Phase separations in binary and ternary cholesterol-phospholipid mixtures

A pair of recent studies has reopened debate on the subject of phase separations in model bilayer mixtures of cholesterol (Choi) and dipalmitoyl-phosphatidylcholine (DPPC). Fluorescence microscopy methods have not been able to detect phase separations in binary DPPC-Chol mixtures that have been inferred from NMR studies. However, micron-scale phaseseparated liquid domains are observed by fluorescence in ternary mixtures of DPPC, Choi, and diphytanoyl-phosphatidylcholine (DiPhyPC). Here, a model of condensed complexes of Choi and DPPC is used to account for these results. In particular, it is shown that the orientation of tie-lines in ternary mixtures of DiPhyPC/DPPC/Chol is compatible with phase separation in binary DPPC/Chol mixtures.