Nanometer-scale molecular organization in lipid membranes studied by time-of-flight secondary ion mass spectrometry.

The organization of lipid membranes plays an important role in a wide range of biological processes at different length scales. Herein, the authors present a procedure based on time-of-flight secondary ion mass spectrometry (ToF-SIMS) to characterize the nanometer-scale ordering of lipids in lipid membrane structures on surfaces. While ToF-SIMS is a powerful tool for label-free analysis of lipid-containing samples, its limited spatial resolution prevents in-depth knowledge of how lipid properties affect the molecular assembly of the membrane. The authors overcome this limitation by measuring the formation of lipid dimers, originating in the same nanometer-sized primary ion impact areas. The lipid dimers reflect the local lipid environment and thus allow us to characterize the membrane miscibility on the nanometer level. Using this technique, the authors show that the chemical properties of the constituting lipids are critical for the structure and organization of the membrane on both the nanometer and micrometer length scales. Our results show that even at lipid surface compositions favoring two-phase systems, lipids are still extracted from solid, gel phase, domains into the surrounding fluid supported lipid bilayer surrounding the gel phase domains. The technique offers a means to obtain detailed knowledge of the chemical composition and organization of lipid membranes with potential application in systems where labeling is not possible, such as cell-derived supported lipid bilayers.