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 ...

We quantify the Kapitza thermal conductance of solid–liquid interfaces between self-assembled monolayers (SAMs) and liquid water using nonequilibrium molecular dynamics simulations. We focus on understanding how surface chemistry, nanoscale roughness, and the direction of heat flow affect interfacial thermal conductance. In agreement with calculations by Shenogina et al. (Phys. Rev. Lett., 2009, 102, 156101) for SAMs with homogeneous headgroup chemistries, we find that for mixed −CF3/–OH SAMs, thermal conductance increases roughly linearly with the fraction of −OH groups on the surface. Increasing nanoscale roughness increases solid–water contact area, and therefore the apparent thermal conductance. However, the inherent thermal conductance, which accounts for the increased contact area, shows only small and subtle variations. These variations are consistent with expectations based on recent work on the effects of nanoscale roughness on interfacial tension (Mittal and Hummer, Faraday Disc., 2010, 146, 341). Finally, we find that SAM–water interfaces show thermal rectification. Thermal conductance is larger when heat flows from the ordered SAM phase to the disordered liquid water phase, and the magnitude of rectification increases with surface hydrophilicity.

Formulation of protein biopharmaceuticals as highly concentrated liquids can improve the drug substance storage and supply chain, improve the target product profile, and allow greater flexibility in dosing methods. The Donnan effect can cause a large offset in pH from the target value established with the diafiltration buffer during the concentration and diafiltration of charged proteins with ultrafiltration membranes. For neutral formulations, the pH will typically increase above the diafiltration buffer pH for basic monoclonal antibodies and decline below the diafiltration buffer pH for acidic Fc-fusion proteins. In this study, new equations for the Donnan effect during the diafiltration and concentration of proteins in solutions containing monovalent and divalent ions were derived. The new Donnan models obey mass conservation laws, account for the buffering capacity of proteins, and account for protein-ion binding. Data for the pH offsets of an Fc-fusion protein and a monoclonal antibody were predicted in both monovalent and divalent buffers using these equations. To compensate for the pH offset caused by the Donnan effect, diafiltration buffers with pH and excipient values offset from the ultrafiltrate pool specifications can be used. The Donnan offset observed during the concentration of an acidic Fc-fusion protein was mitigated by operating at low temperature. It is important to account for the Donnan effect during preformulation studies. The excipients levels in an ultrafiltration pool may differ from the levels in a protein solution obtained by adding buffers into concentrated protein solutions due to the Donnan effect.

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