Formation of intermediate gel-liquid crystalline phase on medium-chain phosphatidylcholine bilayers: Phase...

The bilayer phase transitions of medium-chain phosphatidylcholines with linear saturated acyl chains (C_n = 12, 13 and 14) were measured by high-pressure light-transmittance measurements and differential scanning calorimetry to investigate the formation of intermediate gel-liquid crystalline phase called L_x phase. The constructed phase diagrams showed that there existed a distinct region of the L_x phase between ripple gel (P_β') and liquid crystalline (L_α) phase for multilamellar vesicle bilayers of C12PC and C13PC. The L_x phase of the C12PC bilayer was metastable at all pressures and disappeared at a higher pressure. In the C13PC bilayer, the L_x phase was stable and also disappeared at a higher pressure but its region markedly shrunk. By contrast, the L_x phase was not detected for the C14PC bilayer. Effects of other factors such as vesicle size and solvent substitution on the L_x phase of the C13PC bilayer were also examined. A decrease in vesicle size and solvent substitution from water to 50 wt% ethylene glycol solution promoted the L_x-phase formation as opposed to the effects of acyl-chain elongation and pressurization. The fluorescence data of the C13PC bilayer with different vesicle sizes showed that the L_x phase is caused by the difference of local packing in the bilayer. Considering these facts, we concluded that the L_x phase is an intermediate gel-L_α phase that has gel-phase monolayers with negative curvature and L_α-phase monolayers with positive curvature. The formation mechanism of the L_x-phase in stacked bilayers and dispersed vesicles is also explainable by this difference in packing state.