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 (ice and liquid water), has been constructed. The freeze-induced dehydration of DOPE has been described with the aid of the phase diagram.