Investigating lipid-lipid and lipid-protein interactions in model membranes by ToF-SIMS.

With the chemical imaging capability of ToF-SIMS, biological molecules are identified and localized in membranes without any chemical labels. We have developed a model membrane system made with supported Langmuir-Blodgett (LB) monolayers. This simplified model can be used with different combinations of molecules to form a membrane, and thus represents a bottom-up approach to study individual lipid-lipid or lipid-protein interactions. We have used ternary mixtures of sphingomyelin (SM), phosphatidylcholine (PC), and cholesterol (CH) in the model membrane to study the mechanism of domain formation and interactions between phospholipids and cholesterol. Domain structures are observed only when the acyl chain saturation is different for SM and PC in the mixture. The saturated lipid, whether it is SM or PC, is found to be localized with cholesterol, while the unsaturated one is excluded from the domain area. More complicated model membranes which involve a functional membrane protein glycophorin are also investigated and different membrane properties are observed compared to the systems without glycophorin.