Effects of electrostatic interaction on the phase stability and structures of cubic phases of monoolein/oleic acid mixture membranes.

To elucidate effects of electrostatic interactions due to the surface charges on structures and phase stability of cubic phases of lipids, the mixture membranes of monoolein (MO) and oleic acid (OA) (OA/MO membrane) have been investigated by small-angle X-ray scattering method. As increasing OA concentration in the OA/MO membrane, a phase transition from Q(224) to Q(229) phase occurred, and above 1.0 mol% OA, OA/MO membranes were in the Q(229) phase. However, when NaCl concentration in the bulk phase is large, the Q(224) phase is more stable than the Q(229) phase in the OA/MO membranes containing high concentrations of OA. These results indicate that the electrostatic repulsion between the headgroups of these membranes makes the Q(229) phase more stable than the Q(224) phase. We have also found that lowering pH induces a phase transition from Q(229) to Q(224), and at lower pH a phase transition from Q(224) to H(II). These results are discussed in terms of the spontaneous curvature of the monolayer membrane and a lipid packing parameter of the membrane.