Low pH Stabilizes the Inverted Hexagonal II Phase in Dipalmitoleoylphosphatidylethanolamine Membrane.

Dipalmitoleoylphosphatidylethanolamine (DPOPE) membrane is in the L(α) phase in neutral pH at 20 °C. The results of small-angle X-ray scattering (SAXS) indicate that an L(α) to H(II) phase transition in DPOPE membranes occurred at pH 1.9 in the absence of salt, and at pH 2.8 in the presence of 0.5 M KCl, at fully hydrated condition at 20 °C. The spontaneous curvature of DPOPE monolayer membrane did not change with a decrease in pH values. To elucidate the mechanism, we have investigated the effect of the cationic dioctadecyldimethylammonium (DODMA) on the structure and phase behavior of DPOPE membrane. The result shows that DODMA stabilizes the H(II) phase rather than the L(α) phase in DPOPE membrane at its low concentrations. Based on these results, the H(II) phase stability of DPOPE membrane due to low pH is discussed in terms of the spontaneous curvature of the monolayer membrane and the packing energy of acyl chains in the membrane.