Phase diagram of 1,2-dioleoylphosphatidylethanolamine (DOPE):water system at subzero temperatures and at low water contents.

The phase behavior of partially hydrated 1,2-dioleoylphosphatidylethanolamine (DOPE) has been studied using differential scanning calorimetry and X-ray diffraction methods together with water sorption isotherms. DOPE liposomes were dehydrated in the H-II phase at 29 degrees C and in the L-alpha phase at 0 degrees C by vapor phase equilibration over saturated salt solutions. Other samples were prepared by hydration of dried DOPE by vapor phase equilibration at 29 degrees C and 0 degrees C. Five lipid phases (lamellar liquid crystalline, L-alpha; lamellar gel, L-beta; inverted hexagonal, H-II; inverted ribbon, P-delta; and lamellar crystalline, L-c) and the ice phase were observed depending on the water content and temperature. The ice phase did not form in DOPE suspensions containing < 9 wt% water. The L-c phase was observed in samples with a water content of 2-6 wt% that were annealed at 0 degrees C for 2 or more days. The L-c phase melted at 5-20 degrees C producing the H-II phase. The P-delta phase was observed at water contents of < 0.5 wt%. The phase diagram, which includes five lipid phases and two water phases lice and liquid water), has been constructed. The freeze-induced dehydration of DOPE has been described with the aid of the phase diagram. (C) 1999 Elsevier Science B.V. All rights reserved.