The stability and structure of cholesterol-rich codispersions of cholesterol and phosphatidylcholine

In order to investigate the structure and stability of cholesterol-enriched dispersions of phosphatidylcholine (PC), cholesterol/dipalmitoyl phosphatidylcholine (DPPC) mixtures with molar ratios of 4 +/- 0.5/1 to 1/1 were dispersed in water by sonication. These dispersions comprise liposomes and unilamellar vesicles with diameters in the range 200-1800 A. The bilayers which have a repeat distance of 66 A in these particles at 20 degrees C can contain up to 4 mol cholesterol/mol PC when DPPC is used and about half this ratio with egg PC. these dispersions are metastable in that storage at either 4 or 20 degrees C leads to aggregation and precipitation of vesicles; in addition, there is a decrease in the cholesterol/PC molar ratio in the particles and formation of cholesterol monohydrate crystals. Cholesterol is released slowly and PC dispersions containing more than equimolar amounts of cholesterol can be stable for several months. The amount of free cholesterol was determined by differential scanning calorimetry from the heat associated with the transition at 157 degrees C from smectic liquid crystal to liquid cholesterol. Under conditions of gentle mixing, the maximum solubility of cholesterol in DPPC bilayers is 1.0 +/- 0.1 mol/mol PC when the mixture initially contains less than about 3 mol cholesterol/mol DPPC. This is consistent with published equilibrium phase diagrams which show that equimolar PC/cholesterol bilayers are stable in water.