Effect of Electrostatic Interactions on Phase Stability of Cubic Phases of Membranes of Monoolein/Dioleoylphosphatidic Acid Mixtures

Abstract

To elucidate effects of electrostatic interactions resulting from surface charges on structures and phase stability of cubic phases of lipid membranes, membranes of 1-monoolein (MO) and dioleoylphosphatidic acid (DOPA) (DOPA/MO membrane) mixtures have been investigated by small-angle x-ray scattering method. As increasing DOPA concentration in the DOPA/MO membrane at 30 wt% lipid concentration, a phase transition from Q 224 to Q 229 phase occurred at 0.6 mol% DOPA, and at and above 25 mol% DOPA, DOPA/MO membranes were in the L α phase. As NaCl concentration in the bulk phase increased, for 10% DOPA/90% MO membrane in excess water, a Q 229 to Q 224 phase transition occurred at 60 mM NaCl, and then a Q 224 to H II phase transition occurred at 1.2 M NaCl. Similarly, for 30% DOPA/70% MO membrane in excess water, at low NaCl concentrations it was in the L α phase, but at and above 0.50 M NaCl it was in the Q 224 phase, and then at 0.65 M NaCl a Q 224 to H II phase transition occurred. These results indicate that the electrostatic interactions in the membrane interface make the Q 229 phase more stable than the Q 224 phase, and that, at larger electrostatic interactions, the L α phase is more stable than the cubic phases (Q 224 and Q 229). We have found that the addition of tetradecane to the MO membrane induced a Q 224-to-H II phase transition and also that to the 30% DOPA/70% MO membrane induced an L α -to-H II phase transition. By using these membranes, the effect of the electrostatic interactions resulting from the membrane surface charge (DOPA) on the spontaneous curvature of the monolayer membrane has been investigated. The increase in DOPA concentration in the DOPA/MO membrane reduced the absolute value of spontaneous curvature of the membrane. In the 30% DOPA/70% MO membrane, the absolute value of spontaneous curvature of the membrane increased with an increase in NaCl concentration. On the basis of these new results, the phase stability of DOPA/MO membranes can be reasonably explained by the spontaneous curvature of the monolayer membrane and a curvature elastic energy of the membrane.