(549d) Transport Across Fluid Interfaces and Lipid Membranes: Role of Interfacial Fluctuations

Transport of molecules and nanoparticles across liquid-liquid interfaces and flexible membranes of nanometer thickness plays an important role in various technological applications as well as biological processes. The rate of this transport is determined in part by the height of the energy barrier, which depends on the solute affinity for liquids separated by the interface as well as the membrane composition and density. Another factor affecting the solute transport is the local solute diffusivity associated with the Brownian forces acting on the solute. In a homogeneous solvent these forces are created by collisions between the solute and the solvent molecules occurring on the timescale significantly faster than the timescale of the solute motion. This allows one to approximate the Brownian forces as white noise and model the solute motion by a Langevin equation for the translational degrees of freedom of the solute.

Interfacial systems possess an additional source of the Brownian forces, namely forces caused by relatively slow fluctuations of the interfaces. The timescale of these fluctuations is comparable with the timescale of the solute transport. Therefore, these Brownian forces cannot be modeled as white noise and one needs to explicitly include them into the Langevin equation. The solute transport then corresponds to motion on a multi-dimensional energy landscape parametrized both by the solute and the interfacial degrees of freedom.

In this talk we present analysis of transport of a hydrophobic nanoparticle across a lipid bilayer. The system is investigated using coarse-grained molecular dynamics (MD) simulations and the free energy landscape of the system is reconstructed using constrained MD simulations. A simple model for the energy landscape of the solute-membrane system is proposed and validated. This model explicitly accounts for effects of the membrane undulations on the solute transport. The developed model is quite general and is expected to be applicable to a wide range of interfacial systems.