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

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 Q224 to Q229 phase occurred at 0.6 mol% DOPA, and at and above 25 mol % DOPA, DOPA/MO membranes were in the La 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 Q224 to HII 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 La phase, but at and above 0.50 M NaCl it was in the Q 224 phase, and then at 0.65 M NaCl a Q224 to HII 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 La phase is more stable than the cubic phases (Q 224 and Q229). We have found that the addition of tetradecane to the MO membrane induced a Q224-to-HII phase transition and also that to the 30 % DOPA/70 % MO membrane induced an La-to-HII phase transition. By using these membranes, the effect of the electrostatic interactions resulting from the membrane surfac