Pressure studies on two hydrated phospholipids--1,2-dimyristoyl-phosphatidylcholine and 1,2-dipalmitoyl-phosphatidylcholine.

We present results of studies on the effect of pressure on phase transitions in 1,2-dimyristoyl-phosphatidylcholine (DMPC) and 1,2-dipalmitoyl-phosphatidylcholine (DPPC) dispersed in excess water. The P-T diagram of hydrated DMPC shows a Gel III-Gel II-Gel I triple point at 3.5 kbar, 41 degrees C, the Gel III phase being obtained by annealing the sample at high pressure for several hours. In the case of DPPC, a pressure induced phase (X) appears between the Gel II and Gel I phases at approximately 0.93 kbar. With increasing pressure the temperature range of the X phase increases at the expense of that of the Gel I phase until finally at 2.87 kbar, the latter is completely suppressed. The P-T diagram of water-rich DPPC thus has 2 triple points, the Gel II-X-Gel I triple point at 0.93 kbar, 42.5 degrees C and the X-Gel I-liquid crystal triple point at 2.87 kbar, 98.5 degrees C. A pressure induced Gel III-Gel II transition is also observed in DPPC in the pressure range of 1.7-3 kbar.