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

Abstract

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°C and in the L α phase at 0°C by vapor phase equilibration over saturated salt solutions. Other samples were prepared by hydration of dried DOPE by vapor phase equilibration at 29°C and 0°C. Five lipid phases (lamellar liquid crystalline, L α; lamellar gel, L β; inverted hexagonal, H II; inverted ribbon, P δ; 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°C for 2 or more days. The L c phase melted at 5–20°C producing the H II phase. The P δ 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.