Study on the Subgel-Phase Formation Using an Asymmetric Phospholipid Bilayer Membrane by High-Pressure Fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature–pressure phase diagram. The large portion of the lamellar gel (Lβ′), ripple gel (Pβ′), and pressure-induced gel (LβI) phases exist as metastable phases due to the extremely stable subgel (Lc) phase. The stable Lc phase enables us to examine the properties of the Lc phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable LβI phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the Lc phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″min) values changed with pressure, indicating that the Lc phase has highly pressur...