Thermotropic Phase Behavior of Hydrogenated Soybean Phosphatidylcholine-Cholesterol Binary Liposome Membrane

By combination of differential scanning calorimetry (DSC) and fluorescence spectroscopy of 6-propionyl-2-(dimethylamino)naphthalene (Prodan), we elucidated the thermotropic phase behavior of hydrogenated soybean phosphatidylcholine (HSPC)-cholesterol binary liposome membrane which has similar lipid composition to Doxil (R), the widely used liposome product in treatment of various tumors. We found that the characteristic points at cholesterol mole fraction (X-ch)=0.023 and 0.077 correspond to the hexagonal lattice, in which cholesterol molecules are considered to be regularly distributed in all regions of HSPC lipid bilayer with 1:42 and 1:12 units, respectively, as static averaged structures. Apparent endothermic peak disappeared at X-ch=0.40 in the DSC thermograms, indicating the existence of single liquid ordered phase at X-ch>0.40. In addition, fluorescence measurements of Prodan and its lauroyl derivative in poly(ethylene glycol) (PEG)-modified liposomes indicated that PEG modification has a negligible effect on the phase behavior of HSPC-cholesterol binary liposome membrane. These results may provide useful information in developing novel liposome products whose stability and encapsulated drug release are controlled.