Brain-SM/SDPC/Cholesterol Phase Diagram

We report the solved 3-component phase diagram for the biologically relevant mixture of brain-SM/ SDPC (18:0,22:6)-PC/cholesterol. Polyunsaturated acyl chains can have special influences on the mixing and phase behaviors of lipid mixtures. Their high degree of unsaturation affects the physical properties of biomembranes in ways that are still not fully understood, and their high concentration in some membranes makes them key players in membrane structure. FRET techniques and fluorescence microscopy imaging of giant unilamellar vesicles (GUVs) were employed for phase boundary visualization and phase identification of this 3-component mixture. Fluorescent lipid probes having complementary partitioning behavior were used to distinguish between phases. Of particular interest is the “raft” region of Ld + Lo phase coexistence, which shows phase separation by FRET methods, yet GUVs appear uniform under microscopy. Taken together these results indicate nanoscopic Ld + Lo phase separation in brain-SM/ SDPC (18:0,22:6)-PC/cholesterol mixture.