Low concentration of dioleoylphosphatidic acid induces an inverted hexagonal (H II) phase transition in dipalmitoleoylphosphatidylethanolamine membranes

We have investigated the effects of anionic dioleoylphosphatidic acid (DOPA) on the structure and phase behavior of dipalmitoleoylphosphatidylethanolamine (DPOPE) membranes by small-angle X-ray scattering. The results of X-ray diffraction experiments indicate that an L(alpha) to H(II) phase transition in DPOPE membranes occurred at 2.5 mol% DOPA, and above 4.0 mol% they were completely in the H(II) phase. And in the presence of 0.5 M KCl, the critical concentration of DOPA was decreased to 0.6 mol%. These results show that low concentrations of DOPA stabilize the H(II) phase rather than the L(alpha) phase in DPOPE membranes. The absolute spontaneous curvature of DPOPE membrane was gradually decreased with an increase in DOPA concentrations. On the basis of these results, the H(II) phase stability in DPOPE membranes due to low DOPA concentrations is discussed by the spontaneous curvature of monolayer membrane, the packing energy of alkyl chains of the membrane and lipid packing parameter.