Session A44: Focus Session: Hydrophobic Interactions and Hydrogen Bonding Networks in Polymeric and Soft Matter Systems

More than one dynamic crossover in protein hydration water

We study by theory, simulations and experiments, the dynamics of the hydrogen bond (HB) network of a percolating layer of water hydrating lysozyme powder. Using dielectric spectroscopy we measure the temperature dependence of the relaxation time of proton charge fluctuations. These fluctuations are associated with the dynamics of the HB network of water molecules adsorbed on the protein surface. Using Monte Carlo simulations and mean-field calculations, we study the dynamics and thermodynamics of a coarse-grained model that successfully reproduces the properties of hydration water. Both experimental and model analyses are consistent with the interesting possibility of two dynamic crossovers, (i) at $\approx$ 252 K, and (ii) at $\approx$ 181 K. Because the experiments agree with the model, we can relate the two crossovers to the presence at ambient pressure of two specific heat maxima. The first is caused by fluctuations in the HB formation, and the second, at a lower temperature, is due to the reordering of the HB network.   

The effect of salt solutes on the relaxation dynamics of water from 65 to 720 GHz

During the past decade, a variety of measurement techniques have provided evidence that ions and other solute molecules effect the structure and dynamics of the water molecules directly surrounding them. Most of these experiments have employed infrared spectroscopy which explores vibrational relaxation of the hydration shell by observing int\textit{ra}molecular vibrations. Terahertz spectroscopy, in contrast is sensitive to int\textit{er}molecular dynamics. Here we use a vector network analyzer based\textit{ terahertz dielectric relaxation spectrometer} operating over the frequency range from 65 to 720 GHz. The literature on relaxation dynamics of water is extensive and variable. But these measurements clarify the situation and confirm that the dynamics of water over this regime are best described in terms of three Debye relaxation processes with the characteristic times of 8.56, 1.1 ps and 179 fs (at 25.0\r{ }C). Remarkably, while the relaxation times themselves are not sensitive to salt concentration, the relative strength of the relaxation modes depends in a systematic way on the solute molarity. We discuss these results by relating the salt concentration dependent strength of the three processes to the dynamics and structure of first three hydration shells. Our measurements shed light on the dynamics of hydration shells around solute molecules in a biologically relevant environment.   

Investigating How Contact Angle Effects the Interaction between Water and a Hydrophobic Surface

By definition hydrophobic substances hate water. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low-density region forms near the surface. Contact angle is a measure of how hydrophobic a surface is. We have employed an automated home-built Surface Plasmon Resonance (SPR) apparatus to investigate the effect of varying the contact angle on the depletion layer   

Mesoscale inhomogeneities in an aqueous ternary system

Aqueous solutions of certain low-molecular-weight organic compounds, such as alcohols, amines, or ethers, which are considered macroscopically homogeneous, show the presence of mysterious mesoscale inhomogeneities, order of a hundred nm in size. We have performed static and dynamic light scattering experiments in an aqueous ternary system consisting of tertiary butyl alcohol and propylene oxide. Tertiary butyl alcohol is completely soluble in water and in propylene oxide, and forms strong hydrogen bonds with water molecules. Based on results of the study, we hypothesize that the mesoscale inhomogeneities are akin to a micro phase separation, resulting from a competition between water molecules and propylene oxide molecules, wanting to be adjacent to amphiphilic tertiary butyl alcohol molecules. Coupling between two competing order parameters, super-lattice binary-alloy-like (``antiferromagnetic'' type) and demixing (``ferromagnetic'' type) may explain the formation of these inhomogeneities. Long-term stability investigation of this supramolecular structure has revealed that these inhomogeneities are exceptionally long-lived non-equilibrium structures that persist for weeks or even months.   

Statistical Mechanics of Thermosensitive Nanoparticle Binding: Quantyfing Hydrophobic Interactions in Bulk Solution

We present a novel method which allows one to quantify the binding energy between complex hydrophobic nanoparticles in bulk aqueous solutions by means of light scattering. We tested the method on the case of thermosensitive nanoparticles made of a solid polymeric core onto which a thermosensitive p-NIPAM microgel shell is grafted. The microgel shrinks above a critical T at which the hydrophobic attraction sets in. By means of a novel statistical mechanics model to interpret the data, we manage to demonstrate that the binding energy as a function of T of thermosensitive nanoparticles behaves like in the case of neat two-level systems, with a rather sharp transition from hard-sphere (hydrophilic) to attractive (hydrophobic) at the critical temperature. The model allows us to make a clear quantitative connection between the binding energy and the entropy change of the grafted microgel upon going from hydrophilic to hydrophobic (in turn related to the microgel structure). The methods presented in this work can be applied to quantify the binding energy of complex biomolecules in bulk solution, which is a major challenge in biophysics nowadays. Reference: A. Zaccone, J.J. Crassous, B. Beri, and M. Ballauff, Phys. Rev. Lett. 107, 168303 (2011).   

Effect of Valence of Counterions on the Structure of Charged Membranes, a Computer Simulation Study

Phospholipids have been investigated for a long period, due to its ability of self-assembling into bilayer structures which resemble biological membranes. But most of the studies have been limited on the neutral phosphatidylcholine based lipids. The understanding of charged membranes (e.g., phosphatidylserine) is very limited due to the repulsion between the charged groups on lipids. In the present work, we investigated the effect of different counter-ions on the structures of charged membranes formed by 1,2-dilauroyl-sn-glycoro-3-phospho-L-serine. Three kinds of counterions were investigated, from monovalent, to divalent, to trivalent ions. Molecular dynamics simulations were performed at all-atom level. We have calculated the area per lipid. And the interaction between counterions and COO$^{-}$ groups was found to dominate over that between counterions and PO$_{4}^{-}$ groups.   

Deconstructing Classical Water Models at Interfaces and in Bulk: Hydrophobic Interactions and Hydrogen Bonding

Using concepts from perturbation and local molecular field theories of liquids we divide the potential of the SPC/E water model into short and long ranged parts. The short ranged parts define a minimal reference network model that captures very well the structure of the local hydrogen bond network in bulk water while ignoring effects of the remaining long ranged interactions. This deconstruction can provide insight into the different roles that the local hydrogen bond network, dispersion forces, and long ranged dipolar interactions play in determining a variety of solvation and other properties of SPC/E and related classical models of water. We use these short ranged models along with local molecular field theory to quantify the influence of these interactions on the structure of hydrophobic interfaces and the crossover from small to large scale hydration behavior. The implications of our findings for theories of hydrophobicity and possible refinements of classical water models will also be discussed.   

A SAFT-based classical density functional for water

We present a new classical density functional for water based on a combination of Statistical Associating Fluid Theory (SAFT-VR) with the Fundamental Measure Theory (FMT) functional for the hard-sphere fluid. In the homogeneous limit, our functional reduces to the the published optimal SAFT model of Clark \emph{et al} [1]. By adding a single fitting parameter, we reproduce the bulk surface tension of water within a wide temperature range. We will present results for hydrophobic hard rods and spheres, including the temperature dependence of the hydrophobic interaction. \\[4pt] [1] G. Clak, A. Haslam, A. Galindo, and G. Jackson, Molecular Physics \textbf{104}, 3561 (2006).   

Morphology and chirality control self-assembly of sickle hemoglobin inside red blood cells

Sickle cells exhibit abnormal morphology and membrane mechanics in the deoxygenated state due to the polymerization of the interior sickle hemoglobin (HbS). In this study, the dynamics of self-assembly behavior of HbS in solution and corresponding induced cell morphologies have been investigated by dissipative particle dynamics approach. A coarse-grained HbS model, which contains hydrophilic and hydrophobic particles, is constructed to match the structural properties and physical description (including crowding effects) of HbS. The hydrophobic interactions are shown to be necessary with chirality being the main driver for the formation of HbS fibers. In the absence of chain chirality, only the self-assembled small aggregates are observed whereas self-assembled elongated step-like bundle microstructures appear when we consider the chain chirality. Several typical cell morphologies (sickle, granular, elongated shapes), induced by the growth of HbS fibers, are revealed and their deviations from the biconcave shape are quantified by the asphericity and elliptical shape factors.   

Secondary structure formation in peptide amphiphile micelles

Peptide amphiphiles (PAs) are capable of self-assembly into micelles for use in the targeted delivery of peptide therapeutics and diagnostics. PA micelles exhibit a structural resemblance to proteins by having folded bioactive peptides displayed on the exterior of a hydrophobic core. We have studied two factors that influence PA secondary structure in micellar assemblies: the length of the peptide headgroup and amino acids closest to the micelle core. Peptide length was systematically varied using a heptad repeat PA. For all PAs the addition of a C12 tail induced micellization and secondary structure. PAs with 9 amino acids formed beta-sheet interactions upon aggregation, whereas the 23 and 30 residue peptides were displayed in an apha-helical conformation. The 16 amino acid PA experienced a structural transition from helix to sheet, indicating that kinetics play a role in secondary structure formation. A p53 peptide was conjugated to a C16 tail via various linkers to study the effect of linker chemistry on PA headgroup conformation. With no linker the p53 headgroup was predominantly alpha helix and a four alanine linker drastically changed the structure of the peptide headgroup to beta-sheet, highlighting the importance of hydrogen boding potential near the micelle core.   

Friction and Hydration Repulsion Between Hydrogen-Bonding Surfaces

The dynamics and statics of polar surfaces are governed by the hydrogen-bonding network and the interfacial water layer properties. Insight can be gained from all-atomistic simulations with explicit water that reach the experimentally relevant length and time scales. Two connected lines of work will be discussed: 1) On surfaces, the friction coefficient of bound peptides is very low on hydrophobic substrates, which is traced back to the presence of a depletion layer between substrate and water that forms a lubrication layer. Conversely, friction forces on hydrophilic substrates are large. A general friction law is presented and describes the dynamics of hydrogen-bonded matter in the viscous limit. 2) The so-called hydration repulsion between polar surfaces in water is studied using a novel simulation technique that allows to efficiently determine the interaction pressure at constant water chemical potential. The hydration repulsion is shown to be caused by a mixture of water polarization effects and the desorption of interfacial water.