Effect of sodium chloride on a lipid bilayer

Electrostatic interactions govern structural and dynamical properties of membranes and can vary considerably with the composition of the aqueous buffer. We studied the influence of sodium chloride on a pure POPC lipid bilayer by fluorescence correlation spectroscopy experiments and molecular dynamics simulations. Increasing sodium chloride concentration was found to decrease the self-diffusion of POPC lipids within the bilayer. Self-diffusion coefficients calculated from the 100 ns simulations agree with those measured on a millisecond timescale, suggesting that most of the relaxation processes relevant for lipid diffusion are faster than the simulation timescale. As the dominant effect, the molecular dynamics simulations revealed a tight binding of sodium ions to the carbonyl oxygens of on average three lipids leading to larger complexes with reduced mobility. Additionally, the bilayer thickens by similar to2 Angstrom, which increases the order parameter of the fatty acyl chains. Sodium binding alters the electrostatic potential, which is largely compensated by a changed polarization of the aqueous medium and a lipid dipole reorientation.