Water Molecules Research Articles

A Proton-Conductive Co(II)-Polyoxometalate Acts as a Precatalyst for Efficient Electrocatalytic Water Oxidation.

A cobalt(II)-containing polyoxometalate, [H3O]5[{Co(H2O)4}3{Na(H2O)4}W12O42]·3H2O (Co-POM), has been isolated in a one-step facile aqueous synthesis and characterized unambiguously using single-crystal X-ray crystallography along with routine spectral analysis. The paratungstate cluster anion [W12O42]12- coordinates with {CoII(H2O)4}2+ and {Na(H2O)4}+ complex cations resulting in the formation of the water-insoluble Co-POM compound having three-dimensional (3-D) extended structure. Motivated by the protonated water molecules existing as the counter cations in Co-POM, herein, we demonstrate the detailed proton conductivity studies of the Co-POM, reaching a value of 1.04 × 10-2 S cm-1 at 80 °C and 98% relative humidity (RH). The temperature- and humidity-dependent proton conductivity in Co-POM is governed by Grotthus mechanism with Ea = 0.25 eV. In addition, we examined the electrochemical behavior of Co-POM, in an alkaline borate buffer where it is found to be electrochemically unstable and acts as a precatalyst (and not a true catalyst) for oxygen evolution reaction (OER). We also discuss the "post-mortem" analysis of the postelectrolysis sample to identify the active species which turns out to be a cobalt oxide material (Co3O4) incorporating small amounts of tungsten. Thus, in the present electrocatalysis work, the Co-POM molecule transforms into an efficient water oxidation catalyst (WOC).

Direct Synthesis of Dimethyl Carbonate from Methanol and CO2 over ZrO2 Catalysts Combined with a Dehydrating Agent and a Cocatalyst

Zirconia nanocrystals as catalysts for the direct synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide have received significant interest recently. In this paper, three zirconia-based catalysts presenting different monoclinic and tetragonal phase contents are prepared and characterized by X-ray diffraction (XRD), N2 adsorption–desorption, transmission electron microscopy (TEM), and temperature-programmed desorption of NH3 and CO2 (NH3-TPD and CO2-TPD). The catalytic performances of these solids are evaluated in terms of DMC production. This production is low when using the bare zirconias, but it is significantly increased in the presence of 1,1,1-trimethoxymethane (TMM) playing the role of a dehydrating agent, which shifts the thermodynamic equilibrium. Moreover, the production of DMC is further improved by adding a second solid catalyst (cocatalyst), the molecular sieve 13X, to accelerate the hydration of TMM. Hence, the molecular sieve 13X plays a dual role by trapping water molecules formed by the reaction of DMC synthesis and providing strong acidic sites catalyzing TMM hydrolysis. To the best of our knowledge, the combination of two solid catalysts in the reaction medium to accelerate the water elimination to obtain higher DMC production from CO2 and methanol has never been reported.

Micrometer-Scale Graphene-Based Liquid Cells of Highly Concentrated Salt Solutions for In Situ Liquid-Cell Transmission Electron Microscopy.

In situ liquid-cell transmission microscopy has attracted much attention as a method for the direct observations of the dynamics of soft matter. A graphene liquid cell (GLC) has previously been investigated as an alternative to a conventional SiN x liquid cell. Although GLCs are capable of scavenging radicals and providing high spatial resolutions, their production is fundamentally stochastic, and a significant compositional change in liquids encapsulated in GLCs has recently been pointed out. We found that graphene-based liquid cells were formed in nano- to micrometer sizes with high reproducibility when the concentration of the encapsulated aqueous salt solution was high. In contrast, when we revisited conventional fabrication methods, water-encapsulated GLC was formed with low yield, and any electron diffraction spots from ice were not confirmed by a cooling experiment. The reason for this was the presence of intrinsic defects in the graphene, the presence of which we confirmed by the etch-pit method. The shrinkage of a water-encapsulated cell and a decrease in the bubble area in an aqueous (NH4)2SO4 solution cell suggested that volatile water molecules and gas molecules can leak from the cells during the fabrication and observation processes. Further revision of the conditions for the formation of liquid cells and a reduction in the number of intrinsic graphene defects are expected to lead to the provision of graphene-based liquid cells capable of encapsulating dilute aqueous solutions or pure water.

8-Hy-droxy-quinolinium tri-chlorido-(pyridine-2,6-di-carb-oxy-lic acid-κ3 O,N,O')copper(II) dihydrate.

The title compound, (C9H8NO)[CuCl3(C7H5NO4)]·2H2O, was prepared by reacting CuII acetate dihydrate, solid 8-hy-droxy-quinoline (8-HQ), and solid pyridine-2,6-di-carb-oxy-lic acid (H2pydc), in a 1:1:1 molar ratio, in an aqueous solution of dilute hydro-chloric acid. The CuII atom exhibits a distorted CuO2NCl3 octa-hedral geometry, coordinating two oxygen atoms and one nitro-gen atom from the tridentate H2pydc ligand and three chloride atoms; the nitro-gen atom and one chloride atom occupy the axial positions with Cu-N and Cu-Cl bond lengths of 2.011 (2) Å and 2.2067 (9) Å, respectively. In the equatorial plane, the oxygen and chloride atoms are arranged in a cis configuration, with Cu-O bond lengths of 2.366 (2) and 2.424 (2) Å, and Cu-Cl bond lengths of 2.4190 (10) and 2.3688 (11) Å. The asymmetric unit contains 8-HQ+ as a counter-ion and two uncoordinated water mol-ecules. The crystal structure features strong O-H⋯O and O-H⋯Cl hydrogen bonds as well as weak inter-actions including C-H⋯O, C-H⋯Cl, Cu-Cl⋯π, and π-π, which result in a three-dimensional network. A Hirshfeld surface analysis indicates that the most important contributions to the crystal packing involving the main residues are from H⋯Cl/Cl⋯H inter-actions, contributing 40.3% for the anion. Weak H⋯H contacts contribute 13.2% for the cation and 28.6% for the anion.

Predicting pKa Values of Para-Substituted Aniline Radical Cations vs. Stable Anilinium Ions in Aqueous Media

The focus of pKa calculations has primarily been on stable molecules, with limited studies comparing radical cations and stable cations. In this study, we comprehensively investigate models with implicit solvent and explicit water molecules, direct and indirect calculation approaches, as well as methods for calculating free energy, solvation energy, and quasi-harmonic oscillator approximation for para-substituted aniline radical cations (R-PhNH2•+) and anilinium cations (R-PhNH3+) in the aqueous phase. Properly including and positioning explicit H2O molecules in the models is important for reliable pKa predictions. For R-PhNH2•+, precise pKa values were obtained using models with one or two explicit H2O molecules, resulting in a root mean square error (RMSE) of 0.563 and 0.384, respectively, for both the CBS-QB3 and M062X(D3)/ma-def2QZVP methods. Further improvement was achieved by adding H2O near oxygen-containing substituents, leading to the lowest RMSE of 0.310. Predicting pKa values for R-PhNH3+ was more challenging. CBS-QB3 provided an RMSE of 0.349 and the M062X(D3)/ma-def2QZVP method failed to calculate pKa accurately (RMSE > 1). However, by adopting the double-hybrid functional method and adding H2O near the R substituent group, the calculations were significantly improved with an average absolute difference (ΔpKa) of 0.357 between the calculated and experimental pKa values. Our study offers efficient and reliable methods for pKa calculations of R-PhNH2•+ (especially) and R-PhNH3+ based on currently mature quantum chemistry software.

Synthesis, structural characterization, and theoretical analysis of novel zinc(ii) schiff base complexes with halogen and hydrogen bonding interactions.

In this article, we present the synthesis and characterization of three zinc(ii) complexes, [ZnII(HL1)2] (1), [ZnII(HL2)2]·2H2O (2) and [ZnII(HL3)2] (3), with three tridentate Schiff base ligands, H2L1, H2L2, and H2L3. The structures of the complexes were confirmed by single-crystal X-ray diffraction analysis. DFT calculations were performed to gain insights into the self-assembly of the complexes in their solid-state structures. Complex 1 exhibits dual halogen-bonding interactions (Br⋯Br and Br⋯O) in its solid-state structure, which have been thoroughly investigated through molecular electrostatic potential (MEP) surface calculations, alongside QTAIM and NCIPlot analyses. Furthermore, complex 2 features a fascinating hydrogen-bonding network involving lattice water molecules, which serves to link the [ZnII(HL2)2] units into a one-dimensional supramolecular polymer. This network has been meticulously examined using QTAIM and NCIplot analyses, allowing for an estimation of the hydrogen bond strengths. The significance of H-bonds and CH⋯π interactions in complex 3 was investigated, as these interactions are crucial for the formation of infinite 1D chains in the solid state.

Multifunctional Water Additive Enabling Stable Cycling of Chloride‐Free Magnesium Metal Batteries Based on Carbonate Electrolyte

AbstractRechargeable magnesium batteries (RMBs) face with the challenge of interphase passivation between electrolytes and Mg anodes. Compared with ether electrolytes, carbonate solvents possess the superior electrochemical stability at cathode side, but their incompatibility with Mg metal, high viscosity, and desolvation energy barrier restrict their practical utilization in RMBs. Herein, the “unwanted‐impurity” water with high concentration is revisited and employed as multifunctional additive in carbonate electrolyte to improve the reversibility of RMBs. Water additive enables the localized deep eutectic effect, reduces the viscosity of carbonate electrolyte, and improves the Mg ion conductivity. The water molecules also participate the solvation sheath of Mg ions, resulting in the reduction of Mg deposition overpotential and inhibition of parasitic reaction. Furthermore, the co‐intercalated water molecules in V2O5 cathode layers enable the stabilization of intercalation structure and supply of additional magnesiophilic sites. Cooperated with the binder‐decorated Mg powder anode, the propylene carbonate electrolyte with water additive endows Mg||Mg symmetric cells and Mg||V2O5 full cells with satisfactory cycling performance and high‐voltage stability. This work revisits the impact of impurity water and provides a practical strategy for the utilization of conventional low‐cost carbonate electrolyte family, broadening the design and formulation of electrolytes for chlorine‐free and high‐voltage RMBs.

Operando Decoding Ion-Conductive Switch in Stimuli-Responsive Hydrogel by Nanodiamond-Based Quantum Sensing.

Thermal-responsive hydrogels are developed as ion-conductive switchs for energy storage devices, however, the molecule mechanism of switch on/off remains unclear. Here, poly(N-isopropylacrylamide-co-acrylamide) hydrogel is synthesized as a model material and nanodiamond (ND) based quantum sensing for phase change study is developed. First, micro-scale phase separation with cross-linked mesh structure after sol-gel transition is visualized in situ and water molecules are trapped by polymer chains and on a chemically "frozen" state. Then, the nano-scale inhomogeneous distributions of viscosity, thermal conductivity and ionic mobility in hydrogel at high temperature are observed by measuring the rotation, translation and zero-field splitting of NDs. Besides, the ionic mobility of hydrogel is found to be dependent not only on temperature but also on polymer concentration. These observations suggested that the physical "wall" induced by inhomogeneous phase separation at microscopic scale blocked the ion conduction pathways, providing a potential intrinsic explanation for ion migration shut-down of ionic hydrogels at high temperature.

Constructing Unsaturated Ru Atom Arrays Confined in Mn Oxides to Boost Neutral Water Reduction

Recently, Ru single atoms supported on carbon nanomaterials have demonstrated ultrahigh activity for acid hydrogen evolution reaction (HER), however their neutral HER activity remains low due to the sluggish kinetics for both the water dissociation step to generate H* intermediates and subsequent H* recombination in neutral electrolytes. Here, we synthesize ordered low‐coordinated Ru atom arrays confined in Mn oxides (i.e., Li4Mn5O12) for concurrently boosting the water dissociation and H* recombination, thus achieving a 6‐fold HER activity enhancement than commercial Pt/C in neutral media. Control experiments indicate that low‐coordinated Ru atoms with strong affinity to oxygen atoms of water molecules facilitate the water dissociation to rapidly generate H*. More importantly, both electrochemical and theoretic results uncover that the array‐like structure allows the activation of two water molecules on two adjacent Ru atoms for enabling direct H*–H* recombination via the Tafel step, while isolated Ru atoms can only activate water one by one for recombining H* via the sluggish Heyrovsky step. Clearly, this work paves new avenues to boosting the electrocatalytic activity by constructing ordered metal atoms assembles with controllable coordination environments.

Quantum Chemical Molecular Dynamics Simulations for Methane‐Water Cages

ABSTRACTThe dynamic stability of various methane‐water clathrate‐like cages (CH4@(H2O)n, n = 16, 18, 20, 22), has been analyzed explicitly considering thermal effects by means of ab initio M06‐2X/6–31+G*/PCM calculations, which make use of Gaussian basis functions. Starting from the equilibrium filled cage structures, classical, dynamic reaction coordinate (DRC) on the Born–Oppenheimer surface, and semiclassical, Born–Oppenheimer plus harmonic zero‐point energy surface (BOMD), molecular dynamics have been carried out. Water molecules have a high tendency to orient covalent O–H bonds tangentially to the hydrophobic surface, thus clathrate‐like arrangements are an acceptable model to fully hydrate methane. If the cage size is such as to minimize core repulsion, due to electron cloud overlap, and to maximize host–guest van der Waals attractions, the clathrate‐like structures have a life‐time of two picoseconds in classical DRC simulations. The inclusion of quantum kinetic energy in BOMD simulations results in less structured cages with a reduced amount of hydrogen bond network. The preferential tangential orientation of the O‐H bonds is largely maintained, although few of them point toward the methane for a very short time in BOMD simulations. The reduced configurational space of water molecules hydrating hydrophobic moiety is highlighted, thus any satisfactory molecular modeling has to account for it.

Enhanced Hydrogen Bonding Through Strong Water‐Locking Additives for Long‐Term Cycling of Zinc Ion Batteries

AbstractThe promising energy storage devices, zinc ion batteries (ZIBs), face challenges such as dendritic growth and side reactions, which hinder their application and development. As a polar group, hydroxyl groups can utilize hydrogen bonding to stably anchor water molecules, preventing contact between water and the anode. Moreover, they can attract and guide Zn2+ to rapidly and uniformly deposit on the anode. Here, the introduction of multi‐hydroxyl water‐locking additive Lactobionic acid (LA) molecules is proposed into conventional electrolytes. Through an in situ reaction between the highly reactive carboxyl groups on LA molecules and the zinc anode, a stable multi‐hydroxyl protective layer is formed on the anode surface, effectively preventing interface corrosion and dendritic growth. As a result, the Zn||Zn symmetric cell with LA exhibits remarkable performance, cycling for 2300 h under 1 mA cm−2 and 1 mAh cm−2. Even under more rigorous conditions of 10 mA cm−2 and 10 mAh cm−2, it maintains over 800 h of cycling durability. Moreover, in the Zn||NH4V4O10 full cell configuration, an impressive capacity retention rate of 80.35% after 2000 cycles at a current density of 5 A g−1. This innovative method can open a new avenue for designing high‐performance ZIBs.

Defect Engineering in a Nanoporous Thulium-Organic Framework in Catalyzing Knoevenagel Condensation and Chemical CO2 Fixation.

Defect engineering is an extremely effective strategy for modifying metal-organic frameworks (MOFs), which can break through the application limitations of traditional MOFs and enhance their functionality. Herein, we report a highly robust nanoporous thulium(III)-organic framework, {[Tm2(BDCP)(H2O)5](NO3)·3DMF·2H2O}n (NUC-105), with [Tm(COO)2(H2O)]n chains and [Tm2(COO)4(H2O)8] dinuclears as metal nodes and 2,6-bis(2,4-dicarboxylphenyl)-4-(4-carboxylphenyl)pyridine (BDCP) linkers. In NUC-105, each of the four chains of [Tm(COO)2]n and the two rows of [Tm2(COO)4(H2O)8] units is unified by the organic skeleton, resulting in a rectangular nanochannel with dimensions of 15.35 Å × 11.29 Å, which leads to a void volume of 50%. It is worth mentioning that the [Tm2(COO)4(H2O)8] cluster is very rare in terms of its higher level of associated water molecules, implying that the activated host framework can serve as a strong Lewis acid. NUC-105a exhibited great heterogeneous catalytic performance for CO2 cycloaddition with epoxides under the reaction conditions (0.60 mol % NUC-105a, 5.0 mol % n-Bu4NBr, 65 °C, 5 h), ensuring exclusive selectivity and high conversion rates. In addition, NUC-105a's strong catalytic impact on the Knoevenagel condensation of aldehydes and malononitrile can be attributed to the collaboration between the drastically unsaturated Lewis acidic Tm3+ centers and Lewis basic pyridine groups.

Biomimetic Layered Hydrogel Coating for Enhanced Lubrication and Load-Bearing Capacity

Biomimetic hydrogel lubrication coatings with high wettability and low friction show great promise in tissue engineering, wound dressing, drug delivery, and intelligent sensing. Inspired by the hierarchical structure of natural cartilage, a layered hydrogel coating was constructed to functionalize rigid polyetheretherketone (PEEK). The layered hydrogel coating features a structural design comprising a top soft layer and a middle robust layer. The porous structure of the top soft hydrogel layer stores water molecules, providing surface lubrication, while the dense structure of the middle robust hydrogel layer offers load-bearing capacity. These synergistic effects of the gradient hydrogel layer endow the PEEK substrate with an ultra-low coefficient of friction (COF~0.010 at 5 N load), good load-bearing capacity (COF~0.031 at 10 N load), and excellent wear resistance (COF < 0.05 at 5 N load after 20,000 sliding cycles). This study introduces a novel design paradigm for robust hydrogel coatings with exceptional lubricity, displaying the potential application in cartilage replacement materials.

Synthesis and characterization of rhenium and Technetium-99 m tricarbonyl and dicarbonyl complexes with quinaldic acid and Arsenic/Phosphorus derivatives

The synthesis and characterization of neutral tricarbonyl fac-[Re/99mTc(quin)(X)(CO)3] and dicarbonyl cis–trans-[Re/99mTc(quin)(X)2(CO)2] mixed ligand complexes with quinaldic acid (quin) as the bidentate ligand and trimethoxyphosphine [P(OCH3)3], tris(hydroxymethyl)-phosphine [P(CH2OH)3], triphenylphosphine (PPh3), and triphenylarsine (AsPh3) as the monodentate ligands (X) is described. The synthesis of the [2 + 1] tricarbonyl complexes proceeds by displacement of the water molecule of the fac-[Re/99mTc(quin)(H2O)(CO)3] by the monodentate ligand. Interestingly, the synthesis of the [2 + 1 + 1] dicarbonyl complexes was achieved only for PPh3 after replacing the CO group trans to PPh3 with a second PPh3 molecule. The latter complex was also obtained by refluxing quinaldic acid with the trans-mer-[Re(PPh3)2(Cl)(CO)3] precursor in toluene. Rhenium complexes were prepared in satisfactory yields and fully characterized. At the technetium-99 m level, complexes were produced in high radiochemical yields and characterized by comparative chromatographic analysis using the analogous rhenium complexes. Complexes have shown varying stability against transchelation by cysteine and histidine in agreement with the decreasing σ-donating capacity of the monodentate ligand (P(OCH3)3 > P(CH2OH)3 > PPh3 > AsPh3. The stable complexes with PPh3 showed high lipophilicity (LogP 2.90 and 3.10, respectively) due to the lipophilic nature of the monodentate ligands. The σ-donor and π-acceptor capacity of the monodentate ligand strongly influences the formation and stability of the complexes, and these characteristics should be considered before choosing the appropriate ligand for radiopharmaceuticals design.

Density functional theory (DFT) study of water autoionization in solvated clusters.

We have implemented a cluster-continuum method using density functional theory to model water clusters and various charged species derived from water. The two aims of this study are to determine the minimal basis required for proper modeling of water autoionization and to determine the minimum number of explicit water molecules required to properly model the energetics of solvation. The thermodynamics of water autoionization converge following a modified power law to deliver chemically accurate values of the Gibbs energy change for autoionization with tractably small clusters. Convergence is slower and not exponential as assumed in previous work. We identify the n = 21 set of clusters as the first effectively bulk water like clusters that can capture the energetic influence of both the first and second solvation shells. In this cluster, a water molecule is encapsulated in the center of a closed shell of other water molecules that hydrogen bond to form five-membered rings. The total energy change for clusters with n ≥ 21 calculated using both the RPBE-D3 and ωB97X-D exchange-correlation functionals with the 6-311+G** basis set is shown to deliver good approximations to the free energy change at 298K. This is true even though neither functional models the individual enthalpy or entropy contributions particularly well.

Insight into the complexation of uranium with Bacillus sp. dwc-2 by multi-spectroscopic approaches: FT-IR, TRLF and XAFS spectroscopies

The micromechanisms of the interactions between uranium and microorganisms, especially chemical bonds and bonding numbers between uranium and microbe are not yet clear. In this work, uranium coordination with functional groups in/on Bacillus sp. dwc-2 has been rationally explored by X-ray absorption fine structure (XAFS), Fourier transform infrared (FT-IR), time-resolved laser-induced fluorescence (TRLF) spectroscopies. The results indicate that under acidic condition, 66.8 % of O atoms in coordination shell of U(VI) equatorial plane are provided by organic phosphoryl groups via monodentate coordination, 18.8 % and 15.5 % are provided by water molecules and carboxyl groups via monodentate and bidentate coordination, respectively. While under alkaline condition, these proportions change to 53.1 %, 34.4 % and 22.0 %, respectively. Under neutral condition, U(VI) is mainly deposited by complexing with inorganic phosphates released by microbial cells. Under anaerobic condition, partial U(VI) can be reduced to U(IV) which primarily complex with phosphorus containing groups via monodentate and bidentate coordination, existing as amorphous solids or coordination polymers. This is the first exploration of the binding mechanism between uranium and microorganisms at a semi-quantitative level under different conditions. Our studies will further enhance the contribute to the understanding of coordination mechanism of uranium with microorganism, and demonstrate the potential bioremediation value of this bacterium.

Zeolites as Solid Solvents: Explaining the Chloromethane Hydrolysis over Metal-Exchanged Zeolite Y by DFT Calculations.

We report a theoretical DFT study of the reaction pathways for chloromethane hydrolysis over metal-exchanged zeolites (Li+, Na+, K+, and Mg2+). A cluster of 78 T atoms (T referring to Si or Al atoms), comprising the zeolite Y super cavity coupled with the sodalite cage and three hexagonal prism units (Si(78-x)Al x O129H54; x = 1 or 2), was used in the calculations. The study was carried out using the ONIOM method. The high layer was computed at M062X/6-31++G(d,p), whereas the low layer was computed at the PM6 level of theory. The energy profile was obtained by single-point calculations at the M062X/6-31++G(d,p) for the entire structure. The first step studied was the adsorption of chloromethane on the zeolite, which involves an ion-dipole interaction between the metal cation and the chlorine atom. Then, two mechanistic pathways were investigated: one via formation of an adsorbed methoxide and the other involving a direct, one-step, nucleophilic attack of the water molecule to the adsorbed chloromethane. In all systems studied, the direct mechanism showed a lower energy of activation than the route involving the methoxide intermediate. The same behavior was also observed for chloromethane methanolysis to afford dimethyl ether on the metal-exchanged zeolites. The theoretical results are in agreement with the experiments of chloromethane hydrolysis over metal-exchanged zeolite Y, explaining the formation of dimethyl ether upon chloromethane methanolysis on zeolites of low or no acidity. The transition state for the direct route resembles a SN2 process assisted by the surface, reinforcing the concept of zeolites as solid solvents, providing a polar nanoenvironment that favors and assists the formation of ionic transition states and intermediates.

Glycine as a stabilizing osmolyte for Renilla luciferase: A kinetic and molecular dynamics analysis

Renilla luciferase, a luminescent enzyme, is utilized in gene expression analysis and biosensor technology, and extensive research has been conducted on its structure and function. Glycine is an osmolyte that plays a key role in protein stabilization against denaturation. However, its impact on enzyme properties is unpredictable. This study aimed to investigate the effect of glycine on the kinetics and stability of Renilla luciferase. The data revealed that glycine at a concentration range of 0.1 to 0.6 M improved enzyme kinetics and thermal stability. The highest catalytic efficiency was observed at a concentration of 0.5 M. Molecular dynamics simulations demonstrated that in the presence of 0.5 M glycine, substrate access to the enzyme’s active site was enhanced while the root-mean-square fluctuation (RMSF) of the protein backbone was reduced. Additionally, the analysis of protein-water hydrogen bonding interactions showed an increase in the hydrogen bonding between water molecules and Renilla luciferase. The present study may be used for the formulation of Renilla luciferase for commercial purposes.

Hydronium Ion Transport across the Liquid/Liquid Interface Assisted by a Phase-Transfer Catalyst: Structure and Thermodynamics Using Molecular Dynamics Simulation.

Molecular dynamics simulations are used to examine the thermodynamic and structural aspects of the transfer of the classical hydronium ion (H3O+) across a water/1,2-dichloroethane (DCE) interface assisted by the phase-transfer catalyst (PTC) tetrakis(pentafluorophenyl) borate anion (TPFB-). The free energy of transfer from water to DCE of the H3O+-TPFB- ion pair is calculated to be 6 ± 1 kcal/mol, significantly less than that of the free hydronium ion (17 ± 1 kcal/mol). The ion pair is relatively stable at the interface and in the organic phase when it is accompanied by three water molecules with a small barrier to dissociation that supports its utility as a PTC. An examination of the hydration structure that accompanies the transfer of the ion pair shows that the ion pair, like the free hydronium ion, is transferred with the assistance of a finger-like water structure.

The Membrane Dipole Potential and the Roles of Interfacial Water and Lipid Hydrocarbon Chains.

Understanding membrane charge transport processes, including the actions of ion channels, pumps, carriers, and membrane-active peptides, requires a description of the electrostatics of the lipid bilayer. We have simulated a library of different lipid chemistries to reveal the impact of the headgroup, glycerol backbone, and hydrocarbon chains on the membrane dipole potential. We found a strong dependence of the potential on lipid packing, but this was not caused by the packing of lipid polar components, due to cancellation of their electric fields by electrolyte. In contrast, lipid tail contributions were determined by area per lipid, arising from two countering effects. Increased area per lipid leads to chain tilting that increases methylene dipole projections to strengthen the electric field within the bilayer, while at the same time decreasing the electric field from terminal methyl groups. Moreover, electric fields from some nonterminal groups and the terminal methyl group can extend beyond the bilayer center and be canceled by the opposing leaflet. This interleaflet field annulment explains the experimental reduction in dipole potential for unsaturated and branched lipid bilayers, by as much as ∼200 mV, as well as experiments that substitute chain carbons with sulfur. Replacing ester with ether groups (eliminating two carbonyl groups) causes a significant reduction in potential, also by ∼200 mV, in agreement with experiment. We show that the effect can be largely attributed to the loss of aligned water molecules in the glycerol backbone region, lowering the potential inside the bilayer core. When only one of the two carbonyls is removed (using a hybrid ester-ether lipid or a single-chain lipid), most of this reduction in potential was lost, with the single carbonyl group able to maintain full hydration in the interfacial region. While headgroup chemistry can have a major effect (by as much as ±100 mV relative to phosphatidylcholine), anionic headgroups either decrease or increase the dipole potential, with the variation involving perturbation in hydrogen-bonded water molecules and changes in packing of lipid tails. Overall, these results suggest that membrane electrostatics are dominated by aligned water molecules at the polar-hydrocarbon interface and, surprisingly, by the charge distribution of the nonpolar lipid tails, and not the packing of headgroup and glycerol carbonyl dipoles.