Maximum solubilities of sterols and cholesterol induced domain formation in lipid membranes

Sterol is a major component of cell membranes in living cells. A characteristic sterol is cholesterol in animal cell membranes, stigmasterol in plants, and ergosterol in fungi. The maximum solubility of sterol in a lipid bilayer is the highest mole fraction of sterol that can be incorporated into a lipid bilayer before sterol crystals precipitate. We have investigated how the maximum solubility of cholesterol depends on the sizes of lipid head groups in POPC/POPE/cholesterol lipid bilayers and on the charges of head groups in POPS/cholesterol and POPG/cholesterol lipid bilayers. We found that the maximum solubility of cholesterol increases linearly as a function of the ratio POPC/(POPE+POPC). Our fluorescence microscopy measurements reveal phase separations and large domain formation in the mixture of POPC/POPE lipid bilayers with high amount of cholesterol. We measured maximum solubility limits of ergosterol and stigmasterol molecules in PC lipid bilayers. Our results show that minor differences in sterol structure could result in large differences in sterol-phospholipid interactions. The maximum solubility of di16:0DAG in POPC lipid bilayers in the presence of cholesterol is measured using light scattering technique. Optical microscopy experiment detected the existence of DAG crystals and confirmed solubility limits obtained by light scattering measurement. Our results show that light scattering is a reliable method to measure the maximum solubility.