Low concentration of dioleoylphosphatidic acid induces an inverted hexagonal (HII) phase transition in dipalmitoleoylphosphatidylethanolamine membranes Original Research Article

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 Lα to HII phase transition in DPOPE membranes occurred at 2.5 mol% DOPA, and above 4.0 mol% they were completely in the HII 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 HII phase rather than the Lα 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 HII 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.