Formation of intermediate gel-liquid crystalline phase on medium-chain phosphatidylcholine bilayers: Phase transitions depending on the bilayer packing

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.

Membrane States of Saturated Glycerophospholipids: A Thermodynamic Study of Bilayer Phase Transitions

Bilayer membranes formed by phospholipids vary in their membrane states by undergoing phase transitions in response to various external environmental factors. Pressure is one of these important environmental factors, but there are very few studies on the effects of pressure on phospholipid bilayer membranes. It is possible to deepen our understanding of the membrane states of phospholipid bilayer membranes by combining information regarding temperature- and/or ligand-responsivity with that regarding pressure-responsivity. In this review, we thermodynamically characterize the bilayer phase transitions of three kinds of saturated glycerophospholipids, each with a different polar head group (phosphatidyl-ethanolamine (PE), -choline (PC) or -glycerol (PG)), and explain their various membrane states depending on temperature and pressure. Both temperature- and pressure-responsivity reveal inherent features of these bilayer membranes: the metastability of the gel phase for PE bilayer membranes, the polymorphism of the gel phases for PC bilayer membranes and morphological changes in bilayer aggregates for PG bilayer membranes.

How Do Membranes Respond to Pressure?

Bilayers formed by phospholipids are fundamental structures of biological membranes. The mechanical perturbation brought about by pressure significantly affects the membrane states of phospholipid bilayers. In this chapter, we focus our attention on the pressure responsivity for bilayers of some major phospholipids contained in biological membranes. At first, the membrane states and phase transitions of phospholipid bilayers depending on water content, temperature and pressure are explained by using the bilayer phase diagrams of dipalmitoylphosphatidylcholine (DPPC), which is the most familiar phospholipid in model membrane studies. Subsequently, the thermotropic and barotropic bilayer phase behavior of various kinds of phospholipids with different molecular structures is discussed from the comparison of their temperature--pressure phase diagrams to that of the DPPC bilayer. It turns out that a slight change in the molecular structure of the phospholipids produces a significant difference in the bilayer phase behavior. The systematic pressure studies on the phase behavior of the phospholipid bilayers reveal not only the pressure responsivity for the bilayers but also the role and meaning of several important phospholipids existing in real biological membranes.