Phase behavior and glass transition of 1,2-dioleoylphosphatidylethanolamine (DOPE) dehydrated in the presence of sucrose

Abstract

The effect of sucrose on the phase behavior of 1,2-dioleoylphosphatidylethanolamine (DOPE) as a function of hydration was studied using differential scanning calorimetry and X-ray diffraction. DOPE/sucrose/water dispersions were dehydrated at osmotic pressures ( Π) ranging from 2 to 300 MPa at 30°C and 0°C. The hexagonal II-to-lamellar gel (H II→L β) thermotropic phase transition was observed during cooling in mixtures dehydrated at Π≤35 MPa. After dehydration at Π≥57 MPa, the H II→L β thermotropic phase transition was precluded when sucrose entered the rigid glassy state while the lipid was in the H II phase. Sucrose also hindered the H II-to-lamellar crystalline (L c), and H II-to-inverted ribbon (P δ) lyotropic phase transitions, which occurred in pure DOPE. Although the L c phase was observed in dehydrated 2:1 (mole ratio) DOPE/sucrose mixtures, it did not form in mixtures with higher sucrose contents (1:1 and 1:2 mixtures). The impact of sucrose on formation of the ordered phases (i.e., the L c, L β, and P δ phases) of DOPE was explained as a trapping of DOPE in a metastable H II phase due to increased viscosity of the sucrose matrix. In addition, a glass transition of DOPE in the H II phase was observed, which we believe is the first report of a glass transition in phospholipids.