Effect of electrostatic interactions on phase stability of cubic phases of biomembranes

Abstract

We investigated effect of electrostatic interactions due to surface charges on structures and stability of cubic phases of monoolein (MO) membrane using the small-angle X-ray scattering method. Firstly, we changed the surface charge density of the membrane by using dioleoylphosphatidic acid (DOPA). As increasing DOPA concentration in the membrane at 30 wt % lipid concentration, a Q224 to Q229 phase transition occurred at 0.6 mol % DOPA, and at and above 25 mol %, DOPA/MO membranes were in the Lα phase. NaCl in the bulk phase reduced the effect of DOPA. These results indicate that as the electrostatic interactions increase, the most stable phase changes as follows: Q224 → Q229 → Lα. The increase in DOPA concentration reduced the absolute value of spontaneous curvature of the membrane, |H0|. Secondly, we changed the surface charge of the membrane by adding a de novo designed peptide, which has net positive charges and a binding site on the electrically neutral membrane interface. The peptide-1 (WLFLLKKK) induced a Q224 to Q229 phase transition in the MO membrane at low peptide concentration. As NaCl concentration increases, the MO/peptide-1 membrane changed from Q229 to Q224 phase. The increase in peptide-1 concentration reduced |H0|. Based on these results, the stability of the cubic phases and the mechanism of phase transition between cubic phase and Lα phase are discussed.