Impact of protein denaturants and stabilizers on water structure

It is of great interest to determine how solutes such as urea, sugars, guanidinium salts, and trimethylamine N-oxide affect the stability, solubility, and solvation of globular proteins. A key hypothesis in this field states that solutes affect protein stability indirectly by making or breaking water structure. We used a new technique, pressure perturbation calorimetry, to measure the temperature dependence of a solute's partial compressibility. Using fundamental thermodynamic relations, we converted these data to the pressure dependence of the partial heat capacity to examine the impact of protein stabilizing and denaturing solutes on water structure by applying the classic two-state mixture model for water. Contrary to widely held expectations, we found no correlation between a solute's impact on water structure and its effect on protein stability. Our results indicate that efforts to explain solute effects should focus on other hypotheses, including those based on preferential interaction and excluded volume.     

Hydration of a synthetic clay with tetrahedral charges: a multidisciplinary experimental and numerical study

The interaction of water with a synthetic saponite clay sample, with a layer charge of 1 per unit cell (0.165 C m(-2)), was investigated by following along water adsorption and desorption in the relative pressure range from 10(-6) to 0.99 (i) the adsorbed amount by gravimetric and near-infrared techniques, (ii) the basal distance and arrangement of water molecules in the interlayer by X-ray and neutron diffraction under controlled water pressure, and (iii) the molecular structure and interaction of adsorbed water molecules by near-infrared (NIR) and Raman spectroscopy under controlled water pressure. The results thus obtained were confronted with Grand Canonical Monte Carlo (GC/MC) simulations. Using such an approach, various well-distinct hydration ranges can be distinguished. In the two first ranges, at low water relative pressure, adsorption occurs on external surfaces only, with no swelling associated. The next range corresponds to the adsorption of water molecules around the interlayer cation without removing it from its position on top of the ditrigonal cavity of the tetrahedral layer and is associated with limited swelling. In the following range, the cation is displaced toward the mid-interlayer region. The interlamellar spacing thus reached, around 12.3 A, corresponds to what is classically referred to as a "one-layer hydrate," whereas no water layer is present in the interlayer region. The next hydration range corresponds to the filling of the interlayer at nearly constant spacing. This leads to the formation of a well-organized network of interlayer water molecules with significant interactions with the clay layer. The structure thus formed leads to a complete extinction of the d001 line in D2O neutron diffraction patterns that are correctly simulated by directly using the molecular configurations derived by GC/MC. The next range (0.50 < P/P0 < 0.80) corresponds to the final swelling of the structure to reach d spacing values of 15.2 A (usually referred to the "two-layer hydrate"). It is associated with the development of a network of liquidlike water molecules more structured than in bulk water. The final hydration range at high relative pressure mainly corresponds to the filling of pores between clay particles.     

Crucial importance of translational entropy of water in pressure denaturation of proteins

We present statistical thermodynamics of pressure denaturation of proteins, in which the three-dimensional integral equation theory is employed. It is applied to a simple model system focusing on the translational entropy of the solvent. The partial molar volume governing the pressure dependence of the structural stability of a protein is expressed for each structure in terms of the excluded volume for the solvent molecules, the solvent-accessible surface area (ASA), and a parameter related to the solvent-density profile formed near the protein surface. It is argued that the entropic effect originating from the translational movement of water molecules plays critical roles in the pressure-induced denaturation. We also show that the exceptionally small size of water molecules among dense liquids in nature is crucial for pressure denaturation. An unfolded structure, which is only moderately less compact than the native structure but has much larger ASA, is shown to turn more stable than the native one at an elevated pressure. The water entropy for the native structure is higher than that for the unfolded structure in the low-pressure region, whereas the opposite is true in the high-pressure region. Such a structure is characterized by the cleft and/or swelling and the water penetration into the interior. In another solvent whose molecular size is 1.5 times larger than that of water, however, the inversion of the stability does not occur any longer. The random coil becomes relatively more destabilized with rising pressure, irrespective of the molecular size of the solvent. These theoretical predictions are in qualitatively good agreement with the experimental observations.     

Structure and decompression melting of a novel, high-pressure nanoconfined 2-D ice

Molecular dynamics (MD) simulations of water confined in nanospaces between layers of talc (system composition Mg(3)Si(4)O(10)(OH)(2) + 2H(2)O) at 300 K and pressures of approximately 0.45 GPa show the presence of a novel 2-D ice structure, and the simulation results at lower pressures provide insight into the mechanisms of its decompression melting. Talc is hydrophobic at ambient pressure and temperature, but weak hydrogen bonding between the talc surface and the water molecules plays an important role in stabilizing the hydrated structure at high pressure. The simulation results suggest that experimentally accessible elevated pressures may cause formation of a wide range of previously unknown water structures in nanoconfinement. In the talc 2-D ice, each water molecule is coordinated by six O(b) atoms of one basal siloxane sheet and three water molecules. The water molecules are arranged in a buckled hexagonal array in the a-b crystallographic plane with two sublayers along [001]. Each H(2)O molecule has four H-bonds, accepting one from the talc OH group and one from another water molecule and donating one to an O(b) and one to another water molecule. In plan view, the molecules are arranged in six-member rings reflecting the substrate talc structure. Decompression melting occurs by migration of water molecules to interstitial sites in the centers of six-member rings and eventual formation of separate empty and water-filled regions.     

Molecular Dynamics Simulation of Methane Hydrate Decomposition in the Presence of Alcohol Additives

The decomposition process of methane hydrate in pure water and methanol aqueous solution was studied by molecular dynamics simulation. The effects of temperature and pressure on hydrate structure and decomposition rate are discussed. The results show that decreasing pressure and increasing temperature can significantly enhance the decomposition rate of hydrate. After adding a small amount of methanol molecules, bubbles with a diameter of about 2 nm are formed, and the methanol molecules are mainly distributed at the gas-liquid interface, which greatly accelerates the decomposition rate and gas-liquid separation efficiency. The radial distribution function and sequence parameter analysis show that the water molecules of the undecomposed hydrate with ordered ice-like configuration at a temperature of 275 K evolve gradually into a long-range disordered liquid structure in the dynamic relaxation process. It was found that at temperatures above 280 K and pressures between 10 atm and 100 atm, the pressure has no significant effect on hydrate decomposition rate, but when the pressure is reduced to 1 atm, the decomposition rate increases sharply. These findings provided a theoretical insight for the industrial exploitation of hydrates.     

Phase changes of CO2 hydrate under high pressure and low temperature

High pressure and low temperature experiments with CO(2) hydrate were performed using diamond anvil cells and a helium-refrigeration cryostat in the pressure and temperature range of 0.2-3.0 GPa and 280-80 K, respectively. In situ x-ray diffractometry revealed that the phase boundary between CO(2) hydrate and water+CO(2) extended below the 280 K reported previously, toward a higher pressure and low temperature region. The results also showed the existence of a new high pressure phase above approximately 0.6 GPa and below 1.0 GPa at which the hydrate decomposed to dry ice and ice VI. In addition, in the lower temperature region of structure I, a small and abrupt lattice expansion was observed at approximately 210 K with decreasing temperature under fixed pressures. The expansion was accompanied by a release of water content from the sI structure as ice Ih, which indicates an increased cage occupancy. A similar lattice expansion was also described in another clathrate, SiO(2) clathrate, under high pressure. Such expansion with increasing cage occupancy might be a common manner to stabilize the clathrate structures under high pressure and low temperature.     

On the mechanism of negative compressibility in layered compounds

A mechanism of negative compressibility occurring in compressed layered compounds in the presence of a pressure transmitting fluid medium is discussed within a simple model. It takes into account the excluded volume effects and soft fluid–matrix repulsion. It is demonstrated that a non-monotonic behavior of the inter-layer spacing with the applied pressure results from a competition between the applied pressure and the internal one induced by the adsorbed fluid. Recent experimental data on the graphite oxide “structure breathing” under compression in the presence of water are analyzed in the light of these theoretical results.     

"Effective" negative surface tension: a property of coated nanoaerosols relevant to the atmosphere

Atmospheric aerosols play a very important role in atmospheric processes and have a major influence on the global climate. In this paper, we report results of a molecular dynamics study on the unique properties of organic-coated water droplets. In particular, we find that, for particles preferring an inverted micelle structure, the lower chain-chain interaction, with increasing radial distance from the water-organic interface, results in a negative internal radial pressure profile for the organic layer. As a result, a coated particle behaves as though the surface tension is "negative" and implies that such a particle will inherently have an inverse Kelvin vapor pressure effect, resulting in increased water condensation.     

Near-infrared spectra of water and aqueous electrolyte solutions at high pressures

The near-infrared spectra (9500 to 11000 cm^–1) of pure water and aqueous solutions of alkali halides, MgCl₂, NaClO₄, and R₄NBr were measured at temperatures between 10 and 55°C and pressures up to 500 MPa. From the analysis of the absorption spectra the following conclusions are drawn. (1) The ice I-like open structure is destroyed and the packed structure is formed as the pressure is increased. (2) The open structure of water is destroyed by the addition of alkali halides and MgCl₂ and water molecules are restricted around the ions by ion-dipole interactions. This results in a loosening of the O–H bond. (3) The perchlorate ion destroys the open structure of water and the ion-dipole interaction with water is insignificant. (4) The Bu₄N⁺ ion forms water structure around the ion similar to that of the clathrate open structure.     

The importance of tetrahedrally coordinated molecules for the explanation of liquid water properties.

Ab initio calculations on molecular clusters and a quantum statistical model are used to probe the structure of liquid water and its anomalies. Characteristic temperature dependent mixtures of ring and three-dimensional, voluminous water clusters provide the famous density maximum. The mixture model also reproduces the shift of the density maximum as a function of pressure and isotopic substitution. This finding is consistent with femtosecond spectroscopy data suggesting that two distinct molecular species exist in liquid water. The given structures also reproduce the oxygen-oxygen pair correlation function and the vibrational IR spectrum of liquid water. The results underline the importance of three-dimensional, tetrahedrally coordinated structures for the understanding of water anomalies and the existence of two liquid phases in the supercooled region.     

Effects of temperature and pressure on the near-infrared spectra of HOD in D2O

The near-infrared absorption spectra (9500 to 11000 cm^–1) of HOD, 20 mol% in D₂O were measured at temperatures between 4 and 55°C and pressures up to 500 MPa. From the analysis of the spectra, the following conclusions are drawn. (1) At temperatures below about 38°C, the ice I-like bulky structure is destroyed to form the dense structure which reflects the high-pressure ice-like structure as the pressure is increased. (2) At temperatures above about 38°C, the bulky structure hardly remains at atmospheric pressure and the formation of dense structure proceeds monotonically with increasing pressure. The results and conclusion obtained in the present paper agrees with those obtained for pure H₂O water in the previous investigation.     

Structure of liquid ethylene glycol

The structure of liquid ethylene glycol (EG) was studied by the vibrational spectroscopy and isothermal compressibility techniques. Raman spectra were recorded at 296 K, IR spectra were measured at 296 and 90 K, and the isothermal compressibility was measured over a pressure range of 0.1–300 MPa. The results obtained were compared with analogous data for water. The structure of liquid EG is discussed using the available literature data on the conformation of its molecule in the gas phase and X-ray diffraction data for crystalline EG. It was concluded that liquid EG has a three-dimensional network of hydrogen bonds, which is more uniform and less mobile compared to water, a feature that explains why the viscosity of EG is high.     

纤维素中空纤维膜气体加湿性能的研究

采用纤维素N甲基吗啉N氧化物(NMMO)水三元纺丝体系,以去离子水为芯液,自来水为凝胶浴,湿法纺制了纤维素中空膜.经自然干燥后该膜的轴向、径向都明显收缩,断面呈现均质致密结构.干膜在水中会明显溶胀,重新润湿后具有气密性.考察了加湿水温、水气压力差等因素对膜的水渗透通量的影响,并初步测试了膜对质子交换膜燃料电池(PEMFC)反应气体H2和O2的加湿性能.实验结果表明该膜透水性能较优,气体加湿效果明显,具有应用于PEMFC反应气体加湿系统的潜力.     

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide)

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.     

Comparison of methods for characterizing microporous membranes for plasma separation

To design membranes suitable for therapeutic use, the relationship between membrane structure and permeability needs to be studied. In this work, the solute permeability of small tubular membranes for plasma separation was determined by using radioisotope-labeled solutes. Through analysis of data on solute and pure water permeability and on water content, by means of a tortuous pore model that we have proposed, we can obtain pore diameter, surface porosity and tortuosity. Membrane structure was also analyzed by mercury intrusion and scanning electron microscopy, and the results were compared with each other. The mercury intrusion method is unsuitable for the structural analysis of polymer membranes because of the damage and/or expansion resulting from highly elevated pressure. The tortuous pore model is recommended for the elucidation of membrane structure.     

Molecular simulation of nanoclusters of gas hydrates in a water shell. The mechanical state of the system

The molecular dynamics method is employed to study hydrates of methane (sI), and krypton hydrate (sII), as well as an ice nanocluster in a supercooled water shell. The main attention is focused on the local structure and the mechanical state of two-phase nanosized systems, which is described using the local pressure tensor. Analysis of the temperature dependence of the local pressure allows one to compare two possible mechanisms responsible for the anomalous stability of gas hydrates at ambient pressure. According to the first mechanism, the water shell plays the role of a barrier that prevents the gas from escaping from the hydrate core. The second mechanism implies that the water shell generates additional pressure, which transfers the hydrate to a thermodynamically stable state. Results of molecular dynamics simulation indicate that both mechanisms are simultaneously involved in the stabilization of the hydrate nanocluster.     

羟基磷灰石界面水行为的分子模拟

本文采用分子动力学模拟(MD)方法研究了羟基磷灰石(HAP)(001)和(100)晶面上的水分子行为，发现HAP晶面间的水是处在高电场和高内压的环境下，并可在晶面处形成2～3层高度结构化的水层，这些水具有有序结构和类冰固化特征。其中在HAP晶体的[001]方向具有较强的极性，相对于[100]方向能诱导产生更多的有序结构化水层。研究发现HAP-水界面处钙和磷酸根位点分布和水分子的吸附位点相关，并且水在HAP界面上的吸附形式具有多样性。该工作揭示了HAP界面结构化水层的形成及其结构细节特征。HAP晶面附近的结构化水层可阻止溶液离子自由出入晶面，对HAP颗粒在水溶液中的动力学稳定性具有重要的影响。     

Phase Diagram of the Xe–H2O System up to 15 kbar

The phase equilibria in the Xe–H2O system have been studied by the DTA technique under hydrostatic pressures up to 15 000 bar in a temperature range from -25 °C to 100 °C. We have shown that the cubic structure I xenon hydrate forming at ambient pressure does not undergo any phase transitions under the conditions studied. The temperature of its decomposition into water solution and gas (fluid) increases from 27 °C at 25 bar to 78.2 °C at 6150 bar. At higher pressures the hydrate decomposes into water solution and solid xenon. In the temperature range from 6800 to 9500 bar the decomposition temperature (79.0–79.5 °C) is practically independent of pressure, while further pressure increase results in a slow decrease to 67 °C at 15 000 bar.     

Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.     

Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.