen bonds in the solvation shell of the hydronium ion is found to be only 25%-30%. The hydroxide ion is found to be solvated mainly as a complex anion where it accepts four H-bonds through its oxygen atom and the hydrogen atom of the hydroxide ion remains free all the time. Here, the presolvation of the hydroxide ion to accept a proton requires that one of its hydrogen bonds is broken and the proton comes from a neighboring water molecule with two acceptor and one donor hydrogen bonds. The coordination number reduction by breaking of a hydrogen bond is a slow process, and also the population of water molecules with two acceptor and one donor hydrogen bonds is only 20%-25% of the total number of water molecules. All these factors together tend to slow down the hydroxide ion migration rate in two-dimensional water layers compared to that in three-dimensional bulk water.

Dynamics of Water in Hierarchical Mesoporous H-ZSM-5 by Fast Field-Cycling NMR Relaxometry

The dynamics of water molecules adsorbed in hierarchical mesoporous H-ZSM-5 was investigated by fast field-cycling NMR relaxometry. Three distinct sites in H-ZSM-5 that can host water molecules, designated a, b, and c, have been identified; no significant exchange of water molecules among these sites was observed. Investigations into the relative populations of molecules confined at sites c (Pc) and b (Pb) revealed that the population ratio (Pc/Pb) does not change significantly over the temperature and water-content ranges investigated. Furthermore, the motion of water at site b is very slow and shows almost no dependence on temperature. While H-ZSM-5 has pores of two different sizes (mesopores and micropores), the low population and observed slow motion of water at site b indicates a strong interaction between water molecules and the zeolite materials, which is possible only when water is adsorbed on the surface. The temperature of the phase transition of water provides insight into the properties of water adsorption. The transition temperature of water confined at site a varies with water content in the sample, indicating that it is located in a larger space; thus, site a represents mesopores. The transition temperature of water and the population of water molecules at site c are independent of water content, indicating that this site has been saturated; therefore, site c is the micropore, which is saturated because of capillary condensation.

The effect of hydration on the thermal behaviour of hydrophilic non-aqueous gels stabilised by Carbopol 974P

A thermal analysis of the effect of hydration of non-aqueous polymer-stabilised gels was investigated using differential scanning calorimetry (DSC). The interaction of water with the polymer and its distribution within the gel are critical to the physicochemical behaviour of the gel, and consequently affects the utility of the gel matrix as a drug delivery vehicle. Addition of water at levels up to and including 50% (w/w) did not result in an observable freezing event in the thermogram. However, at 60 and 80% (w/w) water, phase transitions were observed, the magnitude of which were found to be independent of the annealing time within the range used. The observed melting enthalpies increased as the water concentration increased for all formulations, but were always smaller than that of pure water. There was no evidence of multiple transitions that might be attributed to different populations of water molecules. However, the results demonstrate that DSC can be employed to differentiate between freezable and non-freezable water, in these particular formulations.

Hydration Water and Swelling Behavior of Magadiite. The H+, Na+, K+, Mg2+, and Ca2+ Exchanged Forms

The hydration behavior of synthetic magadiite exchanged with H+, Na+, K+, Mg2+, and Ca2+ was investigated by combining thermal analyses, water adsorption gravimetry, X-ray diffraction, and spectroscopic techniques, including 29Si NMR, infrared, and Raman under various pressures. Thermal analyses reveal distinct water populations, the number of which depends on the valency of the cation. Except for H-magadiite, which does not swell, water adsorption isotherms exhibit two steps, corresponding to two increases of the interlayer distance. These phenomena are shifted toward higher relative pressures in the case of divalent exchanged samples. For Na- and K-magadiite, the adsorption of the first water molecules generates a water molecule in a C1 symmetry, interacting simultaneously with both the cation and the surface. For higher relative pressure, two new populations of water molecules appear: (i) water molecules linked to the cation and (ii) doubly hydrogen-bonded molecules. In the case of Mg2+, the first hydr...

Evaluation of the energy barrier for dehydration of homoionic (Li, Na, Cs, Mg, Ca, Ba, Alx(OH)yz+ and La)-montmorillonite by a differentiation method

AbstractThe thermal energy necessary for the removal of molecular water from Li+-, Na+-, Cs+-, Mg2+-, Ca2+-, Ba2+-, Alx(OH)yz+- and La3+-homoionic montmorillonite powders was determined by thermogravimetric analysis under atmospheric pressure. The weight loss curves and their derivatives exhibit one or several features related to the various populations of water molecules. The activation energy for water removal, which is the sum of the adsorption energy and the activation energy for diffusion, was calculated in each case using a simple first-order differentiation method. The results allow the physisorbed water and the water coordinated to the cations to be clearly separated. For the later population and with the exception of the Na-clay, a good correlation was found between the temperatures and activation energies for water removal and the polarizing power of the cations. Comparison with the results of mechanical tests performed on similar samples suggests that the creep of smectite clays is not controlled by mobility of the individual water molecules but by the mobility of the interlayer cations surrounded by their hydration shell.

Magnetic Resonance Imaging of Water in Flower Buds of Blueberry

Dormant and chilled highbush blueberry (Vaccinium corymbosum L.) flower buds were examined by magnetic resonance imaging (MRI). T2 relaxation times of water molecules were too short to create images from flowers within buds that were dormant and had received no chilling, but they were sufficiently long to create images from buds that had their chilling requirement satisfied. To explain the change in relaxation times, we concluded that water is present in a motionally restricted form in flowers of dormant blueberry buds and in a freer form in flowers of buds after the chilling requirement has been satisfied. T2 values for chilled blueberry buds indicated that one population of water molecules with a detectable T2 time was present in flowers of chilled buds with a relaxation time of ≈8 to 15 ms.

Energetics for preferential solvation of a chromium complex in aqueous organic solvent

Line broadening of NMR signals of solvent components induced by a paramagnetic chromium complex has been measured at varying temperatures to reveal the energetics for preferential solvation of the complex by a component of a mixture of an organic solvent (acetonitrile or acetone) and water. At water mole fractions lower than 0.8, [Cr(SCN)6]3– is preferentially solvated by the organic solvent molecules, mostly owing to lowering of the enthalpy of the organic solvent in the solvation layer compared to that of the bulk solution. However, at water mole fractions higher than 0.8, the population of water molecules in the solvation layer increases markedly because of the increase in entropy. Such thermodynamic properties of the solvation are correlated with the microscopic structure of mixed solvents. In contrast, in an acetonitrile (or aceteone)–benzene mixture, no preferential solvation was observed for Cr(acac)3.