Effect of electrostatic interactions on phase stability of cubic phases of membranes of monoolein/dioleoylphosphatidic acid mixtures

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-alpha phase. As NaCl concentration in the bulk phase increased, for 10% DOPA/90% MO membrane in excess water, a Q229 to Q224 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 Lq phase, but at and above 0.50 M NaCl it was in the Q224 phase, and then at 0.65 M NaCl a Q224 to H-II phase transition occurred. These results indicate that the electrostatic interactions in the membrane interface make the Q229 phase more stable than the Q224 phase, and that, at larger electrostatic interactions, the Lq phase is more stable than the cubic phases (Q224 and Q229). 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 La-to-H 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.