A method for detection of permeation events in Molecular Dynamics simulations of lipid bilayers

The cell membrane is one of the most important structures of life. Understanding its functioning is essential for several human knowledge areas, mainly how it controls the efflux of substances between the cytoplasm and the environment. Being a complex structure, composed of several classes of compounds such as lipids, proteins, sugars, etc., a convenient way to mimic it is through a phospholipid bilayer. The Molecular Dynamics simulation of lipid bilayers in solution is the main computational approach to model the cell membrane. In this work, we present a method to detect permeation events of molecules through the lipid bilayer, characterizing its crossing time and trajectory. By splitting the simulation box into well-defined regions, the method distinguishes the passage of molecules through the bilayer from artifacts produced by crossing molecules through the simulation box edges when using periodic boundary conditions. We apply the method to study the spontaneous permeation of water molecules through bilayers with different lipid compositions and modeled with different force fields. Our method successfully characterizes the permeation events, and the results obtained show that the frequency and time of permeation are independent of the force field used to model the phospholipids. Besides, it is observed that the increase in the concentration of cholesterol molecules in lipid bilayers induces the reduction of permeation events due to its compacting action on the bilayer, making it denser and, therefore, hindering the diffusion of water molecules inside it. The computational tool to perform the method discussed here is available on https://github.com/crobertocamilo/MD-permeation.