Hydrophobic effects and modeling of biophysical aqueous solution interfaces

An Integral Equation Study of the Hydrophobic Interaction between Graphene Plates

The hydrophobic association of two parallel graphene sheets is studied using the 3D-RISM HNC integral equations with several theoretical methods for the solvent distribution functions. The potential of mean force is calculated to study the effects of the aqueous solvent models and methods on the plates as a function of distance. The results of several integral equations (IE) are compared to MD simulations for the same model. The 3D-IEs are able to qualitatively reproduce the nature of the solvent effects on the potential of mean force but not quantitatively. The local minima in the potential of mean force occur at distances allowing well defined layers of solvent between the plates but are not coincident with those found in simulation of the same potential regardless of the theoretical methods tested here. The dewetting or drying transition between the plates is generally incorrectly dependent on steric effects with these methods even for very hydrophobic systems without solute-solvent attractions, in contradiction with simulation.

High-definition self-assemblies driven by the hydrophobic effect: synthesis and properties of a supramolecular nanocapsule

High definition self-assemblies, those that possess order at the molecular level, are most commonly made from subunits possessing metals and metal coordination sites, or groups capable of partaking in hydrogen bonding. In other words, enthalpy is the driving force behind the free energy of assembly. The hydrophobic effect engenders the possibility of (nominally) relying not on enthalpy but entropy to drive assembly. Towards this idea, we describe how template molecules can trigger the dimerization of a cavitand in aqueous solution, and in doing so are encapsulated within the resulting capsule. Although not held together by (enthalpically) strong and directional non-covalent forces, these capsules possess considerable thermodynamic and kinetic stability. As a result, they display unusual and even unique properties. We discuss some of these, including the use of the capsule as a nanoscale reaction chamber and how they can bring about the separation of hydrocarbon gases.

Structure and depletion at fluorocarbon and hydrocarbon/water liquid/liquid interfaces

The results of x-ray reflectivity studies of two oil/water (liquid/liquid) interfaces are inconsistent with recent predictions of the presence of a vaporlike depletion region at hydrophobic/aqueous interfaces. One of the oils, perfluorohexane, is a fluorocarbon whose superhydrophobic interface with water provides a stringent test for the presence of a depletion layer. The other oil, heptane, is a hydrocarbon and, therefore, is more relevant to the study of biomolecular hydrophobicity. These results are consistent with the subangstrom proximity of water to soft hydrophobic materials.

Intrinsic structure of hydrophobic surfaces: the oil-water interface

We investigate the water-oil interface using molecular dynamics simulations of realistic models of alkanes and water. The intrinsic density profiles are computed using a methodology that removes the smoothing effect of the capillary waves. We show that at 300 K the intrinsic width of the gap separating the oil and water phases spans little more than one water molecule diameter, and undergoes very weak short-ranged fluctuations, indicating that the water-oil interface is a rigid molecular structure at ambient temperature. Only near the drying transition (above 500 K for dodecane), the gap features uncoupled fluctuations of the oil and water surfaces, as expected in a typical drying structure. We find that the intrinsic structure of water next to the oil phase is remarkably similar to the bare water-vapor interface.

Temperature and length scale dependence of tetraalkylammonium ion-amide interaction

We have studied the temperature and length scale dependence of the energetics of the pair interaction of well-established hydrophobic solutes tetraalkylammonium bromides with hydrophilic formamide (FA) and hydrophobic hexamethylphosphoric triamide (HMPT). Our results do indicate the anomalous length scale dependence of the tetraalkylammonium cation-amide interaction in water. As the cation size is increased, the unfavorable enthalpy of interaction is increased rather linearly until the maximum is reached, after which there appears to be a reversal of the trend. We believe that this phenomenon arises from the impossibility of water to maintain its H-bond network near large tetraalkylammonium cations that leads to the formation of a somewhat disordered solute hydration shell. The energetic cost for overlapping this shell with the amide hydration shell in water is noticeably smaller than that for tetraalkylammonium cations of a moderated size.

Attractive surface force in the presence of dissolved gas: a molecular approach

Despite widespread evidence of the influence of dissolved air on hydrophobic interaction, the mechanisms of observed effects are still unknown. Although some experiments indicate that adsorbed gases can modify the structure of water next to hydrophobic surfaces, gas effects on measured forces have been observed only at large surface separations. Gas-specific depletion of water at a hydrophobic surface has been detected but was not reproduced in subsequent measurements. We use computer simulations to study short-ranged hydrophobic attraction in the absence and presence of dissolved gas and monitor gas adsorption at molecular resolution inaccessible in experiments. Although we observe a significant accumulation of dissolved gases at hydrophobic surfaces, even in supersaturated gas solutions surface concentrations remain too low to induce any significant change in the local structure of water and short-range surface forces. We present direct calculations of the hydrophobic force between model hydrocarbon plates at separations between 1.5 and 4 nm. Although stronger, the calculated solvation force has a similar decay rate as deduced from recent surface force apparatus measurements at a somewhat lower contact angle. Within the statistical uncertainty, short-range attraction is not affected by the presence of dissolved nitrogen, even in supersaturated solution with a gas fugacity as high as 30 atm. Comparisons of the adsorption behavior of N2, O2, CO2, and Ar reveal similar features in contrast to the peculiar suppression of water depletion reported for an Ar solution in a neutron reflectivity experiment. Our calculations reveal a notable difference between pathways to the capillary evaporation of pure water and gas-phase nucleation in confined supersaturated gas solutions.

Diffusion and residence time of hydrogen peroxide and water in crowded protein environments

Reactive oxygen species (ROS) have important functions in cell signaling and, when present at overly high levels, may cause oxidation of important biological molecules. Kinetic models to study diffusion of ROS inside of mitochondria often assume dynamics similar to that in solution. However, it is well-known that separation of proteins in the cytosol or inside of mitochondria, where ROS are most predominant, can be smaller than 1 nm. Diffusion of small molecules can be better regarded as a percolation process. In this article, we report results of diffusivity and residence of water and hydrogen peroxide in the proximity of proteins. In carrying out this study, we found some issues with the conventional way of computing residence times by means of survival time correlation functions. The main problem is that particles remaining on the surface of a protein for long times and for which one has very poor statistics contribute significantly to the short time behavior of the survival time correlation function. We mathematically describe this problem and propose methodology to overcome it.

Ellipsometric study of depletion at oil-water interfaces

Ellipsometry is exquisitely sensitive to density variations across a fluid-fluid interface. The coefficient of ellipticity at the interface between water and a series of nonpolar and polar oils is the opposite sign to that predicted for an interface roughened by thermal capillary waves. For pure hydrocarbons, the coefficient of ellipticity is correlated with the refractive index of the oil, but is largely independent of the molecular architecture of the oil phase, ruling out molecular alignment at the interface as the major cause of the deviation from the capillary-wave model. The introduction of a "drying" layer between the oil and water can explain the experimental data. The thickness of the drying layer, modeled as a slab with a relative permittivity of unity, was only 0.3-0.4 A, which is close to that expected simply from the hard sphere repulsion of a hydrocarbon surface. For polar oils, the coefficient of ellipticity decreases as the interfacial tension decreases, consistent with the reduction in thickness of the hard-sphere exclusion region on account of the formation of hydrogen bonds to water.

Dynamics of Water in the Hydration Shells of C60: Molecular Dynamics Simulation Using a Coarse-Grained Model

Dynamics of water in the solvation shells of a fullerene molecule as obtained from a coarse-grained (CG) model for the C60-water interaction has been presented and compared with the same obtained from the atomistic model. While in the CG model the interaction between a fullerene and a water has been represented by a simple two-body central potential as obtained from a coarse-graining of the interactions of a C60 molecule with water, in the atomistic description all the interactions between the atoms of a C60 and a water molecule have been explicitly taken into account. Extensive molecular dynamics simulations of a C60 molecule in water have been performed in isothermal-isobaric ensembles. Translational and reorientational mobilities as well as residence time of water in the solvation shells of a fullerene molecule have been obtained by calculating the corresponding time correlation functions from simulation trajectories. Comparison of the dynamical behaviors obtained from the CG and the atomistic models shows overall good agreement. The nature of the relaxation and the trend that the dynamics becomes slower with the decreasing solute-water distance as obtained from the atomistic model have, in general, been reproduced by the CG model.

Water properties and potential of mean force for hydrophobic interactions of methane and nanoscopic pockets studied by computer simulations

We consider model systems consisting of a methane molecule and hemispherical pockets of subnanometer radii whose walls are made of hydrophobic material. The potential of mean force for process of translocation of the methane molecule from bulk water into the pockets' interior is obtained, based on an explicit solvent molecular dynamics simulations. Accompanying changes in water density around the interacting objects and spatial distribution of solvent's potential energy are analyzed, allowing for interpretation of details of hydrophobic interactions in relation to hydrophobic hydration properties. Applicability of surface area-based models of hydrophobic effect for systems of interest is also investigated. A total work for the translocation process is not dependent on pocket's size, indicating that pocket desolvation has little contribution to free energy changes, which is consistent with the observation that solvent density is significantly reduced inside "unperturbed" pockets. Substantial solvent effects are shown to have a longer range than in case of a well investigated methane pair. A desolvation barrier is present in a smaller pocket system but disappears in the larger one, suggesting that a form of a "hydrophobic collapse" is observed.

Interfacial water at hydrophobic and hydrophilic surfaces: depletion versus adsorption

The structure of the water-solid interface for widely varying surface properties is investigated with Monte Carlo simulations using the SPC/E water model. Of particular interest is the relation between the wetting coefficient as a measure of the hydrophobicity of the substrate and the density depletion close to the solid surface. The substrates are modeled as rigid ordered lattices of sites that interact with water molecules through an orientation-independent Lennard-Jones potential of varying strength. Hydrophilic character is obtained by addition of polar hydroxyl groups on the substrate surface, and the influence of density, spatial distribution, and angular orientation of the polar groups on the interfacial water structure is studied.

Enthalpy-entropy contributions to the potential of mean force of nanoscopic hydrophobic solutes

Entropic and enthalpic contributions to the hydrophobic interaction between nanoscopic hydrophobic solutes, modeled as graphene plates in water, have been calculated using molecular dynamics simulations in the isothermal-isobaric (NPT) ensemble with free energy perturbation methodology. We find the stabilizing contribution to the free energy of association (contact pair formation) to be the favorable entropic part, the enthalpic contribution being highly unfavorable. The desolvation barrier is dominated by the unfavorable enthalpic contribution, despite a fairly large favorable entropic compensation. The enthalpic contributions, incorporating the Lennard-Jones solute-solvent terms, largely determine the stability of the solvent separated configuration. We decompose the enthalpy into a direct solute-solute term, the solute-solvent interactions, and the remainder that contains pressure-volume work as well as contributions due to solvent reorganization. The enthalpic contribution due to changes in water-water interactions arising from solvent reorganization around the solute molecules is shown to have major contribution in the solvent induced enthalpy change.

Contrasting nonaqueous against aqueous solvation on the basis of scaled-particle theory

Normal hexane is adopted as a typical organic solvent for comparison with liquid water in modern theories of hydrophobic hydration, and detailed results are worked-out here for the C-atom density in contact with a hard-sphere solute, rhoCG(R), for the full range of solute radii. The intramolecular structure of an n-hexane molecule introduces qualitative changes in G(R) compared to scaled-particle models for liquid water. Also worked-out is a revised scaled-particle model implemented with molecular simulation results for liquid n-hexane. The classic scaled-particle model, acknowledging the intramolecular structure of an n-hexane molecule, is in qualitative agreement with the revised scaled-particle model results, and is consistent in sizing the methyl/methylene sites which compose n-hexane in the simulation model. The classic and revised scaled-particle models disagree for length scales greater than the radius of a methyl group, however. The liquid-vapor surface tension of n-hexane predicted by the classic scaled-particle model is too large, though the temperature variation is reasonable; this contrasts with the classic scaled-particle theory for water which predicts a reasonable magnitude of the water liquid-vapor surface tension, but an incorrect sign for the temperature derivative at moderate temperatures. Judging on the basis of the arbitrary condition that drying is indicated when G(R)<1, hard spheres dry at smaller sizes in n-hexane than in liquid water.

Theoretical study of the role of solvent H2O in neopentyl and pinacol rearrangements

The neopentyl and the pinacol rearrangements as examples of Wagner-Meerwein rearrangements were investigated by the use of DFT calculations. As the first reaction, a model of neopentyl chloride (1b) and (H2O)12 was employed. In the reaction, the patterns of C--Cl scission, methyl migration, and C--OH formation were analyzed. The calculations have shown that the 2-methyl-2-butanol (6) is formed in two steps with the transient intermediate, neopentyl alcohol (3). The first step is the nucleophilic substitution reaction and is the rate-determining one. The second step is the dual migration of methyl and OH2 groups. The primary and tertiary carbocations were calculated to be absent in the neopentyl rearrangement starting from the hydrolysis. As the second reaction, the pinacol rearrangement of two substrates 2,3-dimethyl-2,3-butanediol (7) and 2,3-diphenyl-2,3-butanediol (12) was investigated. Acidic aqueous solvent was modeled by H3O+ and 12H2O. The reaction paths were promoted by a hydrogen-bond circuit of H3O+(H2O)2 and were determined as completely concerted processes. Protonated species and carbocations as intermediates also do not intervene during the pinacol rearrangement. Active functions of proton relays along the hydrogen bonds in the two rearrangements were demonstrated.

Depth Profiling of Water Molecules at the Liquid−Liquid Interface Using a Combined Surface Vibrational Spectroscopy and Molecular Dynamics Approach

The studies presented here combine experimental and computational approaches to provide new insights into how water structures and penetrates into the organic phase at two different liquid-liquid systems: the interfaces of carbon tetrachloride-water (CCl4-H2O) and 1,2-dichloroethane-water (DCE-H2O). In particular, molecular dynamics simulations are performed to generate computational spectral intensities of the CCl4-H2O and DCE-H2O interfaces that are directly comparable with experimental measurements. These simulations are then applied toward the generation of spectral profiles, responses that vary as functions of both frequency and interfacial depth. These studies emphasize the similarities and differences in the structure, orientation, and bonding of interfacial water as a function of interfacial depth for these two liquid-liquid systems and demonstrate the differing behavior of water monomers that penetrate into the organic phase.

Water properties inside nanoscopic hydrophobic pocket studied by computer simulations

The structure and dynamics of water in the vicinity of the hemispherical hydrophobic pocket of 8 A radius were examined via molecular dynamics simulations in NVT ensemble. Density, hydrogen bonding properties, and residence times of water molecules were projected on two-dimensional planes providing a spatial description of water behavior. We found that the average water density is significantly depleted relative to bulk value. A detailed analysis of pocket occupancy revealed fluctuations between states of completely empty pocket and a pocket filled with a bulklike fluid, which seem to result from collective behavior of water molecules. Free energy differences accompanying these fluctuations are rather small, suggesting that the given pocket radius is close to the critical one for transition between gas and liquid phases in the considered system. We show that the situation is different in the case of a simple Lennard-Jones fluid. These results indicate that changing the surface curvature from flat to concave may lead to qualitative difference in water behavior in its vicinity. We think that our studies may also put some light on binding site desolvation process which is necessary to understand to make correct predictions of binding energies.

The dewetting transition and the hydrophobic effect

A molecular-level description of the behavior of water in hydrophobic spaces is presented in terms of the coupled effects of solute size and atomic solute-solvent interactions. For model solutes with surface areas near those of protein contacts, we identify three different regions of solute-water interaction to be associated with three distinctly different structural characteristics of water in the intersolute region: dry, oscillating, and wet. A first orderlike phase transition is confirmed from the wet to dry state bridged by a narrow region with liquid-vapor oscillations in the intersolute region as the strength of the solute-water attractive dispersion interaction decreases. We demonstrate that the recent idea that cavitation in the intersolute region of nanoscopic solutes is preceded by the formation of a vapor layer around an individual solute is not the general case. The appearance of density waves pulled up around and outside of a nanoscopic plate occurs at lower interaction strengths than are required to obtain a wet state between such plates. We further show that chemically reasonable estimates of the interaction strength lead to a microscopically wet state and a hydrophobic interaction characterized by traps and barriers to association and not by vacuum induced collapse.

A pH-study of n-dodecyl-beta-D-maltoside foam films

The influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-beta-d-maltoside (beta-C12G2) was studied. Foam film measurements were carried out with the thin film pressure balance (TFPB) technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure Pc and, with the latter, complete disjoining pressure versus thickness curves (Pi-h curves) were measured. Most of the results were obtained for 10(-4) and 10(-5) M beta-C12G2 solutions that contained 10(-3) M electrolyte. Measurements were carried out in a pH range from 3 to 9. The major results are the following: (1) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs approximately cmc. This holds true for both low and high pH values. (2) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (3) The pH of the isoelectric point increases with increasing surfactant concentration. (4) The q0-pH curve of beta-C12G2 shows two pH ranges (3-5.5 and 7-10) in which the surface charge density q0 is pH-insensitive, while a significant change of q0 was observed between pH=5.5 and 7.0. A possible explanation is given.

On the dissolution of vapors and gases

The captive bubble technique in combination with axisymmetric drop shape analysis (ADSA-CB) and with micro gas chromatography is used to study the dynamics of dissolution of different gases and vapors in water in situ. The technique yields the changes in the interfacial tension and bubble volume and surface. As examples, the dissolution of methanol and hexane vapors, inhaled anesthetic vapors, and gases, that is, diethyl ether, chloroform, isoflurane, enflurane, sevoflurane, desflurane, N2O, and xenon, and as nonimmobilizers perfluoropentane and 1,1,2-trichloro-1,2,2-trifluoro-ethane (R113) were investigated. The examination of interfacial tension-time and bubble volume-time functions permits us to distinguish between water-soluble and -insoluble substances, gases, and vapors. Vapors and gases generally differ in terms of the strength of their intermolecular interactions. The main difference between dissolution processes of gases and vapors is that, during the entire process of gas dissolution, no surface tension change occurs. In contrast, during vapor dissolution the surface tension drops immediately and decreases continuously until it reaches the equilibrium surface tension of water at the end of dissolution. The results of this study show that it is possible to discriminate anesthetic vapors from anesthetic gases and nonimmobilizers by comparing their dissolution dynamics. The nonimmobilizers have extremely low or no solubility in water and change the surface tension only negligibly. By use of newly defined molecular dissolution/diffusion coefficients, a simple model for the determination of partition coefficients is developed.

Sequence-Dependent Binding of Dipeptides by an Artificial Receptor in Water

An artificial dipeptide receptor (1) was designed and observed to bind the deprotonated dipeptide Ac-D-Ala-D-Ala-OH in buffered water with K = 33,100 M(-1), whereas other dipeptides such as Ac-Gly-Gly-OH or Ac-D-Val-D-Val-OH were bound less efficiently, by factors of more than 10 (K < 3000 M(-1)). The efficient binding and the pronounced sequence selectivity are the result of a combination of strong electrostatic contacts and size-discriminating hydrophobic interactions. To provide such a combination, a guanidiniocarbonylpyrrole cation was attached to a novel cyclotribenzylene-substituted alanine derivative 5, to provide a hydrophobic bowl-shaped cavity just large enough to bind a methyl group but not any larger alkyl chains, thus causing the receptor to prefer alanine to valine. We describe the synthesis of 1 and the evaluation of its complexation properties in UV and fluorescence titration studies.

Peptide dendrimers as artificial enzymes, receptors, and drug-delivery agents

The dendritic architecture applied to peptides provides a practical entry into globular macromolecules resembling proteins. A modular design was chosen using a divergent synthesis on solid support alternating proteinogenic alpha-amino acids with branching diamino acids, producing peptide dendrimers with a molecular weight of 3-5 kDa. Initial studies focused on models for hydrolases and produced esterase peptide dendrimers featuring histidine as the key catalytic residue. Variations of amino acid composition and the branching diamino acid led to enantioselective catalysts. Rate accelerations of k(cat)/k(uncat) = 90,000 were obtained when the design was changed to monomeric peptide dendrimers alternating two amino acids with the branching unit. A combinatorial approach was developed allowing for the preparation of large libraries (>60,000 members), which were screened for B12 binding and catalytic activity. The peptide dendrimers were also investigated for drug delivery. Glycopeptide dendrimers conjugated to colchicine selectively inhibited the proliferation of targeted cells, whereas colchicine alone displayed high toxicity.

Hydrophobic and Ionic Interactions in Nanosized Water Droplets

A number of situations such as protein folding in confined spaces, lubrication in tight spaces, and chemical reactions in confined spaces require an understanding of water-mediated interactions. As an illustration of the profound effects of confinement on hydrophobic and ionic interactions, we investigate the solvation of methane and methane decorated with charges in spherically confined water droplets. Free energy profiles for a single methane molecule in droplets, ranging in diameter (D) from 1 to 4 nm, show that the droplet surfaces are strongly favorable as compared to the interior. From the temperature dependence of the free energy in D = 3 nm, we show that this effect is entropically driven. The potentials of mean force (PMFs) between two methane molecules show that the solvent separated minimum in the bulk is completely absent in confined water, independent of the droplet size since the solute particles are primarily associated with the droplet surface. The tendency of methanes with charges (M(q+) and M(q-) with q(+) = |q(-)| = 0.4e, where e is the electronic charge) to be pinned at the surface depends dramatically on the size of the water droplet. When D = 4 nm, the ions prefer the interior whereas for D < 4 nm the ions are localized at the surface, but with much less tendency than for methanes. Increasing the ion charge to e makes the surface strongly unfavorable. Reflecting the charge asymmetry of the water molecule, negative ions have a stronger preference for the surface compared to positive ions of the same charge magnitude. With increasing droplet size, the PMFs between M(q+) and M(q-) show decreasing influence of the boundary owing to the reduced tendency for surface solvation. We also show that as the solute charge density decreases the surface becomes less unfavorable. The implications of our results for the folding of proteins in confined spaces are outlined.

Fundamental measure theory of hydrated hydrocarbons

To calculate the solvation of hydrophobic solutes, we have developed a method based on the fundamental measure treatment of density functional theory. This method allows us to carry out calculations of density profiles and the solvation energy for various hydrophobic molecules with high accuracy. We have applied the method to the hydration of various hydrocarbons (linear, branched and cyclic). The calculations of the entropic and enthalpic parts are also carried out. We have examined the question of the temperature dependence of the entropy convergence. Finally, we have calculated the mean force potential between two large hydrophobic nanoparticles immersed in water.

Hydration of Hydrophobic Solutes Treated by the Fundamental Measure Approach

We have developed a method to calculate the hydration of hydrophobic solutes by the fundamental measure theory. This method allows us to carry out calculations of the density profile and the hydration energy for hydrophobic molecules. An additional benefit of the method is the possibility to calculate interaction forces between solvated nanoparticles. Based on the designed method, we calculate hydration of spherical solutes of various sizes from one angstrom up to several nanometers. We have applied methods to evaluate the free energies, the enthalpies, and the entropies of hydrated rare gases and hydrocarbons. The obtained results are in agreement with available experimental data and simulations.

Hydrophobic and hydrophilic yoctowells

Rigid molecular monolayers made of alpha,omega-diamido lipids form yoctoliter-sized gaps ("yoctowells", 1 yL = 10(-24) L or 1 nm3) around porphyrin islands on smooth surfaces. Their hydrophobic walls adsorb cyclic edge amphiphiles, e.g., trans-1,2-cyclohexanediol, cellobiose, and tyrosine, which fill-up the wells slowly and irreversibly by a process called "kinetic trapping". Wells with oligoethylene or oligoamide walls are effective as 3D-crown ethers or oligoamide barrels for reversible "thermodynamic trapping" of amines or amides. Three porphyrins A, B, and C were sorted as stacks within the yoctowells, and a methylammonium ring was established at their rim to fixate a fourth molecule D at a longer distance. Yoctowells are easy to prepare, characterize, and modify and provide simple models of biological modules.