Structural and energetic model of the mechanisms for reduced self-diffusion in a lipid bilayer with increasing ionic strength.

Ionic concentration of the buffer strongly affects properties of a lipid membrane, such as membrane durability (e.g., in electroporation experiments), lateral diffusion coefficient, and zeta potential. The effect of ionic strength is studied by Monte Carlo simulations based on the improved Pink model with explicitly included interactions between lipid heads. We examine the energetic profile of the membrane, conformation of lipid molecules, and molecular interactions. The study is provided for dipalmitoyl-phosphatidylcholine (DPPC) membrane in the gel (300 K) and fluid (330 K) temperatures for the ionic strength in the range 10-3000 mM at several values of dielectric constant. At high ionic strength, the simulations indicate an increase of the membrane stability due to the screening of the repulsive forces between lipid heads, more stable conformation of lipid chains, and denser packing of the molecules. These effects may account for reduced lateral diffusion in the membrane, as observed in experiments. The simulation also suggests that chains tend to assume a more straightened configuration and the number of standing polar heads increases, which may contribute to thickening of the membrane. An increase of the head tilt dependent on ionic strength may account for the greater value of zeta potential. The model shows stronger electropermebilization of the membrane in external electric field when ionic strength is low.