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, ؊CH3) 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.hydration ͉ hydrophilic ͉ hydrophobic ͉ wetting ͉ fluctuations H ydrophobicity, reflected in the low solubility of nonpolar solutes or in their tendency to aggregate in water, is known to play an important role in many biological and colloidal self-assembly processes (1-4). Yet defining it precisely is challenging, and its molecular signatures remain unclear. Macroscopically, hydrophobicity is often characterized by measuring the droplet contact angle, with surfaces showing angles greater than 90°termed hydrophobic. Water beads up into droplets on hydrophobic surfaces and spreads on hydrophilic ones. Translating these ideas into the molecular domain presents special challenges. In a recent perspective, Granick and Bae (5) highlight the ambiguity in defining hydrophobicity at molecular length scales, such as near proteins or nanotubes, where droplet contact angle measurements are not possible.At the molecular level, hard-sphere solutes have served as excellent models for studies of hydrophobicity, with their hydration thermodynamics capturing the solubility of noble gases as a function of temperature (6, 7), pressure (8), and salt addition (9, 10). With increasing solute length scale, the elegant theory by Lum, Chandler, and Weeks (11) as well as computer simulations (12, 13) predict a gradual dewetting of the solute. Near large solutes or a hard-wall, water density is small and vapor-like, and the wall-water interface resembles a water vapor-liquid interface (14).Realistic solutes exert van der Waals and/or electrostatic interactions and pull the water interface closer, ...

Water near hydrophobic surfaces is like that at a liquid–vapor interface, where fluctuations in water density are substantially enhanced compared to those in bulk water. Here we use molecular simulations with specialized sampling techniques to show that water density fluctuations are similarly enhanced, even near hydrophobic surfaces of complex biomolecules, situating them at the edge of a dewetting transition. Consequently, water near these surfaces is sensitive to subtle changes in surface conformation, topology, and chemistry, any of which can tip the balance towards or away from the wet state, and thus significantly alter biomolecular interactions and function. Our work also resolves the long-standing puzzle of why some biological surfaces dewet and other seemingly similar surfaces do not.

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.

Approaches to quantify wetting at the macroscale do not translate to the nanoscale, highlighting the need for new methods for characterizing hydrophobicity at the small scale. We use extensive molecular simulations to study the hydration of homo and heterogeneous self-assembled monolayers (SAMs) and of protein surfaces. For homogeneous SAMs, new pressure-dependent analysis shows that water displays higher compressibility and enhanced density fluctuations near hydrophobic surfaces, which are gradually quenched with increasing hydrophilicity, consistent with our previous studies. Heterogeneous surfaces show an interesting context dependence--adding a single -OH group in a CH3 terminated SAM has a more dramatic effect on water in the vicinity compared to that of a single CH3 group in an -OH background. For mixed -CH3/-OH SAMs, this asymmetry leads to a non-linear dependence of hydrophobicity on the surface concentration. We also present preliminary results to map hydrophobicity of protein surfaces by monitoring local density fluctuations and binding of probe hydrophobic solutes. These molecular measures account for the behavior of protein's hydration water, and present a more refined picture of its hydrophobicity map. At least for one protein, hydrophobin-II, we show that the hydrophobicity map is different from that suggested by a commonly used hydropathy scale.

Interfaces are a most common motif in complex systems. To understand how the presence of interfaces affects hydrophobic phenomena, we use molecular simulations and theory to study hydration of solutes at interfaces. The solutes range in size from subnanometer to a few nanometers. The interfaces are self-assembled monolayers with a range of chemistries, from hydrophilic to hydrophobic. We show that the driving force for assembly in the vicinity of a hydrophobic surface is weaker than that in bulk water and decreases with increasing temperature, in contrast to that in the bulk. We explain these distinct features in terms of an interplay between interfacial fluctuations and excluded volume effects-the physics encoded in Lum-Chandler-Weeks theory [Lum K, Chandler D, Weeks JD (1999) J Phys Chem B 103:4570-4577]. Our results suggest a catalytic role for hydrophobic interfaces in the unfolding of proteins, for example, in the interior of chaperonins and in amyloid formation. binding | hydrophobicity | thermodynamics H ydrophobic effects are ubiquitous and often the most significant forces of self-assembly and stability of nanoscale structures in liquid matter, from phenomena as simple as micelle formation to those as complex as protein folding and aggregation (1, 2). These effects depend importantly on length scale (3-5). Water molecules near small hydrophobic solutes do not sacrifice hydrogen bonds, but have fewer ways in which to form them, leading to a large negative entropy of hydration. In contrast, hydrogen bonds are broken in the hydration of large solutes, resulting in an enthalpic penalty. For hydrophobic solutes in bulk water at standard conditions, the cross-over from one regime to the other occurs at around 1 nm (3-6) and marks a change in the scaling of the solvation free energy from being linear with solute volume to being linear with exposed surface area. In bulk water, this crossover provides a framework for understanding the assembly of small species into a large aggregate.Typical biological systems contain a high density of interfaces, including those of membranes and proteins, spanning the entire spectrum from hydrophilic to hydrophobic. Whereas water near hydrophilic surfaces is bulk-like in many respects, water near hydrophobic surfaces is different, akin to that near a liquid-vapor interface (3-5, 7-9). Here, we consider how these interfaces alter hydrophobic effects. Specifically, to shed light on the thermodynamics of hydration at, binding to, and assembly at interfaces, we study solutes with a range of sizes at various self-assembled monolayer interfaces over a range of temperatures using molecular simulations and theory.Our principal results are that, although the hydration thermodynamics of hydrophobic solutes at hydrophilic surfaces is similar to that in bulk, changing from entropic to enthalpic with increasing solute size, it is enthalpic for solutes of all length scales near hydrophobic surfaces. Further, the driving force for hydrophobically driven assembly in the vicinity of hydrophobic ...