Surrounding Water Molecules Research Articles

Janus regulation of ice growth by hyperbranched polyglycerols generating dynamic hydrogen bonding

In this study, a new phenomenon describing the Janus effect on ice growth by hyperbranched polyglycerols, which can align the surrounding water molecules, has been identified. Even with an identical polyglycerol, we not only induced to inhibit ice growth and recrystallization, but also to promote the growth rate of ice that is more than twice that of pure water. By investigating the polymer architecture and population, we found that the stark difference in the generation of quasi-structured H2O molecules at the ice/water interface played a crucial role in the outcome of these opposite effects. Inhibition activity was induced when polymers at nearly fixed loci formed steady hydrogen bonding with the ice surface. However, the formation-and-dissociation dynamics of the interfacial hydrogen bonds, originating from and maintained by migrating polymers, resulted in an enhanced quasi-liquid layer that facilitated ice growth. Such ice growth activity is a unique property unseen in natural antifreeze proteins or their mimetic materials.

Identification of Novel Antifreeze Peptides from Takifugu obscurus Skin and Molecular Mechanism in Inhibiting Ice Crystal Growth.

Foodborne hydrolyzed antifreeze peptides have been widely used in the food industry and the biomedical field. However, the components of hydrolyzed peptides are complex and the molecular mechanism remains unclear. This study focused on identification and mechanism analysis of novel antifreeze peptides from Takifugu obscurus skin by traditional methods and computer-assisted techniques. Results showed that three peptides (EGPRAGGAPG, GDAGPSGPAGPTG, and GEAGPAGPAG) possessed cryoprotection via reducing the freezing point and inhibiting ice crystal growth. Molecular docking confirmed that the cryoprotective property was related to peptide structure, especially α-helix, and hydrogen bond sites. Moreover, the antifreeze peptides were double-faces, which controlled ice crystals while affecting the arrangement of surrounding water molecules, thus exhibiting a strong antifreeze activity. This investigation deepens the comprehension of the mechanism of antifreeze peptides at molecular scale, and the novel efficient antifreeze peptides can be developed in antifreeze materials design and applied in food industry.

Detecting the First Hydration Shell Structure around Biomolecules at Interfaces.

Understanding the role of water in biological processes remains a central challenge in the life sciences. Water structures in hydration shells of biomolecules are difficult to study in situ due to overwhelming background from aqueous environments. Biological interfaces introduce additional complexity because biomolecular hydration differs at interfaces compared to bulk solution. Here, we perform experimental and computational studies of chiral sum frequency generation (chiral SFG) spectroscopy to probe chirality transfer from a protein to the surrounding water molecules. This work reveals that chiral SFG probes the first hydration shell around the protein almost exclusively. We explain the selectivity to the first hydration shell in terms of the asymmetry induced by the protein structure and specific protein–water hydrogen-bonding interactions. This work establishes chiral SFG as a powerful technique for studying hydration shell structures around biomolecules at interfaces, presenting new possibilities to address grand research challenges in biology, including the molecular origins of life.

Regulation Mechanism of Trivalent Cations on Friction Coefficient of a Poly(Vinylphosphonic Acid) (PVPA) Superlubricity System

The application range of superlubricity systems can be extended effectively by realizing an adjustable friction coefficient. In this study, a stable poly(vinylphosphonic acid) (PVPA) superlubricity system was developed using sodium chloride (NaCl) solution as the lubricant. A sudden increase in the friction coefficient occurred when a trivalent salt solution was introduced to the base lubricant during the friction process. The changes in surface microstructure and interfacial molecular behavior induced by trivalent cations were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and molecular dynamics simulation. The regulation mechanism of trivalent cations with respect to the friction coefficient of the PVPA superlubricity system was explored. Although La3+ and Fe3+ in solutions exist in two forms (La3+ and Fe(OH)3), both can destroy the stable coating structure through a bridging effect, resulting in a sudden increase in the friction coefficient. The ability of various cations to attract the surrounding water molecules is an important reason for the varying degree of change of the friction coefficient. In addition, the degree of sudden increase in the friction coefficient is dependent on the concentration of trivalent cations. There is an extreme concentration at which the maximum sudden increase degree in friction coefficient can be obtained. This study provides insights into the realization of oil-based superlubricity through interface regulation.

Kinetic study of fulvic acid on methane hydrate formation and decomposition in clay-rich silty sediments

Humic substances and salts are widely distributed in marine sediments, a phenomenon that undoubtedly increases the complexity of recovering methane from hydrate. In this work, kinetic experiments were carried out to determine the role of fulvic acid (FA) on methane hydrate formation and decomposition in marine sediments. We used the mixed clay-rich silty with pore water, containing 3.0 wt% NaCl, to simulate the sediments, with experimental pressure (12.7–14.6 MPa) and temperature (8.6–16.2 °C) conditions adopted from the hydrate reservoir of Shenhu area, South China Sea (SHCS). Results revealed that FA has favorable/unfavorable effects on methane hydrate formation in the sediments, in a concentration-dependent manner. FA solutions, at concentrations of 6.0 and 10.0 wt%, inhibited the formation of methane hydrate, because the high-water holding capacity itself could reduce water activity. Conversely, 2.0 wt% FA solution accelerated methane hydrate formation by arranging surrounding water molecules with H-bonds. Interestingly, it was found that the addition of FA could significantly weaken NaCl's inhibitory effect on methane hydrate formation, while water conversion ratios increased ∼2/3-fold relative to NaCl without FA solution containing systems. These results suggested that presence of FA in marine sediments promoted hydrate reformation during reservoir production. Besides, we found that there is a synergistic effect between FA and NaCl in the process of methane decomposition.

Interfacial layers between ion and water detected by terahertz spectroscopy.

Dynamic fluctuations in the hydrogen-bond network of water occur from femto- to nanosecond timescales and provide insight into the structural/dynamical aspects of water at ion-water interfaces. Employing terahertz spectroscopy assisted with molecular dynamics simulations, we study aqueous chloride solutions of five monovalent cations, namely, Li, Na, K, Rb, and Cs. We show that ions modify the behavior of the surrounding water molecules and form interfacial layers of water around them with physical properties distinct from those of bulk water. Small cations with high charge densities influence the kinetics of water well beyond the first solvation shell. At terahertz frequencies, we observe an emergence of fast relaxation processes of water with their magnitude following the ionic order Cs > Rb > K > Na > Li, revealing an enhanced population density of weakly coordinated water at the ion-water interface. The results shed light on the structure breaking tendency of monovalent cations and provide insight into the properties of ionic solutions at the molecular level.

Complex investigation of H-bond in Water-N-methylacetamide system: Volumetric properties, DFT, IR, MD analysis

• Behavior of the volumetric properties of the binary water-NMA mixture was studied over wide pressure ranges. • Hydrogen bonds in NMA–water mixtures were analyzed using computer chemistry and vibrational spectroscopy. • The analysis of the strength of the hydrogen bond between amides and surrounding water molecules is carried out. • The hydrogen atoms of water and amide compete as hydrogen bond donors. A combined approach of computational chemistry, vibrational spectroscopy experiments, and density measurements was used to study the structural properties of water-N-methylacetamide mixtures over the entire concentration range. The structure of N-methylacetamide dimers and hydrogen-bonded complexes of N-methylacetamide with water molecules was investigated by the quantum chemical method on the basis of the density functional theory. The vibrational spectra of aqueous solutions of N-methylacetamide were recorded. Based on quantum chemical calculations, the bands of the vibrational spectrum were identified, which made it possible to trace the change in hydrogen bonding in the system. At increased N-methylacetamide concentration, the hydrogen bond network rearranges. The number of hydrogen bonds between water molecules decreases, water molecules bind to N-methylacetamide molecules, changing the nature of its environment. Aqueous mixture containing N-methylacetamide shows strong deviations of volumetric properties from ideality. For the further analysis of the data obtained, molecular dynamics simulation of this system was conducted at various temperatures and pressures. It was shown that the resulting hydrogen bonds are of medium strength, but their strength depends on the water molecule environment, which is proved by NBO and QTAIM data. A network of hydrogen bonds is formed in N-methylacetamide aqueous solution, which is noticeably destroyed with an increase in temperature and slightly changes with an increase in pressure.

Boiling heat transfer of nanofluids on the graphene membrane: A molecular dynamics simulation

The molecular dynamics method was used to study the boiling heat transfer of the base fluid (consisting only of water molecules) and nanofluid film on the graphene membrane. The nanofluid consists of water molecules and Cu nanoparticles of three sizes (0.8 nm, 1.2 nm, and 2 nm). Compared with the boiling heat transfer of the base fluid on the graphene membrane at high superheat, the influence of nanofluids with different particle sizes on boiling heat transfer was studied. The effects of the particle size, the water temperature at the radial distance of the particle surface, the molecular mass density distribution, the evaporation rate, and the heat flux on the boiling are analyzed. The heat transfer at the interface between nanoparticles and graphene mainly relies on phonons. The results show that the nanoparticles act as an indirect heat source to heat the surrounding water molecules and strengthen the boiling heat transfer at the solid-liquid interface, and the thermal conductivity at the interface increases with the increase of particle size. The atomic vibration frequencies between the graphene membrane and nanoparticles are well coupled in the low-frequency region, which enhances the boiling heat transfer.

The roles of functional groups of antifreeze protein in inhibition of hydrate growth

The search for green and efficient inhibitors to solve the hydrate blockage problem is a widespread concern in natural gas hydrate exploitation and oil/gas transportation. Tenebrio molitor antifreeze protein (TmAFP) as a biodegradable substance with a high value of thermal hysteresis activity has an excellent inhibition effect on ice growth. Considering that gas hydrates are ice-like solids, we performed molecular dynamics simulations on TmAFP to study its inhibition behavior on methane hydrate growth. Our results show that TmAFP adsorbs on hydrate growth surface by providing several functional groups as “central gas” to be trapped in half hydrate cages where the methyl groups of threonine residues have the highest chance. Moreover, the amide groups of amino acids can also be engaged in half cages. Since these amino acids are uncharged and polar, we infer that they interact with the half-cage with mainly hydrogen bonding. Surprisingly, all adsorption sites were non-ice binding sites and TmAFP can disrupt surrounding water molecules before adsorption, unlike the behavior of previously well-studied fish antifreeze proteins. However, functional groups of the backbone of the ice-binding surface on the β-sheet do not act as the adsorption sites. The reason is that the backbone is not easily deformable and the amino acid group spacing in this part does not match the adjacent hydrate cage spacing. This implies that the structure of the antifreeze protein has a crucial role in hydrate growth inhibition. When seeking efficient antifreeze proteins for hydrate growth inhibition, functional groups as well as their relative positions should be simultaneously considered. These studies guide the development of green and efficient kinetic hydrate growth inhibitors.

Assessment of structural changes in proteins and surrounding water molecules in solution according to SAXS and MD data

Assessment of structural changes in proteins and surrounding water molecules in solution according to SAXS and MD data

The catalytic mechanism of the mitochondrial methylenetetrahydrofolate dehydrogenase/cyclohydrolase (MTHFD2)

Methylenetetrahydrofolate dehydrogenase/cyclohydrolase (MTHFD2) is a new drug target that is expressed in cancer cells but not in normal adult cells, which provides an Achilles heel to selectively kill cancer cells. Despite the availability of crystal structures of MTHFD2 in the inhibitor- and cofactor-bound forms, key information is missing due to technical limitations, including (a) the location of absolutely required Mg2+ ion, and (b) the substrate-bound form of MTHFD2. Using computational modeling and simulations, we propose that two magnesium ions are present at the active site whereby (i) Arg233, Asp225, and two water molecules coordinate , while together with Arg233 stabilize the inorganic phosphate (Pi); (ii) Asp168 and three water molecules coordinate , and further stabilizes Pi by forming a hydrogen bond with two oxygens of Pi; (iii) Arg201 directly coordinates the Pi; and (iv) through three water-mediated interactions, Asp168 contributes to the positioning and stabilization of , and Pi. Our computational study at the empirical valence bond level allowed us also to elucidate the detailed reaction mechanisms. We found that the dehydrogenase activity features a proton-coupled electron transfer with charge redistribution connected to the reorganization of the surrounding water molecules which further facilitates the subsequent cyclohydrolase activity. The cyclohydrolase activity then drives the hydration of the imidazoline ring and the ring opening in a concerted way. Furthermore, we have uncovered that two key residues, Ser197/Arg233, are important factors in determining the cofactor (NADP+/NAD+) preference of the dehydrogenase activity. Our work sheds new light on the structural and kinetic framework of MTHFD2, which will be helpful to design small molecule inhibitors that can be used for cancer treatment.

Structural Evolution of a Polystyrene-Block-Poly(Ethylene Oxide) Block Copolymer in Tetrahydrofuran/Water Cosolvents.

This study aims to quantitatively investigate the effect of water content on the self-assembly behavior of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) in tetrahydrofuran/water cosolvents by small-angle X-ray scattering. PS-b-PEO chains preferentially form fractal aggregates at a dilute concentration in neat tetrahydrofuran (THF). By adding a small amount of water into THF, PS-b-PEO forms gelled networks. The gelled networks have correlated inhomogeneities, which were generated through mesophase separation. These gelled networks are not present when PS-b-PEO is dissolved in THF/methanol and THF/ethanol cosolvents. The substitution of water with 12 M HCl reduces the viscosity of the gelled networks. Those results indicate that the gelled networks of PS-b-PEO need hydrogen bonds formed from surrounding water molecules to be bridging agents, which connect different PEO block chains together. Upon increasing the water content in THF/water cosolvents, dispersed micelles with a core-shell conformation or aggregated micelles preferentially coexist with fractal aggregates.

Enantioselective Organocatalyzed Michael Addition of Isobutyraldehyde to Maleimides in Aqueous Media.

Thiourea was introduced into (R,R)-1,2-diphenylethylenediamine as an organocatalyst to promote the reaction between isobutyraldehydes and maleimides. Enantioselective Michael addition reaction was carried out as an eco-friendly method using water as the solvent. As a result of the reaction between isobutyraldehyde and maleimide, ≥97% yield and 99% enantioselectivity were obtained at a low catalyst loading of 0.01 mol%. The solvent effect can be explained by theoretical calculations that indicate the participation of a transition state, in which the CF3 substituent of the catalyst is a hydrogen bond activated by the surrounding water molecules. This discovery enabled the use of low catalyst loading in the organic reactions of chiral substances for pharmaceutical applications. Furthermore, a solvent effect for Michael reaction of the organocatalysts was proposed, and the organic reaction mechanisms were determined through quantum calculations.

Versatile Macrocyclic Platform for the Complexation of [natY/90Y]Yttrium and Lanthanide Ions.

We report a macrocyclic ligand (H3L6) based on a 3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane platform containing three acetate pendant arms and a benzyl group attached to the fourth nitrogen atom of the macrocycle. The X-ray structures of the YL6 and TbL6 complexes reveal nine coordination of the ligand to the metal ions through the six nitrogen atoms of the macrocycle and three oxygen atoms of the carboxylate pendants. A combination of NMR spectroscopic studies (1H, 13C, and 89Y) and DFT calculations indicated that the structure of the YL6 complex in the solid state is maintained in an aqueous solution. The detailed study of the emission spectra of the EuL6 and TbL6 complexes revealed Ln3+-centered emission with quantum yields of 7.0 and 60%, respectively. Emission lifetime measurements indicate that the ligand offers good protection of the metal ions from surrounding water molecules, preventing the coordination of water molecules. The YL6 complex is remarkably inert with respect to complex dissociation, with a lifetime of 1.7 h in 1 M HCl. On the other hand, complex formation is fast (∼1 min at pH 5.4, 2 × 10–5 M). Studies using the 90Y-nuclide confirmed fast radiolabeling since [90Y]YL6 is nearly quantitatively formed (radiochemical yield (RCY) > 95) in a short time over a broad range of pH values from ca. 2.4 to 9.0. Challenging experiments in the presence of excess ethylenediaminetetraacetic acid (EDTA) and in human serum revealed good stability of the [90Y]YL6 complex. All of these experiments combined suggest the potential application of H3L6 derivatives as Y-based radiopharmaceuticals.

Mechanisms of cation-induced superlubricity transition of poly(vinylphosphonic acid) coatings

Water-based superlubricity systems have unique advantages, such as low friction and environmental friendliness. While control of water-based superlubricity is highly attractive, it is also challenging. The superlubricity transition behavior of poly(vinylphosphonic acid) (PVPA) lubricated using a NaCl solution when mixed with multiple lubricants was investigated in this study. The mechanisms of superlubricity transition were investigated by exploring the internal structural changes of a PVPA coating and the molecular behaviors at the PVPA interface affected by cations through X-ray photoelectron spectroscopy (XPS) depth profiling, quartz crystal microbalance with dissipation (QCM-D), and molecular dynamics simulation. The type and magnitude of the superlubricity transition caused by cations with different valence states are clearly different. The introduced monovalent cation that can stably adsorb on the PVPA surface, exhibits a strong attraction toward the surrounding water molecules, instantaneously strengthening the lubrication at the PVPA interface. These changes at the interface are reversible and would cause a sudden decrease in the coefficient of friction (COF). Adsorbing an introduced divalent cation on the PVPA molecular chain leads to the transformation of the PVPA coating from a stable network structure to an unstable bridging structure. The sudden increase in the COF, caused by the altered internal structure of the coating, is irreversible. This study provides theoretical support for the adaptive control of water-based superlubricity and intelligent lubrication.

Insight into the self-aggregation behavior of lignite and anthracite in water: Atomic-level research using experiments and molecular dynamics simulations

To enable effective treatment of coal slime and prevent its release as an environmental pollutant, the interaction between coal particles in aqueous solutions must be well understood to enhance flocculation and sedimentation of the coal. In this study, the interaction between coal particles was explored using experiments and molecular simulations, which revealed the self-aggregation behavior of lignite and anthracite at different coalification levels in water. Atomic force microscopy results indicated that the anthracite–anthracite adhesion force is greater than that of lignite–lignite, and the adhesion force dominates particle interactions. The surface electrostatic potential range of lignite (-85.57–87.67 kcal/mol) is much higher than that of anthracite (-63.68–59.60 kcal/mol) and the lignite surface provides more polar regions for adsorbing water molecules. Lignite and anthracite rely on π–π accumulation to form a dimer skeleton structure. The dispersion interaction, dominated by π–π interactions, is key to promoting cluster formation and maintaining a stable cluster. The coal molecules proceed through three stages of movement, forming a cluster structure by dissipating the surrounding water molecules. The hydrophobic surface of the cluster molecules is exposed, gradually concealing the hydrophilic groups, which maintains the stability of coal self-aggregates. This process affects the water-molecule distribution, damaging the water bridge structure formed between the water molecules in coal molecule interlayers. This results in a decrease in the average number of water molecules surrounding a single lignite or anthracite molecule, and a change in the water-molecule orientation. Therefore, to enhance the flocculation of coal particles in slime water, it is necessary to increase the hydrophobicity of low-rank coal particles.

Low-Temperature Mixing of Polar Hydrogen Bond-Forming Molecules in Amorphous Solid Water

The level of mixing of guest molecules within low-temperature amorphous solid water (ASW) (as a host) is of interest and relevance to model studies of photochemistry that take place in the interstellar medium (ISM). In this study, we explore how the physical properties of guest molecules, methanol and ammonia, which strongly interact with ASW films, affect the level of mixing and distribution of these molecules throughout the ASW film at adsorption temperatures of 35–100 K, while they are deposited on a Ru(0001) substrate under ultra-high vacuum conditions. Both methanol and ammonia have gas-phase dipole moments similar to that of water, and they are both capable of forming hydrogen bonds with water. The level of mixing and dispersion of these molecules in ASW films is explored through the isothermal noninvasive contact potential difference (ΔCPD) measurements and temperature programmed contact potential difference experiments (TP-ΔCPD). Upon adsorption at 35 K, both molecules form hydrogen bonds with the surrounding water molecules but do not move too far from their initial location in the ASW film. This is confirmed by the observation of an “inverse volcano” process, where guest molecules initially placed within the interaction range of the ruthenium substrate migrate to the substrate upon water crystallization instead of desorbing to the vacuum as expected in a typical “volcano” process. The adsorption temperature at which extensive mixing is achieved is correlated to the glass transition and crystallization temperatures of the guest molecules, which in this case is lower for both guest molecules than that of water. Homogeneous mixing is only achieved when the films are heated above the glass transition temperature of the host (water) molecules.

Contribution of the Induced-Dipole Interaction to Methane Aggregation in Water.

Apolar molecules in the gas phase have no dipole moments. However, when placed in an aqueous environment, they acquire a dipole moment induced by the electric fields of the surrounding water molecules. Could these induced dipole moments, not present in the gas phase but present in solution, play an important role in the hydrophobic interaction between two apolar molecules? In particular, for two methane molecules, our results show that the interaction between the induced-dipole moments only very weakly plays a role in the aggregation of a pair of methane molecules in water. The induced-dipole-induced-dipole interaction has a magnitude as large as 1 kcal/mol for certain mutual orientations of the induced dipole moments, which is larger than the magnitude of the free energy of aggregation of the methane solutes in water. However, when averaged over all physically occurring conformations for a fixed intersolute separation, this interaction averages to an insignificant value (magnitude less than 0.01 kcal/mol) except, possibly, for some very short intermolecular separation.

Hydrolysis of Al3+ in Aqueous Solutions: Experiments and Ab Initio Simulations

An experimental and computational study on the hydrolysis of Al3+ in aqueous solutions is here reported. Speciation model and formation constants were determined by potentiometric titrations at T = 298.15 K, 0.1 ≤ I/mol L−1 ≤ 1 in aqueous NaCl, NaNO3, NaCl/NaNO3 solutions. The dependence of formation constants on ionic strength is reported in all the ionic media over the range of 0.1–1.0 mol L−1. Under the studied experimental conditions, the formation of Al3(OH)45+ and Al13(OH)327+ species is observed in all the investigated ionic media and ionic strengths. The formation constants of the species formed by Al3+ with Cl− were determined together with the dependence on the ionic strength. Moreover, with the aim of unveiling the molecular structure of the formed Al complexes, quantum-mechanical calculations and state-of-the-art ab initio molecular dynamics simulations under explicit solvation were executed. These computations show, for the first time, the highly cooperative role played by the surrounding water molecules in neutralising mononuclear systems–such as AlCl2+ and AlClOH+–and the hydrolytic polynuclear system, Al3(OH)45+.

Energy Redistribution Following CO2 Formation on Cold Amorphous Solid Water.

The formation of molecules in and on amorphous solid water (ASW) as it occurs in interstellar space releases appreciable amounts of energy that need to be dissipated to the environment. Here, energy transfer between CO2 formed within and on the surface of amorphous solid water (ASW) and the surrounding water is studied. Following CO(1Σ+) + O(1D) recombination the average translational and internal energy of the water molecules increases on the ps time scale by 15–25% depending on whether the reaction takes place on the surface or in an internal cavity of ASW. Due to tight coupling between CO2 and the surrounding water molecules the internal energy exhibits a peak at early times which is present for recombination on the surface but absent for the process inside ASW. Energy transfer to the water molecules is characterized by a rapid ps and a considerably slower ns component. Within 50 ps a mostly uniform temperature increase of the ASW across the entire surface is found. The results suggest that energy transfer between a molecule formed on and within ASW is efficient and helps to stabilize the reaction products generated.