Comparing the Permeability of DPPC Bilayers in the Gel Phase When Surrounded By Water and Ethanol

Lipid membranes function as barriers for all living cells, they separate and protect the complex actions of biological molecules within, from the surrounding environment. Therefore, understanding the fundamental transport phenomena across such barriers may unveil the chemical underpinnings of cell-environment interactions. On a much larger scale, stacked layers of these membranes form the protective, outermost coating of human skin known as the stratum corneum. Consisting of free fatty acids, cholesterol, and saturated long-chain ceramides, this layer prevents water loss, and fortifies against unwanted foreign substances. In this experiment simulations of these membranes were created in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) and thermodynamically equilibrated. For the sake of simplicity only DPPC phospholipids (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) were modeled in the following simulations. The DPPC lipids were formed in the densely packed gel phase bilayer allowing them little mobility at atmospheric pressure and 305K. Then a series of simulated tests were run on the bilayers to test their permeability. Density and percent occupied area were calculated. Programs to pull simulated water molecules through the bilayer were designed. A simple experiment using pure water and an 11% ethanol solution surrounding the bilayer was carried out to test the free energy profile of the water molecule as it moved through the bilayer.