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.
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.
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.
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