Low-pH-Induced Lamellar to Bicontinuous Primitive Cubic Phase Transition in Dioleoylphosphatidylserine/Monoolein...

Electrostatic interactions (EIs) play important roles in the structure and stability of inverse bicontinuous cubic (Q_II) phases of lipid membranes. We examined the effect of pH on the phase of dioleoylphosphatidylserine (DOPS)/monoolein (MO) membranes at low ionic strengths using small-angle X-ray scattering (SAXS). We found that the phase transitions from lamellar liquid-crystalline (L_α) to primitive cubic (Q_II^P) phases in DOPS/MO (2/8 molar ratio) membranes occurred in buffers containing 50 mM NaCl at and below the final pH of 2.75 as the pH of the membrane suspension was decreased from a neutral value. The kinetic pathway of this transition was revealed using time-resolved SAXS with a stopped-flow apparatus. The first step is a rapid transition from the L_α phase to the hexagonal II (H_II) phase, and the second step is a slow transition from the H_II phase to the Q_II^P phase. We determined the rate constants of the first step, k₁, and of the second step, k₂, by analyzing the time course of SAXS intensities quantitatively. The k₁ value increased with temperature. The analysis of this result provided the values of its apparent activation energy, which were constant over temperature but increased with pH. This can be explained by an EI effect on the free energy of the transition state. In contrast, the k₂ value decreased with temperature, indicating that the true activation energy increased with temperature. These experimental results were analyzed using the theory of the activation energy of phase transitions of lipid membranes when the free energy of the transition state depends on temperature. On the basis of these results, we discussed the mechanism of this phase transition.