Water Channel Formation and Ion Transport in Linear and Branched Lipid Bilayers

Abstract

The composition of natural lipid membranes varies greatly depending on the type of organism, and it has been observed that small variations in lipid composition affect dramatically the membrane properties, such as structural stability and solute permeability. One of the variations is the methyl-branched lipids commonly found in archaea and bacteria. Using molecular dynamics simulations, we studied the influence of methyl branching on the electric-field induced formation of water channels in lipid bilayers and ion transports through them. We employed a double lipid bilayer setup to create within a periodic box two water compartments separated by those two bilayers. One of the compartments contains an excess of cations, while the other an equal excess of anions. This setup creates an initial transmembrane potential controlled by the ion concentration in each water compartment. We compared the response of diphytanoylphosphatidylcholine (DPhPC) lipid bilayers that have multiple methyl-branches with that of the straight-chain dipalmitoylphosphatidylcholine (DPPC) lipids. We found that compared to the straight-chain DPPC lipids, branched DPhPC lipids require a higher critical transmembrane potential and a longer time for the membrane to break down, followed by water channel formation, and transport of anions and cations through the channel. We demonstrated that while adding methyl branches reduces the lateral diffusion of the lipids, different transport properties of branched lipids are mainly due to the bulkiness of the branched lipid tails resulting in different water channel morphologies. The transmembrane potential creates toroidal pores in the straight-chain lipid bilayers, but barrel stave pores in the branched-lipid bilayers the formation of which requires a higher transmembrane potential. Our results provided a deeper understanding of the ion transport process through lipid bilayer membranes and shed light on the transport of various molecules across the lipid membranes.