Effect of High Pressure on a Lipid Non-Bilayer Phase

Abstract

We have developed a new optical method to monitor the lamellar to hexagonal_II phase transition of lipids under high pressure. This method depends on the changes in the transmittance property of a pelleted sample inside a windowed pressure cell. The pelleted sample is opaque to a focused He-Ne laser beam below the phase transition whereas the compacted lipid becomes translucent above the phase transition temperature. The change in transmitted light intensity is about an order of magnitude and allows us to identify the onset of the phase transition to within 0.1°C. We have found a large hysteresis in the heating and cooling scans of egg phosphatidylethanolamine that is independent of the rate of scan. In addition, pressure increases the phase transition temperature of the lamellar to hexagonal_II phase indicating that the hexagonalu phase is destabilized with respect to the lamellar phase under pressure.