Low pH Induces an Interdigitated Gel to Bilayer Gel Phase Transition in Dihexadecylphosphatidylcholine Membrane

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (LβI) phase in excess water at 20°C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from LβI phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in Lβ′ phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature (Tm) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced LβI to bilayer gel phase transition in DHPC-MLV and the increase in its Tm is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases.