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

Abstract

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) 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 L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) 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 L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.

Comprehensive characterization of temperature- and pressure-induced bilayer phase transitions for saturated phosphatidylcholines containing longer chain homologs

“…On the other hand, considerably large values and the acyl chain-length dependence of the H and V values cannot be explained by only the hydration change near the head group. Recently, we have found [54] that bilayers of some asymmetric PCs with different linear acyl chains in length are suitable for a study of the L c phase formation because they form the L c phase in a much shorter time than CnPCs, and because the formation is further accelerated by the application of pressure. Our previous high-pressure fluorometric study on a myristoylpalmitoylphosphatidylcholine bilayer suggested that the asymmetric PC molecules in the L c phase are probably arranged in staggered structure where polar head groups of the molecules are alternately protruding with respect to the membrane surface.…”

Thermotropic and Barotropic Phase Behavior of Phosphatidylcholine Bilayers

“…In general, it is well known that the L c phase formation of the bilayers of symmetric PCs with saturated acyl chains is very slow [26] and the formation requires a thermal treatment called annealing before experiments. On the other hand, the MPPC and PMPC can form the L c phase easily on account of the metastability of the L β ′ phase, in particular, the L c phase formation of the MPPC bilayer is very fast and the fast formation enables us to study the state of the L c phase in detail by a method of high-pressure fluorometry [38]. Moreover, there is a two-phase coexistence region (gray-colored area in the figure) of the L β ′ and L β I phases in the SPPC bilayer.…”