Computer Simulation of Cosolvent Effects on Hydrophobic Hydration

Theory of Gas Solubility and Hydrophobic Interaction in Aqueous Electrolyte Solutions

Ion-specific effects on the solubility of nonpolar solutes and on the solute-solute hydrophobic interaction in aqueous electrolyte solutions are studied on the basis of a continuum theory that incorporates the excluded volume of the molecules using the four-component (water, cations, anions, and solutes) Boublı́k-Mansoori-Carnahan-Starling-Leland model and ion hydration (electrostriction) using the Born model. We examine how the ordering of ions in the salt effect on the solubility as measured by the Sechenov coefficient K_S changes with varying sizes of ions and solutes. Our calculation reproduces the general trend of experimentally measured K_S and also provides insight into the irregular behavior of K_S for lithium ion. The correlation between K_S and the salt effect on the hydrophobic interaction that has been pointed out earlier is accounted for by an explicit connection between K_S and the salt-enhanced-association coefficient C_I in the expansion of the second osmotic virial coefficient B(n_s) = B(0) - C_In_s + ··· in powers of the salt density n_s at fixed pressure and temperature. The quadratic relation CI≈KS2/4 is derived for ions and solutes that are not very large.

Ion Size Dependences of the Salting-Out Effect: Reversed Order of Sodium and Lithium Ions

A general trend of the salting-out effect on hydrophobic solutes in aqueous solution is that the smaller the size of a dissolved ion, the larger the effect of reducing the solubility of a hydrophobe. An exception is that Li⁺, the smallest in alkali metal ions, has a notably weaker effect than Na⁺. To understand the reversed order in the cation series, we performed molecular dynamics simulations of aqueous solutions of salt ions and calculated the Setschenow coefficient of methane with the ionic radius of either a cation or an anion varied in a wide range. It is confirmed that the Setschenow coefficient is correlated with the packing fraction of salt solution, as observed in earlier studies, and also correlated with the partial molar volume of an ion. Analyses of correlation function integrals, packing fractions of solvation spheres, and orientations of water molecules surrounding an ion reveal the key differences in microscopic properties between the cation and anion series, which give rise to the reversed order in the cation series of the partial molar volumes of ions and ultimately that of the Setschenow coefficients.

Kirkwood-Buff-Derived Force Field for Peptides and Proteins: Philosophy and Development of KBFF20

A new classical nonpolarizable force field, KBFF20, for the simulation of peptides and proteins is presented. The force field relies heavily on the use of Kirkwood-Buff theory to provide a comparison of simulated and experimental Kirkwood-Buff integrals for solutes containing the functional groups common in proteins, thus ensuring intermolecular interactions that provide a good balance between the peptide-peptide, peptide-solvent, and solvent-solvent distributions observed in solution mixtures. In this way, it differs significantly from other biomolecular force fields. Further development and testing of the intermolecular potentials are presented here. Subsequently, rotational potentials for the ϕ/ψ and χ dihedral degrees of freedom are obtained by analysis of the Protein Data Bank, followed by small modifications to provide a reasonable balance between simulated and observed α and β percentages for small peptides. This, the first of two articles, describes in detail the philosophy and development behind KBFF20.

Aqueous Mixtures of Urea and Trimethylamine-N-oxide: Evidence for Kosmotropic or Chaotropic Behavior?

Trimethylamine-N-oxide (TMAO) and urea are commonly produced in many extremophilic microorganisms that live in harsh environments. In view of high temperature, high pressure, or high salt content, TMAO is known as a protein structure stabilizer, whereas urea destabilizes protein structures even under ambient conditions. Despite clear evidence, destabilizers are often regarded as chaotropes, meaning water-structure breakers, whereas kosmotropes as water-structure makers are classified as stabilizers. Using atomistic molecular dynamics simulations, we study aqueous mixtures of TMAO and urea in various biologically relevant concentrations to gain insight into the molecular details of their mutual cross-interactions and their influence on water dynamics and structure. Our results for binary and ternary solutions in combination with different mixing ratios show that both co-solutes strengthen the water network in terms of dynamic and structural aspects. Slight differences in the water binding behavior between both species result in only negligible compensation effects. The outcomes of our simulations thus question the validity and the ill-considered use of attributes like kosmotropic or chaotropic substances for stabilizers and destabilizers.

Preferential Binding of Urea to Single-Stranded DNA Structures: A Molecular Dynamics Study

In nature, a wide range of biological processes such as transcription termination and intermolecular binding depend on the formation of specific DNA secondary and tertiary structures. These structures can be both stabilized or destabilized by different cosolutes coexisting with nucleic acids in the cellular environment. In our molecular dynamics simulation study, we investigate the binding of urea at different concentrations to short 7-nucleotide single-stranded DNA structures in aqueous solution. The local concentration of urea around a native DNA hairpin in comparison to an unfolded DNA conformation is analyzed by a preferential binding model in light of the Kirkwood-Buff theory. All our findings indicate a pronounced accumulation of urea around DNA that is driven by a combination of electrostatic and dispersion interactions and accomplished by a significant replacement of hydrating water molecules. The outcomes of our study can be regarded as a first step into a deeper mechanistic understanding toward cosolute-induced effects on nucleotide structures in general.

The promotion and suppression of DNA charge neutralization by the cosolute ectoine

Ectoine, a cosolute and osmolyte, is used by extremophilic microorganisms to maintain an osmotic equilibrium of cells with their surrounding medium under conditions of extreme salinity or thermal and pressure stresses. It is also considered a protectant of biomolecules such as protein and DNA in cells. In the present study, we investigate its influence on DNA charge neutralization and compaction through dynamic light scattering (DLS), atomic force microscopy (AFM) and single molecular magnetic tweezers (MT). We found that ectoine can promote DNA charge neutralization induced by multivalent cations at mild cosolute concentration in solution. When the concentration of ectoine is high enough, however, a mixed effect of promotion and suppression can be found under the same ionic conditions. In this case, the electrophoretic mobility (EM) of DNA is promoted in the region of low cation concentration, while suppressed in the region of high counterionic concentration. The charge neutralization of DNA by ectoine is also related to DNA compaction. The promotion and suppression of DNA compaction by ectoine was observed by AFM imaging. The condensed structure of DNA becomes more compact and then loose once more with the increasing concentration of ectoine. Meanwhile, the condensing forces of DNA measured by magnetic tweezers shows the same trend as does the DNA EM. We explained the experimental findings through the combined effect of two intrinsic features of ectoine, preferential exclusion and enhancement of the dielectric constant of the medium.

Ectoine can promote DNA charge neutralization at mild cosolute concentration in solution. When the concentration of ectoine is high enough, however, a mixing effect of promotion and suppression can be found in the same ionic condition.

Mixtures of LiTFSI and urea: ideal thermodynamic behavior as key to the formation of deep eutectic solvents?

We studied the dynamic and structural properties of deep eutectic solvents composed of LiTFSI salts in presence of urea.

Properties of Ion Complexes and Their Impact on Charge Transport in Organic Solvent-Based Electrolyte Solutions for Lithium Batteries: Insights from a Theoretical Perspective

Electrolyte formulations in standard lithium ion and lithium metal batteries are complex mixtures of various components. In this article, we review molecular key principles of ion complexes in multicomponent electrolyte solutions in regards of their influence on charge transport mechanisms. We outline basic concepts for the description of ion–solvent and ion–ion interactions, which can be used to rationalize recent experimental and numerical findings concerning modern electrolyte formulations. Furthermore, we discuss benefits and drawbacks of empirical concepts in comparison to molecular theories of solution for a more refined understanding of ion behavior in organic solvents. The outcomes of our discussion provide a rational for beneficial properties of ions, solvent, co-solvent and additive molecules, and highlight possible routes for further improvement of novel electrolyte solutions.

Aqueous ionic liquids in comparison with standard co-solutes : Differences and common principles in their interaction with protein and DNA structures

Ionic liquids (ILs) are versatile solvents for a broad range of biotechnological applications. Recent experimental and simulation results highlight the potential benefits of dilute ILs in aqueous solution (aqueous ILs) in order to modify protein and DNA structures systematically. In contrast to a limited number of standard co-solutes like urea, ectoine, trimethylamine-N-oxide (TMAO), or guanidinium chloride, the large amount of possible cation and anion combinations in aqueous ILs can be used to develop tailor-made stabilizers or destabilizers for specific purposes. In this review article, we highlight common principles and differences between aqueous ILs and standard co-solutes with a specific focus on their underlying macromolecular stabilization or destabilization behavior. In combination with statistical thermodynamics theories, we present an efficient framework, which is used to classify structure modification effects consistently. The crucial importance of enthalpic and entropic contributions to the free energy change upon IL-assisted macromolecular unfolding in combination with a complex destabilization mechanism is described in detail. A special focus is also set on aqueous IL-DNA interactions, for which experimental and simulation outcomes are summarized and discussed in the context of previous findings.

Aqueous ionic liquids and their influence on peptide conformations: denaturation and dehydration mechanisms

Low concentrated aqueous ionic liquids (ILs) and their influence on protein structures have attracted a lot of interest over the last few years. This can be mostly attributed to the fact that aqueous ILs, depending on the ion species involved, can be used as protein protectants or protein denaturants. Atomistic molecular dynamics (MD) simulations are performed in order to study the influence of different aprotic ILs on the properties of a short hairpin peptide. Our results reveal distinct binding and denaturation effects for 1-ethyl-3-methylimidazolium (EMIM) in combination with different anions, namely, chloride (CL), tetrafluoroborate (BF4) and acetate (ACE). The simulation outcomes demonstrate that the studied ILs with larger anions reveal a more pronounced accumulation behavior of the individual ion species around the peptide, which is accomplished by a stronger dehydration effect. We can relate these findings to the implications of the Kirkwood-Buff theory, which provides a thermodynamic explanation for the denaturation strength in terms of the IL accumulation behavior. The results for the spatial distribution functions, the binding energies and the local/bulk partition coefficients are in good agreement with metadynamics simulations in order to determine the energetically most stable peptide conformations. The free energy landscapes indicate a decrease of the denaturation strength in the order EMIM/ACE, EMIM/BF4 and EMIM/CL, which coincides with a decreasing size of the anion species. An analysis of the potential binding energies reveals that this effect is mainly of enthalpic nature.

Influence of compatible solute ectoine on distinct DNA structures: thermodynamic insights into molecular binding mechanisms and destabilization effects

We study ectoine-induced destabilization effects on DNA hairpins by a combination of atomistic molecular dynamics simulations, experiments, and theoretical approaches.

Electrolyte solvents for high voltage lithium ion batteries: ion correlation and specific anion effects in adiponitrile

Combined atomistic molecular dynamics, quantum chemical, and experimental study regarding the properties of two lithium conducting salts in high voltage electrolyte solvent adiponitrile.

How Water's Properties Are Encoded in Its Molecular Structure and Energies

How are water's material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth's living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies-water's solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions-hydroxide and protons-diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water's molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water's orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties.

Aqueous ionic liquids and their effects on protein structures: an overview on recent theoretical and experimental results

Ionic liquids (ILs) are used in a variety of technological and biological applications. Recent experimental and simulation results reveal the influence of aqueous ionic liquids on the stability of protein and enzyme structures. Depending on different parameters like the concentration and the ion composition, one can observe distinct stabilization or denaturation mechanisms for various ILs. In this review, we summarize the main findings and discuss the implications with regard to molecular theories of solutions and specific ion effects. A preferential binding model is introduced in order to discuss protein-IL effects from a statistical mechanics perspective. The value of the preferential binding coefficient determines the strength of the ion influence and indicates a shift of the chemical equilibrium either to the native or the denatured state of the protein. We highlight the role of water in order to explain the self-association behavior of the IL species and discuss recent experimental and simulation results in the light of the observed binding effects.

Hofmeister Effect on PNIPAM in Bulk and at an Interface: Surface Partitioning of Weakly Hydrated Anions

The effect of sodium fluoride, sodium trichloroacetate, and sodium thiocyanate on the stability and conformation of poly(N-isopropylacrylamide), in bulk solution and at the gold-aqueous interface, is investigated by differential scanning calorimetry, dynamic light scattering, quartz crystal microbalance, and atomic force microscopy. The results indicate a surface partitioning of the weakly hydrated anions, i.e., thiocyanate and trichloroacetate, and the findings are discussed in terms of anion-induced electrostatic stabilization. Although attractive polymer-ion interactions are suggested for thiocyanate and trichloroacetate, a salting-out effect is found for sodium trichloroacetate. This apparent contradiction is explained by a combination of previously suggested mechanisms for the salting-out effect by weakly hydrated anions.

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

Atomistic molecular dynamics simulations of aqueous ionic liquid mixtures were performed in order to compare the resulting Kirkwood–Buff integrals with experimental data and the corresponding integrals derived by an inverse Kirkwood–Buff approach.

Effects of monohydric alcohols and polyols on the thermal stability of a protein

The thermal stability of a protein is lowered by the addition of a monohydric alcohol, and this effect becomes larger as the size of hydrophobic group in an alcohol molecule increases. By contrast, it is enhanced by the addition of a polyol possessing two or more hydroxyl groups per molecule, and this effect becomes larger as the number of hydroxyl groups increases. Here, we show that all of these experimental observations can be reproduced even in a quantitative sense by rigid-body models focused on the entropic effect originating from the translational displacement of solvent molecules. The solvent is either pure water or water-cosolvent solution. Three monohydric alcohols and five polyols are considered as cosolvents. In the rigid-body models, a protein is a fused hard spheres accounting for the polyatomic structure in the atomic detail, and the solvent is formed by hard spheres or a binary mixture of hard spheres with different diameters. The effective diameter of cosolvent molecules and the packing fractions of water and cosolvent, which are crucially important parameters, are carefully estimated using the experimental data of properties such as the density of solid crystal of cosolvent, parameters in the pertinent cosolvent-cosolvent interaction potential, and density of water-cosolvent solution. We employ the morphometric approach combined with the integral equation theory, which is best suited to the physical interpretation of the calculation result. It is argued that the degree of solvent crowding in the bulk is the key factor. When it is made more serious by the cosolvent addition, the solvent-entropy gain upon protein folding is magnified, leading to the enhanced thermal stability. When it is made less serious, the opposite is true. The mechanism of the effects of monohydric alcohols and polyols is physically the same as that of sugars. However, when the rigid-body models are employed for the effect of urea, its addition is predicted to enhance the thermal stability, which conflicts with the experimental fact. We then propose, as two essential factors, not only the solvent-entropy gain but also the loss of protein-solvent interaction energy upon protein folding. The competition of changes in these two factors induced by the cosolvent addition determines the thermal-stability change.

Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study

The concentration-dependent binding of urea to PNIPAM influences the chain conformation as a result of the subtle interplay between hydration properties and urea repartition around the polymer surface.

Stabilizing effect of TMAO on globular PNIPAM states: preferential attraction induces preferential hydration

We study the effect of the organic co-solute trimethylamine N-oxide (TMAO) on the volume phase transition of microgel particles made from poly(N-isopropylacrylamide) (PNIPAM) using dynamic light scattering (DLS) and all-atom molecular dynamics (MD) simulations.

Peptides in the presence of aqueous ionic liquids: tunable co-solutes as denaturants or protectants?

We studied the stability of a small β-hairpin peptide under the influence of an aqueous 1-ethyl-3-methylimidazolium acetate solution via all-atom molecular dynamics simulations. A significant structure dependent binding of ACE to the peptide was identified as the main reason for the denaturation of the native conformation.

Kirkwood-Buff analysis of aqueous N-methylacetamide and acetamide solutions modeled by the CHARMM additive and Drude polarizable force fields

Kirkwood-Buff analysis was performed on aqueous solutions of N-methylacetamide and acetamide using the Chemistry at HARvard Molecular Mechanics additive and Drude polarizable all-atom force fields. Comparison of a range of properties with experimental results, including Kirkwood-Buff integrals, excess coordination numbers, solution densities, partial molar values, molar enthalpy of mixing, showed both models to be well behaved at higher solute concentrations with the Drude model showing systematic improvement at lower solution concentrations. However, both models showed difficulties reproducing experimental activity derivatives and the excess Gibbs energy, with the Drude model performing slightly better. At the molecular level, the improved agreement of the Drude model at low solute concentrations is due to increased structure in the solute-solute and solute-solvent interactions. The present results indicate that the explicit inclusion of electronic polarization leads to improved modeling of dilute solutions even when those properties are not included as target data during force field optimization.

Anionic and cationic Hofmeister effects on hydrophobic and hydrophilic surfaces

Using a two-step modeling approach, we address the full spectrum of direct, reversed, and altered ionic sequences as the charge of the ion, the charge of the surface, and the surface polarity are varied. From solvent-explicit molecular dynamics simulations, we extract single-ion surface interaction potentials for halide and alkali ions at hydrophilic and hydrophobic surfaces. These are used within Poisson-Boltzmann theory to calculate ion density and electrostatic potential distributions at mixed polar/unpolar surfaces for varying surface charge. The resulting interfacial tension increments agree quantitatively with experimental data and capture the Hofmeister series, especially the anomaly of lithium, which is difficult to obtain using continuum theory. Phase diagrams that feature different Hofmeister series as a function of surface charge, salt concentration, and surface polarity are constructed from the long-range force between two surfaces interacting across electrolyte solutions. Large anions such as iodide have a high hydrophobic surface affinity and increase the effective charge magnitude on negatively charged unpolar surfaces. Large cations such as cesium also have a large hydrophobic surface affinity and thereby compensate an external negative charge surface charge most efficiently, which explains the well-known asymmetry between cations and anions. On the hydrophilic surface, the size-dependence of the ion surface affinity is reversed, explaining the Hofmeister series reversal when comparing hydrophobic with hydrophilic surfaces.

Hydrophobicity of proteins and interfaces: insights from density fluctuations

Macroscopic characterizations of hydrophobicity (e.g., contact angle measurements) do not extend to the surfaces of proteins and nanoparticles. Molecular measures of hydrophobicity of such surfaces need to account for the behavior of hydration water. Theory and state-of-the-art simulations suggest that water density fluctuations provide such a measure; fluctuations are enhanced near hydrophobic surfaces and quenched with increasing surface hydrophilicity. Fluctuations affect conformational equilibria and dynamics of molecules at interfaces. Enhanced fluctuations are reflected in enhanced cavity formation, more favorable binding of hydrophobic solutes, increased compressibility of hydration water, and enhanced water-water correlations at hydrophobic surfaces. These density fluctuation-based measures can be used to develop practical methods to map the hydrophobicity/philicity of heterogeneous surfaces including those of proteins. They highlight that the hydrophobicity of a group is context dependent and is significantly affected by its environment (e.g., chemistry and topography) and especially by confinement. The ability to include information about hydration water in mapping hydrophobicity is expected to significantly impact our understanding of protein-protein interactions as well as improve drug design and discovery methods and bioseparation processes.

Designing heteropolymers to fold into unique structures via water-mediated interactions

Hydrophobic homopolymers collapse into globular structures in water driven by hydrophobic interactions. Here we employ extensive molecular dynamics simulations to study the collapse of heteropolymers containing one or two pairs of oppositely charged monomers. We show that charging a pair of monomers can dramatically alter the most stable conformations from compact globular to more open hairpin-like. We systematically explore a subset of the sequence space of one- and two-charge-pair polymers, focusing on the locations of the charge pairs. Conformational stability is governed by a balance of hydrophobic interactions, hydration and interactions of charge groups, water-mediated charged-hydrophobic monomer repulsions, and other factors. As a result, placing charge pairs in the middle, away from the hairpin ends, leads to stable hairpin-like structures. Turning off the monomer-water attractions enhances hydrophobic interactions significantly leading to a collapse into compact globular structures even for two-charge-pair heteropolymers. In contrast, the addition of salt leads to open and extended structures, suggesting that solvation of charged monomer sites by salt ions dominates the salt-induced enhancement of hydrophobic interactions. We also test the ability of a predictive scheme based on the additivity of free energy of contact formation. The success of the scheme for symmetric two-charge-pair sequences and the failure for their flipped versions highlight the complexity of the heteropolymer conformation space and of the design problem. Collectively, our results underscore the ability of tuning water-mediated interactions to design stable nonglobular structures in water and present model heteropolymers for further studies in the extended thermodynamic space and in inhomogeneous environments.

Receptacle model of salting-in by tetramethylammonium ions

Water is a poor solvent for nonpolar solutes. Water containing ions is an even poorer solvent. According to standard terminology, the tendency of salts to precipitate oils from water is called salting-out. However, interestingly, some salt ions, such as tetramethylammonium (TMA), cause instead the salting-in of hydrophobic solutes. Even more puzzling, there is a systematic dependence on solute size. TMA causes the salting-out of small hydrophobes and the salting-in of larger nonpolar solutes. We study these effects using NPT Monte Carlo simulations of the Mercedes-Benz (MB) + dipole model of water, which was previously shown to account for hydrophobic effects and ion solubilities in water. The present model gives a structural interpretation for the thermodynamics of salting-in. The TMA structure allows deep penetration by a first shell of waters, the dipoles of which interact electrostatically with the ion. This first water shell sets up a second water shell that is shaped to act as a receptacle that binds the nonpolar solute. In this way, a nonpolar solute can actually bind more tightly to the TMA ion than to another hydrophobe, leading to the increased solubility and salting-in. Such structuring may also explain why molecular ions do not follow the same charge density series as atomic ions do.

Reversed anionic Hofmeister series: the interplay of surface charge and surface polarity

We describe a two-scale modeling approach toward anion specificity at surfaces of varying charge and polarity. Explicit-solvent atomistic molecular dynamics simulations at neutral hydrophobic (i.e., nonpolar) and neutral hydrophilic (i.e., polar) self-assembled monolayers furnish potentials of mean force for Na(+) and the halide anions F(-), Cl(-), and I(-) which are then used within Poisson-Boltzmann theory to calculate ionic distributions at surfaces of arbitrary charge for finite ion concentration. On the basis of calculated long-ranged electrostatic forces and coagulation properties, we obtain the direct anionic Hofmeister series at negatively charged hydrophobic surfaces. Reversal takes place when going to negative polar or to positive nonpolar surfaces, leading to the indirect series, while for positive polar surfaces the direct series is again obtained. This is in full accordance with a recent experimental classification of colloidal coagulation kinetics and also reflects the trends of the ion specific solubility properties of proteins. A schematic Hofmeister phase diagram is proposed. Partial series reversal is understood as a transient phenomenon for surfaces of intermediate polarity or charge.

Unfolding of hydrophobic polymers in guanidinium chloride solutions

Guanidinium chloride (GdmCl) is a widely used chemical denaturant that unfolds proteins. Its effects on hydrophobic interactions are, however, not fully understood. We quantify the effects of GdmCl on various manifestations of hydrophobicity--from solvation and interactions of small solutes to folding-unfolding of hydrophobic polymers--in water and in concentrated GdmCl solutions. For comparison, we also perform similar calculations in solutions of NaCl and CsCl in water. Like NaCl and CsCl, GdmCl increases the surface tension of water, decreases the solubility of small hydrophobic solutes, and enhances the strength of hydrophobic interactions at the pair level. However, unlike NaCl and CsCl, GdmCl destabilizes folded states of hydrophobic polymers. We show that Gdm(+) ions preferentially coat the hydrophobic polymer, and it is the direct van der Waals interaction between Gdm(+) ions and the polymer that contributes to the destabilization of folded states. Interestingly, the temperature dependence of the free energy of unfolding of the hydrophobic polymer in water is protein-like, with signatures of both heat and cold denaturation. Addition of GdmCl shifts the cold denaturation temperature higher, into the experimentally accessible region. Finally, translational as well as conformational dynamics of the polymer are slower in GdmCl and correlate with dynamics of water molecules in solution.

Kirkwood-Buff theory of molecular and protein association, aggregation, and cellular crowding

An analysis of the effect of a cosolvent on the association of a solute in solution using the Kirkwood-Buff theory of solutions is presented. The approach builds on the previous results of Ben-Naim by extending the range of applicability to include any number of components at finite concentrations in both closed and semiopen systems. The derived expressions, which are exact, provide a foundation for the analysis and rationalization of cosolvent effects on molecular and biomolecular equilibria including protein association, aggregation, and cellular crowding. A slightly different view of cellular crowding is subsequently obtained. In particular, it is observed that the addition of large cosolvents still favors the associated form even when traditional excluded volume effects are absent.

Characterizing hydrophobicity of interfaces by using cavity formation, solute binding, and water correlations

Hydrophobicity is often characterized macroscopically by the droplet contact angle. Molecular signatures of hydrophobicity have, however, remained elusive. Successful theories predict a drying transition leading to a vapor-like region near large hard-sphere solutes and interfaces. Adding attractions wets the interface with local density increasing with attractions. Here we present extensive molecular simulation studies of hydration of realistic surfaces with a wide range of chemistries from hydrophobic (-CF(3), -CH(3)) to hydrophilic (-OH, -CONH(2)). We show that the water density near weakly attractive hydrophobic surfaces (e.g., -CF(3)) can be bulk-like or larger, and provides a poor quantification of surface hydrophobicity. In contrast, the probability of cavity formation or the free energy of binding of hydrophobic solutes to interfaces correlates quantitatively with the macroscopic wetting properties and serves as an excellent signature of hydrophobicity. Specifically, the probability of cavity formation is enhanced in the vicinity of hydrophobic surfaces, and water-water correlations correspondingly display characteristics similar to those near a vapor-liquid interface. Hydrophilic surfaces suppress cavity formation and reduce the water-water correlation length. Our results suggest a potentially robust approach for characterizing hydrophobicity of more complex and heterogeneous surfaces of proteins and biomolecules, and other nanoscopic objects.

Quantifying accumulation or exclusion of H+, HO-, and Hofmeister salt ions near interfaces

Recently, surface spectroscopies and simulations have begun to characterize the non-uniform distributions of salt ions near macroscopic and molecular surfaces. The thermodynamic consequences of these non-uniform distributions determine the often-large ion-specific effects of Hofmeister salts on a very wide range of processes in water. For uncharged surfaces, where these nonuniform ion distributions are confined to the first few layers of water at the surface, a two-state approximation to the distributions of water and ions, called the salt ion partitioning model (SPM) has both molecular and thermodynamic signiicance. Here, we summarize SPM results quantifying the local accumulation of H+, exclusion of HO-, and range of partitioning behavior of Hofmeister anions and cations near macroscopic and molecular interfaces. These results provide a database to interpret or predict Hofmeister salt effects on aqueous processes in terms of structural information regarding amount and composition of the surface exposed or buried in these processes.

Kirkwood-Buff theory of four and higher component mixtures

Explicit expressions are developed for the chemical potential derivatives, partial molar volumes, and isothermal compressibility of solution mixtures involving four components at finite concentrations using the Kirkwood-Buff theory of solutions. In addition, a general recursion relationship is provided which can be used to generate the chemical potential derivatives for higher component solutions.

Computer simulation of ion cluster speciation in concentrated aqueous solutions at ambient conditions

Dynamic simulations are used to investigate ion cluster formation in unsaturated aqueous NaCl at 25 degrees C. Statistical, structural, and dynamic properties are reported. An effort is made to identify general behaviors that are expected to hold beyond the limitations of the force field. Above approximately 1 M, clusters with more than ten ions begin to form after approximately 10-20 ns of simulation time, but no evidence of irreversible ion aggregation is observed. Cluster survival times are estimated, showing that the kinetics become increasingly complex as salt is added, leading to multiple decay rates. Cluster dipole moment distributions show characteristic peaks that reflect the preferred conformations of clusters in solution. These are modulated by electrostatic and liquid-structure forces and are described in detail for clusters of up to five ions. For a given size and charge, the cluster morphology is independent of salt concentration. Below approximately 2 M, clusters affect the structure of water in their first hydration shells, so dipole moments parallel to the cluster macrodipoles are induced. These effects show a weak dependence with concentration below approximately 2 M, but vanish in the 2-3 M range. A possible connection with the structural transition recently suggested by NMR data in concentrated electrolytes is discussed. The effects of electrostatics on cluster speciation and morphology are discussed based on results from a set of simulations carried out with the ionic charges removed.