Key Role of Choline Head Groups in Large Unilamellar Phospholipid Vesicles for the Interaction with and Rupture by Silica Nanoparticles.

For highly abundant silica nanomaterials, detrimental effects on proteins and phospholipids are postulated as critical molecular initiating events that involve hydrogen-bonding, hydrophobic, and/or hydrophilic interactions. Here, large unilamellar vesicles with various well-defined phospholipid compositions are used as biomimetic models to recapitulate membranolysis, a process known to be induced by silica nanoparticles in human cells. Differential analysis of the dominant phospholipids determined in membranes of alveolar lung epithelial cells demonstrates that the quaternary ammonium head groups of phosphatidylcholine and sphingomyelin play a critical and dose-dependent role in vesicle binding and rupture by amorphous colloidal silica nanoparticles. Surface modification by either protein adsorption or by covalent coupling of carboxyl groups suppresses the disintegration of these lipid vesicles, as well as membranolysis in human A549 lung epithelial cells by the silica nanoparticles. Furthermore, molecular modeling suggests a preferential affinity of silanol groups for choline head groups, which is also modulated by the pH value. Biomimetic lipid vesicles can thus be used to better understand specific phospholipid-nanoparticle interactions at the molecular level to support the rational design of safe advanced materials.

Modelling peptide adsorption energies on gold surfaces with an effective implicit solvent and surface model.

The interaction of proteins and peptides with inorganic surfaces is relevant in a wide array of technological applications. A rational approach to design peptides for specific surfaces would build on amino-acid and surface specific interaction models, which are difficult to characterize experimentally or by modeling. Even with such a model at hand, the large number of possible sequences and the large conformation space of peptides make comparative simulations challenging. Here we present a computational protocol, the effective implicit surface model (EISM), for efficient in silico evaluation of the binding affinity trends of peptides on parameterized surface, with a specific application to the widely studied gold surface. In EISM the peptide surface interactions are modeled with an amino-acid and surface specific implicit solvent model, which permits rapid exploration of the peptide conformational degrees of freedom. We demonstrate the parametrization of the model and compare the results with all-atom simulations and experimental results for specific peptides.

Liquid-liquid phase separation in an inhomogeneous ternary colloid-polymer mixture.

Suspended colloids are often considered as models for molecules, which are sufficiently big so that they can be observed directly in (light) microscopes and for which the effective interaction among each other can be tailored. The Asakura-Oosawa model of ideal colloid-polymer mixtures captures the idea of tuning the interaction between the colloids via a potential, which possesses a range set by the size of the polymers and an attractive strength characterized by the (reservoir) number density of the polymers, which plays the role of an inverse temperature. The celebrated Asakura-Oosawa depletion potential allows one to recreate the bulk phase diagram of a simple fluid by employing a colloid-polymer mixture. This has been verified in theory, by computer simulations, and via experiments. Here, we study the phase behavior of a confined colloid-polymer mixture with two polymer species. The sizes and densities are chosen such that the resulting bulk phase diagram exhibits a second stable critical point within the framework of the classical density functional theory. Our results suggest that a suitably tuned colloid-polymer mixture can be an interesting model system to study fluids with two critical points.

Toward a density-functional theory for the Jagla fluid.

The so-called Jagla fluid is well known to exhibit, in addition to the usual gas-liquid critical point, also a liquid-liquid critical point, as well as a density anomaly. This makes it an interesting toy model for water, for which a liquid-liquid critical point is considered to exist but so far eludes experimental verification due to crystallization occurring in the corresponding metastable, deeply supercooled state. With the Jagla fluid being understood quite well in bulk-mostly via simulation studies-the focus of the present study is to describe the spatially inhomogeneous fluid in terms of classical density-functional theory (DFT) with the aim to be able to control its phase behavior on changing the shape or the nature of the confinement of the fluid. This information might contribute to guide potential experimental tests of the liquid-liquid critical point of actual water. We first determine the bulk phase diagram for the Jagla fluid by using thermodynamical perturbation theory. In doing so we explain why the perturbation theories of Barker and Henderson as well as of Weeks, Chandler, and Anderson fail to describe the Jagla fluid. We then continue to construct a perturbative DFT based on our bulk model, which shows significant improvement over the standard mean-field DFT valid at high temperatures. But ultimately the perturbative DFT breaks down at state points close to the binodal line and at low temperatures. This prevents us from achieving the original aim to study a highly confined, inhomogeneous Jagla fluid close to its liquid-liquid binodal.

Bubble gating in biological ion channels: A density functional theory study.

We study the behavior of a waterlike liquid inside the gate of a biological ion channel following the basic geometry of the well studied potassium channel KcsA. We calculate the three-dimensional density distribution ρ(r) of the liquid within the framework of classical density functional theory and observe the formation of a low density region (bubble) when the gate is narrow. This observation corresponds to a finite-size form of capillary evaporation and supports the so-called bubble-gate theory. From the density profile we also compute the energy landscape of the gate and the energy required to change the gate from a closed (narrow) to an open (wide) state and vice versa.