Bulk Water Molecules Research Articles

Behavior of Water Molecules in PEFCs as Studied by 1H NMR Spectroscopy

Water molecules confined in perfluorinated ionomer membranes which are extensively used for polymer electrolyte fuel cells, presented characteristic 1H NMR spectra reflecting the difference in the morphology. For the hydrated membrane the water molecules at the distinct segments of the polymer undergo rapid exchange to give an apparent single signal. The signal shifts to the higher field with increasing the water content due to the increase of the fraction of the bulk water molecules. On shortage of bulk water the morphological conversion takes place. The membrane of such morphology exhibits a spectrum with separated regions at room temperature regardless of the water content, since the morphology prohibit the rapid exchange among the water molecules localized at the distinct segments. The characteristic spectral changes of water molecules associated with the chemical exchange among the water molecules in the different water domains and the change of the polymer morphology were demonstrated

Simulations of water at the interface with hydrophilic self-assembled monolayers (Review)

Simulations of water at hydrophilic self-assembled monolayer (SAM) surfaces are especially relevant for biological interfaces. Well-defined, atomically smooth surfaces that can be continuously varied are possible with SAMs. These characteristics enable more accurate measurements than many other surfaces with the added advantage of tailoring the surface to treat specific chemical groups. A fundamental question is how solid surfaces affect the structure and dynamics of water. Measurements of the structure and dynamics of water at solid surfaces have improved significantly, but there remain differences among the experiments. In this article, the authors review simulations of water at the interface with hydrophilic SAMs. These simulations find that while the interfacial water molecules are slower than the bulk water molecules, the interfacial dynamics remains that of a liquid. A major biological application of SAMs is for making coatings resistant to protein adsorption. SAMs terminated with ethylene glycol monomers have proven to be excellent at resisting protein adsorption. Understanding the mechanisms behind this resistance remains an unresolved issue. Recent simulations suggest a new perspective of the role of interfacial water and the inseparable interplay between the SAM and the water.

Complex temporal patterns in molecular dynamics: A direct measure of the phase-space exploration by the trajectory at macroscopic time scales

Computer simulated trajectories of bulk water molecules form complex spatiotemporal structures at the picosecond time scale. This intrinsic complexity, which underlies the formation of molecular structures at longer time scales, has been quantified using a measure of statistical complexity. The method estimates the information contained in the molecular trajectory by detecting and quantifying temporal patterns present in the simulated data (velocity time series). Two types of temporal patterns are found. The first, defined by the short-time correlations corresponding to the velocity autocorrelation decay times (< or = 0.1 ps), remains asymptotically stable for time intervals longer than several tens of nanoseconds. The second is caused by previously unknown longer-time correlations (found at longer than the nanoseconds time scales) leading to a value of statistical complexity that slowly increases with time. A direct measure based on the notion of statistical complexity that describes how the trajectory explores the phase space and independent from the particular molecular signal used as the observed time series is introduced.

Calculating Geochemical Reaction Pathways - Exploration of the Inner-Sphere Water Exchange Mechanism in Al(H2O)63+(aq) + nH2O with ab Initio Calculations and Molecular Dynamics

We have simulated exchange of inner-sphere and bulk water molecules for different sizes of Al3+(aq) clusters, Al(H2O)63+ + nH2O for n = 0, 1, 6, or 12, with ab initio and molecular dynamics simulations, in order to understand how robust the ab initio method is for identifying hydrolytic reaction pathways of particular importance to geochemistry. In contrast to many interfacial reactions, this particular elementary reaction is particularly simple and well-constrained by experiment. Nevertheless, we find that a rich array of parallel reaction pathways depend sensitively on the details of the solvation sphere and structure and that larger clusters are not necessarily better. Inner-sphere water exchange in Al3+(aq) may occur through two Langford-Gray dissociative pathways, one in which the incoming and outgoing waters are cis, the other in which they are trans to one another. A large majority of exchanges in the molecular dynamics simulations occurred via the trans mechanism, in contrast to the predictions of the ab initio method. In Al(H2O)63+ + H2O, the cis mechanism has a transition state of 84.3 kJ/mol, which is in good agreement with previous experimental and ab initio results, while the trans mechanism has only a saddle point with two negative frequencies, not a transition state, at 89.7 kJ/mol. In addition to the exchange mechanisms, dissociation pathways could be identified that were considerably lower in energy than experiment and varied considerably between 60 and 100 kJ/mol, depending on the particular geometry and cluster size, with no clear relation between the two. Ab initio calculations using large clusters with full second coordination spheres (n = 12) were unable to find dissociation or exchange transition states because the network of hydrogen bonds in the second coordination sphere was too rigid to accommodate the outgoing inner-sphere water. Our results indicate that caution should surround ab initio simulation of complicated dynamic processes such as hydrolysis, ion exchange, and interfacial reactions that involve several steps. Dynamic methods of simulation need to accompany static methods such as ab initio calculation, and it is best to consider simulated pathways as hypotheses to be tested experimentally rather than definitive properties of the reaction.

Investigation of Hydration Structures of Proteins

Water is a mother liquid of life. To understand why water is indispensable to life, I have been investigating the structures and interactions at protein-water interface, i.e. the hydration structures of proteins, by cryogenic X-ray crystallography and molecular dynamics simulation. Through developing devices and experimental techniques in cryogenic X-ray diffraction experiments, I realized that cryogenic X-ray crystallography is one of techniques effectively observe hydration structures of proteins surface. In addition, the author developed a novel calculation codes to characterize systematically hydration structures of the protein structures determined at around 100 K, as to the amount of water molecules, the hydrogen-bond geometries, the local and the global distribution of them on the surface of proteins. The standard tetrahedral hydrogen bonds of water molecules in bulk water were retained in the hydration structures and contributed to extend three-dimensional network of hydrogen bonds among hydration water molecules and oxygen and nitrogen atoms of protein surface. In cryo-crystallography of multi-subunit or multi-domain proteins, the reorganization of hydration structure occurred with the dynamical motions of proteins. In that, water molecules act as inhibitors for protein motions, glue to stabilize the higher-order structures or lubricant to realize conformational changes by utilizing its tetrahedral arms of hydrogen bonds. Thus, hydration water molecules must have great influences on the dynamics and functions of proteins in aqueous solution.

Thermodynamical properties of reaction intermediates during apoplastocyanin folding in time domain

Two intermediates observed for the folding process of apoplastocyanin (apoPC) were investigated by using a photoinduced triggering system combined with the transient grating and transient lens methods. The thermodynamic quantities, enthalpy, heat capacity, partial volume, and thermal expansion volume changes during the protein folding reaction were measured in time domain for the first time. An interesting observation is the positive enthalpy changes during the folding process. This positive enthalpy change must be compensated by positive entropy changes, which could be originated from the dehydration effect of hydrophobic residues and/or the translational entropy gain of bulk water molecules. Observed negative heat capacity change was explained by the dehydration effect of hydrophilic residues and/or motional confinement of amino acid side chains and water molecules in apoPC. The signs of the volume change and thermal expansion volume were different for two processes and these changes were interpreted in terms of the different relative contributions of the hydration and the dehydration of the hydrophilic residues. These results indicated two-step hydrophobic collapses in the early stage of the apoPC folding, but the nature of the dynamics was different.

Temperature dependence of protein-hydration hydrodynamics by molecular dynamics simulations

The dynamics of water molecules near the protein surface are different from those of bulk water and influence the structure and dynamics of the protein itself. To elucidate the temperature dependence hydration dynamics of water molecules, we present results from the molecular dynamic simulation of the water molecules surrounding two proteins (Carboxypeptidase inhibitor and Ovomucoid) at seven different temperatures ( T = 273 to 303 K, in increments of 5 K). Translational diffusion coefficients of the surface water and bulk water molecules were estimated from 2 ns molecular dynamics simulation trajectories. Temperature dependence of the estimated bulk water diffusion closely reflects the experimental values, while hydration water diffusion is retarded significantly due to the protein. Protein surface induced scaling of translational dynamics of the hydration waters is uniform over the temperature range studied, suggesting the importance protein–water interactions.

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

FIELD EMISSION AND ELECTRICAL PROPERTIES OF IMOGOLITE

We demonstrate field emission (FE) properties of imogolite including turn-on field and life time stability. Imogolite is found to be a good electron emitter under external electric field. The conductance of bound water molecules adsorbed on imogolite surface is measured and compared with bulk water molecules at various pH values, i.e., at various proton concentrations. At low concentration, most protons pass through bound water on imogolite rather than bulk water. At high concentration, on the other hand, protons move more easily through bulk water than bound water. Explanations are given in this paper.

Rates of Oxygen Exchange between the [HxNb6O19]8-x(aq) Lindqvist Ion and Aqueous Solutions

Oxygen-isotope-exchange rates were measured between sites in the Lindqvist-type [H(x)()Nb(6)O(19)](8)(-)(x)()((aq)) polyoxoanion and aqueous solution as a function of pH and temperature. The ion has a central mu(6)-O that is inert to exchange, 12 mu(2)-O(H), and 6 eta-O. The potassium salt of this ion is recrystallized in (17)O-enriched water to (17)O-label the anion, which is then redissolved into isotopically normal water so that the (17)O NMR signals from structural oxygens can be followed as a function of time. Because the central mu(6)-O retains its (17)O signal throughout the experiments, it is clear that the polyoxoanion remains intact during isotopic equilibration of the other structural oxygens. At pH conditions where the [HNb(6)O(19)](7)(-) ion predominates, the mu(2)-O(H) sites isotopically exchange with solution about an order of magnitude more rapidly than the eta-O sites. Yet, we observe that the terminal and bridging oxo sites react at nearly the same rates when the ion is coordinated to 2-3 protons and possibly when it is unprotonated. On the basis of molecular models and experimental kinetic data, we propose metastable polymorphs of the hexaniobate structure where four of the mu(2)-O(H) and eta-O sites are temporarily equivalent and bonded to a coordinatively unsaturated Nb(V). This hypothesized intermediate allows facile access to bulk water molecules for exchange but cannot fully explain the kinetic results and additional experiments on other Lindvist ions are required.

Surface Corrections for Remote Sensing Reflectance in Case 2 Waters of Lake Superior

We establish the average wavelength dependence for rough-surface reflectance S rs(λ) in Lake Superior and determine its magnitude at any individual site by extrapolating the total reflectance or raw remote sensing reflectance R rsT(λ) measured at the surface to the asymptotic limit of zero scattering where S rs(λ) ≅ R rsT(λ). The results show that S rs(λ) differs from the flat-surface Fresnel reflectance of sky radiance used in standard determination of the remote sensing reflectance R rs(λ) attributed to the scattering of light by particles and molecules in bulk water. In waters containing colored dissolved organic matter (CDOM), R rs(λ) can be very low and radiometric measurements at ground level can often lead to negative estimates of R rs(λ) if we assume simple mirror-like reflectance of the sky radiance as the basis for correcting radiometric data for surface reflectance. We examine the differences between Fresnel reflectance and S rs(λ) and estimate the ratio of the concentrations of CDOM versus suspended particles that could produce negative values of R rs(λ) if assume Fresnel reflectance in calculation of R rs(λ).

Dielectric Relaxation Behavior of Aqueous Solutions of Carbobetaines with Varying Intercharge Distances

The dielectric relaxation behavior of aqueous triethylammonioalkanoate (carbobetaine: Et(3)nCB) solutions was examined as a function of frequency from 1.00 x 10(6) to 2.00 x 10(10) Hz (6.28 x 10(6) to 1.26 x 10(11) rad s(-1) in angular frequency); number of intercharge methylene groups, n = 1, 3, 4, 5, and 10; and solute concentration, c. Two major relaxation modes, fast and slow, were found in all solutions examined. Et(3)nCB systems with n = 5 and 10 possessed another, medium, relaxation mode with relaxation time tau(Dh) at high c values (above the contact concentration of solutes) in addition to the fast and slow modes. The fast mode with a relaxation time, tau(w), of approximately 10 ps was attributed to the rotational motion of bulk water molecules. The slow mode with a relaxation time, tau, of 0.08-1 ns, depending on the n value, was attributed to the overall rotational motion of each carbobetaine in aqueous solution. The concentration normalized relaxation strength, Deltaepsilonc(-1), and tau value of the slow relaxation mode increased with increasing n. These findings were quantitatively explained on the basis of changes in the intercharge distance resulting in increased size and dipole moment of the carbobetaines. Above the contact concentration, collisions between solute molecules likely hindered their rotational motions, leading to an increase in tau. The middle relaxation mode found in longer Et(3)nCBs (n = 5 and 10) with a relaxation time, tau(Dh), of approximately 0.2 ns, more than 20 times as long as that of bulk water molecules, tau(w), was attributed to the dehydration of water molecules tightly bound to all Et(3)nCBs examined (including those with n < 5). This mode was not observed in the solutions of Et(3)nCBs with n < 5, since the tau values corresponding to overall rotation were close to or shorter than the tau(Dh) values.

Hydration of Poly(ethylene oxide)s in Aqueous Solution As Studied by Dielectric Relaxation Measurements

The hydration state of poly(ethylene oxide)s (PEOs) in aqueous solutions was investigated using dielectric relaxation measurements at 25 degrees C over a frequency range up to 20 GHz, which is the relaxation frequency of water molecules in a bulk state. The dielectric relaxation spectra obtained indicated decomposition into two major and one minor relaxation modes with relaxation times of 8.3, 22, and 250 ps, respectively. The two major modes were attributed to rotational relaxation of water molecules belonging to the bulk state and water molecules hydrogen bonded to ethylene oxide (EO) monomer units. The number of hydration water molecules per EO unit depended on the molar mass of PEO (M) and reached a constant value of 3.7 at M > 1500, which agrees with the value obtained by other experiments.

Molecular dynamics study of water penetration in staphylococcal nuclease

The ionization properties of Lys and Glu residues buried in the hydrophobic core of staphylococcal nuclease (SN) suggest that the interior of this protein behaves as a highly polarizable medium with an apparent dielectric constant near 10. This has been rationalized previously in terms of localized conformational relaxation concomitant with the ionization of the internal residue, and with contributions by internal water molecules. Paradoxically, the crystal structure of the SN V66E variant shows internal water molecules and the structure of the V66K variant does not. To assess the structural and dynamical character of interior water molecules in SN, a series of 10-ns-long molecular dynamics (MD) simulations was performed with wild-type SN, and with the V66E and V66K variants with Glu66 and Lys66 in the neutral form. Internal water molecules were identified based on their coordination state and characterized in terms of their residence times, average location, dipole moment fluctuations, hydrogen bonding interactions, and interaction energies. The locations of the water molecules that have residence times of several nanoseconds and display small mean-square displacements agree well with the locations of crystallographically observed water molecules. Additional, relatively disordered water molecules that are not observed crystallographically were found in internal hydrophobic locations. All of the interior water molecules that were analyzed in detail displayed a distribution of interaction energies with higher mean value and narrower width than a bulk water molecule. This underscores the importance of protein dynamics for hydration of the protein interior. Further analysis of the MD trajectories revealed that the fluctuations in the protein structure (especially the loop elements) can strongly influence protein hydration by changing the patterns or strengths of hydrogen bonding interactions between water molecules and the protein. To investigate the dynamical response of the protein to burial of charged groups in the protein interior, MD simulations were performed with Glu66 and Lys66 in the charged state. Overall, the MD simulations suggest that a conformational change rather than internal water molecules is the dominant determinant of the high apparent polarizability of the protein interior.

Water behavior in the conversion process of gel-to-subgel phase in dimyristoylphosphatidylethanolamine–water system as studied by DSC

The gel phase of dimyristoylethanolamine (DMPE)–water system was annealed at temperatures of −5 to +5 °C for periods of 2 weeks and was converted into a more stable state of the so-called L-subgel phase. To elucidate the role of water molecules in the conversion of the gel to the subgel phase, the ice-melting behavior for the DMPE–water samples of varying periods of the annealing was investigated by a differential scanning calorimetry (DSC). The ice-melting DSC curves were deconvoluted according to a computer program for multiple Gaussian curve analysis, and the numbers of freezable interlamellar and bulk water molecules were estimated from respective ice-melting enthalpies of the deconvoluted curves. At the annealing periods of 4 days, water molecules amounting to 3.5 H 2O/lipid were observed to be further incorporated into regions between the bilayers, giving rise to the total number of freezable interlamellar water molecules 7 H 2O/lipid which is fairly larger than that (4 H 2O/lipid) estimated for the gel phase. Finally, the conversion of the gel-to-subgel phases completed at the annealing periods of 10 days. The resultant subgel phase showed the amount of nonfreezable interlamellar water larger by 1 H 2O/lipid than that of the nonannealed gel phase. Furthermore, the present study indicated an essential role of the bulk water molecules, which fill an empty space produced at the lipid bilayer surface in the conversion process of the gel-to-subgel phases.

The effects of confinement on the behavior of water molecules between parallel Au plates of (001) planes

Molecular dynamics simulation is utilized to investigate the behavior of water molecules confined between two Au plates of (001) planes separated by gaps of 24.48, 16.32, 12.24, 11.22, and 10.20 A. The simulation results indicate that the arrangements of the water molecules are dependent on the gap size. For the largest gap size, adsorption of the Au surface creates two permanent water layers in the vicinity of each Au plate. Furthermore, in this case, the gap size is sufficiently large to permit the formation of a central region within which the water molecules are randomly oriented in a similar manner to bulk water molecules. The results indicate that the orientation of the first water layer directly absorbed by the plate surface does not change as the gap size between the two Au plates is reduced. However, the orientations of the O-H bonds in the second water layer parallel to the surface rearrange to form hydrogen bonds between the water layers as the separation between the plates is decreased. Finally, an inspection of the variation of the self-diffusion coefficients with the gap size suggests that the difference between the dynamic properties of the water molecules in the z direction and the x-y plane decreases as the distance between the two Au plates increases.

Density functional theory based molecular-dynamics study of aqueous fluoride solvation

We use density functional theory based molecular-dynamics simulations to study the aqueous solvation of the fluoride anion. Our studies are focused on the first solvation shell and have resulted in detailed information on its structural and dynamical properties. The fluoride ion leads to the formation of a rigid solvation shell, qualitatively consistent with simulation and experimental studies, classifying fluoride as a "structure making" particle. However, quantitatively we find the solvation shell to be less structured and more mobile than predicted from empirical force-field simulation. The influence on the intramolecular electronical and structural properties of water is minimal, as observed for other halogens. We propose two distinct mechanisms for the exchange of bulk and first solvation shell water molecules.

Spectroscopic properties of neodymium(III)-containing polyoxometalates in aqueous solution

The spectroscopic properties of the neodymium(III)-containing polyoxometalates (POMs) [Nd(PW 11O 39) 2] 11−, [Nd(PMo 2W 9O 39) 2] 11−, [Nd(PMo 4W 7O 39) 2] 11−, [Nd(PMo 6W 5O 39) 2] 11−, [Nd(SiMo 2W 9O 39) 2] 13−, [Nd(P 2W 17O 61) 2] 17−, [NdW 10O 36] 9−, [NdP 5W 30O 110] 12− and [NdAs 4W 40O 140] 25− are described. Absorption spectra of aqueous solutions of the complexes have been recorded and the transition intensities are parameterised in terms of the Judd–Ofelt intensity parameters Ω λ ( λ = 2, 4, 6). Marked differences were found between the luminescence lifetimes of the complexes of the type Nd(POM) and those of the type Nd(POM) 2, due to a better shielding of the neodymium(III) ions from the bulk water molecules in the latter type of complexes.

Percolation Model for Proton Immobility in Ice

A simple Ising model and a statistical theory of gel formation in a polymer matrix were combined to explain the effect of temperature on the mobility of protons in ice. Cowin et al observed that protons in ice bulk were immobile at temperatures below 190 K, and suddenly become mobile at temperatures above 190 K (Nature 398, 405 (1999)). We proposed here that the transport of protons is controlled by a percolation mechanism. The proton transport is facilitated by the occurrence of easily reoriented water molecules in the ice bulk. The fraction of this molecule depends on temperature and was calculated using a simple Ising model. The formation of network of these molecules which provides pathways for proton transport was calculated using a statistical theory of gel formation in a polymeric system. Our model succeeded to reproduce the variation of potential difference between the ice film surfaces with respect to temperature as observed by Cowin et al based on a soft landing experiment.

The flux of oxygen from the basal surface of gibbsite (α-Al(OH) 3) at equilibrium

Experiments were conducted on gibbsite to determine whether oxygen-isotope exchange rates at hydroxyl bridges (μ 2-OH) on the basal sheet exhibit similar reactivity trends as in large aluminum polyoxocations, for which high-quality kinetic data exist. We followed the exchange of 18O from the mineral surface to solution by using a high-surface-area solid that had been enriched to tens of percent in 18O. To establish this high enrichment, we initially react the solid hydrothermally with highly enriched H 2 18O in order to tag all oxygens near the mineral surface, and then back exchange the most reactive oxygens with isotopically normal water. This enrichment procedure isolates 18O into the least-reactive sites, which are presumably μ 2-OH on the basal surface. By analogy with aqueous aluminum complexes, including large multimers, the η-OH 2 sites exchange within fractions of a second and should be isotopically normal using this procedure. When suspended in isotopically normal electrolyte solutions, we find that the rates of release of 18O from the mineral fall close to the rates of dissolution. The lack of steady isotopic exchange of μ 2-OH on gibbsite surfaces contrasts with the aluminum polyoxocations, where the μ 2-OH exchange many hundreds of times with bulk water molecules before the molecule dissociates. Additional experiments were conducted in solutions at near-neutral pH to determine the flux of oxygens at conditions near thermodynamic equilibrium. As in more acidic solutions, rates are close to values expected from dissolution of the mineral and there is no evidence for steady exchange of hydroxyl bridges with water molecules in the bulk solution.