Session Q17: Focus Session: Hydrophobic Interactions at Multiple Scales in Biology

Water, Hydrophobic Interactions, and Polymer Collapse

The collapse of a hydrophobic polymer in water is a basic model for many-body hydrophobic interactions, and it holds promise of providing fundamental insights into biomolecular folding transitions. Here, we discuss simulations that probe the effects of monomer length scale along with the strength of monomer-monomer and monomer-water interactions on the thermodynamics of the hydrophobic polymer collapse transition.   

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, rho*G(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.   

Protein folding, stability, and solvation structure in osmolyte solutions hydrophobicity

The hydrophobic effect between solutes in aqueous solutions plays a central role in our understanding of recognition and folding of proteins and self assembly of lipids. Hydrophobicity induces nonideal solution behavior which plays a role in many aspects of biophysics. Work on the use of small biochemical compounds to crowd protein solutions indicates that a quantitative description of their non-ideal behavior is possible and straightforward. Here, we will show what the structural origin of this non-ideal solution behavior is from expression derived from a semi grand ensemble approach. We discuss the consequences of these findings regarding protein folding stability and solvation in crowded solutions through a structural analysis of the m-value or the change in free energy difference of a macromolecule in solution with respect to the concentration of a third component. This effect has recently been restudied and new mechanisms proposed for its origins in terms of transfer free energies and hydrophobicity.   

Effects of lengthscales and attractions on the collapse of hydrophobic polymers in water

Hydrophobic and hydrophilic hydration and interactions play important roles in biological and colloidal self assembly processes. More recently, lengthscale dependences and manybody effects in these interactions have received renewed attention. We will present results from theory and molecular dynamics simulations on hydration of and interactions between solutes and interfaces of varying chemistries (from hydrophobic to hydrophilic) and lengthscales. These simulations combined with those of folding-unfolding of hydrophobic polymers in water and mixed aqueous solutions provide insights relevant to biological assembly.   

Depletion when Water meets a Hydrophobic Surface

What happens when water is forced into contact with a hydrophobic surface? Our previous synchrotron X-ray reflectivity experiments (Phys. Rev. Lett., 2006) reported strong evidence for the existence of an angstrom-thick region of low-density at this interface. Here we report fresh experiments in which ethanol, a wetting fluid, is studied at these same surfaces to quantify the contribution from terminal methyl groups on the hydrophobic surface that are invisible to X-rays. The existence of a depletion layer when water meets a suitably hydrophobic surface is confirmed. Better quantification of its thickness emerges.   

Profile of the Interface between a Hydrophobic Surface and Water

Aqueous interfaces are ubiquitous and play a fundamental role in biology, chemistry, and geology. The structure of water near interfaces is of the utmost importance, including chemical reactivity and macromolecular function. Theoretical work by Chandler et al. on polar-apolar interfaces predicts that a water depletion layer exists between a hydrophobic surface and bulk water for hydrophobes larger than $\sim$20nm2 (a $\sim$4A in radius apolar molecule). Until now, what the interface really looks like remains in dispute since recent experiments give conflicting results: from complete wetting (no water depletion layer) to a water depletion layer. Those experiments that have found a water depletion layer report 40-70\% water in the depletion zone: 40 -70\% and a width of $\sim$3A. However, an alternative interpretation to the profiles exists where no depletion layer is required. By studying hydrophobic SAM surfaces against several water mixtures we obtained the hydrophobic/water profile by phase sensitive neutron reflectivity. With this model independent technique we observe a 2 times wider and drier depletion water layer: 6A thick and 0-25\% water. Given the level of disagreement, I will review the topic of immiscible interfaces and show how phase sensitive reflectometry is unique in obtaining nm resolution profiles without fitting bias.   

Reconstructing the dynamics of water near a model charged surface using inelastic x-ray scattering

Understanding the behavior of water near hydrophobic surfaces is fundamental to many aspects in biology and surface. From high resolution inelastic x-ray scattering measurements of the dynamical structure factor at 3rd generation synchrotron sources, we reconstruct the longitudinal (density) response function of water. We use this data set to investigate how water behaves at polar and non-polar surfaces via linear response theory. Preliminary data on this and on how water wets hydrophobic surface patches of different sizes will be presented.