Thermotropic Properties of Dioleoylphosphatidylethanolamine in Aqueous Dimethyl Sulfoxide Solutions

The effect of dimethyl sulfoxide (DMSO) on the phase behavior and structural parameters of aqueous dispersions of 1,2-dioleoylglycerophosphoethanolamine (DOPE) was characterized using differential scanning calorimetry and time-resolved synchrotron X-ray diffraction. In the range of concentrations of DMSO in water up to 50 wt%, there was a progressive increase in the temperature of the lamellar gel to lamellar liquid–crystal phase transition temperature and a marked decrease in temperature of the transition to the hexagonal II phase. At DMSO concentrations greater than about 25 wt% a direct transition from lamellar gel to hexagonal II structure was observed. A partial phase diagram of the phospholipid in DMSO has been constructed from thermal and X-ray diffraction data. Concentrations of DMSO greater than 50 wt% resulted in the formation of several lamellar crystal phases and interdigitated phases. Phase transition sequences measured on heating included low-temperature exothermic transitions between the interdigitated phase and lamellar crystal phases and an endothermic transition at about 30°C between a lamellar crystal and an interdigitated phase. This latter phase converted directly into an hexagonal II phase at a temperature of about 50°C. On cooling, a second nonbilayer phase (I) first appeared which subsequently reorganized to form a mixture of interdigitated and lamellar crystal phases. The significance of this data in terms of its possible relevance to the biological role of DMSO is discussed.