Abstract

Saturated sphingomyelin (SM) lipids are implicated in lipid rafts in cell plasma membranes. Here we use fluorescence microscopy to observe coexisting liquid domains in vesicles containing SM, an unsaturated phosphatidylcholine lipid (either DOPC or POPC), and cholesterol. We note similar phase behavior in a model membrane mixture without SM ($\mathrm{DOPC}/\mathrm{DPPC}/\mathrm{Chol}$), but find no micron-scale liquid domains in membranes of $\mathrm{POPC}/\mathrm{PSM}/\mathrm{Chol}$. We delineate the onset of solid phases below the miscibility transition temperature, and detail indirect evidence for a three-phase coexistence of one solid and two liquid phases.