Effect of Sodium Chloride on Lipid Bilayers

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 molecular dynamics simulations.


Figure 1: Coordination of sodium ions by on average three lipid carbonyl oxygens and two water molecules.

Increasing sodium chloride concentration was found to decrease the self-diffusion of POPC lipids within the bilayer. Self-diffusion coefficients calculated from the 100 nanosecond simulations agree with those measured on a millisecond time scale using fluorescence correlation spectroscopy, suggesting that most of the relaxation processes relevant for lipid diffusion are faster than the simulation time scale. As the dominant effect, the molecular dynamics simulations revealed a tight binding of sodium ions to the carbonyl oxygens of in average three lipids leading to larger complexes with reduced mobility. Additionally, the bilayer thickens by approx. 2 Å 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.


Dynamics Simulation of Lipid Bilayers

The movie below shows a bilayer system with 512 POPC lipids. A pre-equilibrated POPC bilayer (kindly provided by Peter Tieleman) was used as a start structure. All simulations were performed in a periodic box filled with more than 20.000 water molecules, corresponding to a hydration level of approx. 40 waters/lipid, yielding a total system size of more than 80.000 atoms. All MD simulations were carried out using the GROMACS simulation suite. Electrostatic interactions were calculated with the Particle-Mesh Ewald Method (PME). The temperature was coupled to an external temperature bath at 300 K.


Electrostatic Potential Across a POPC Bilayer

Figure 2: Upper panel: Total electrostatic potential (averaged and symmetrized) across the membrane (system AS without NaCl black, BS with sodium chloride at 110mM concentration red, and CS at a NaCl concentration of 330mM green line). Lower panel: Contributions of water (dipole) potential (solid lines) and of POPC distribution (dashed lines) to the total electrostatic potential.

The total electrostatic potential is only slightly affected by the binding of sodium ions to the lipid headgroups (upper panel). However, the individual contributions (lower panel) differ significantly: The potential drop due to the lipid headgroups is increased, whereas the compensation due to the orientation of water dipoles is decreased. The potential contribution caused by the water dipoles exhibits an additional minimum when NaCl is present. Overall, the region of anisotropic dipole orientation is increased. Therefore, we expect an increased range of hydration phenomena is the presence of salt ions.

Publication

Böckmann, R. A.; Hac, A. E.; Heimburg, T.; Grubmueller, H.: Effect of sodium chloride on a lipid bilayer. Biophysical Journal 85 (3), pp. 1647 - 1655 (2003)
Go to Editor View