Imaging Mass Spectrometry
Biological membranes are 2-dimensional fluids where components are, to a greater or lesser extent, free to diffuse in the plane of the membrane. Despite this fluidity, it is generally believed that local organization is present on some length and timescale, but it is very difficult to characterize this lateral organization. This is one of the grand challenges of structural biology. While no single method is likely to be universally useful, we have developed imaging mass spectrometry using a NanoSIMS instrument.
About this figure: We are using nanoscale secondary ion mass spectrometry (nanoSIMS, shown in A and B) to study lateral organization in model and biological membranes without perturbative labels. For example, we can quantify the composition of coexisting liquid phases in phase-separated supported monolayers using stable isotope labeled lipids (C). We can also take advantage of the distance-dependent recombination of atoms that occurs during the secondary ionization process to measure the distance between isotopically labeled molecules and to detect nanometer-scale lipid clusters in supported lipid bilayers (D).
The NanoSIMS instrument
The NanoSIMS instrument uses a cesium primary ion beam (50-nm spot size) to eject secondary ions from a given sample, allowing for high-resolution, label-free mass imaging, where molecular identity is encoded by isotopic substitution. Our lab was the first to demonstrate that this instrument could be used to image a sample as thin as a lipid bilayer and provide direct quantitative analysis of membrane domains [244, 263]. Recent work has focused on pushing the resolution beyond the roughly 50-nm spot size by measuring the yield of diatomic and triatomic negative ions formed by atom recombination between different molecules that within a few nm of each other [313]. This allows for super-resolved lateral correlation of isotopically labeled molecular species. We use this to gain quantitative information about the lateral organization of lipid bilayers on an unprecedented length scale.
[313] "Atomic Recombination in Dynamic Secondary Ion Mass Spectrometry Probes Distance in Lipid Assemblies: A Nanometer Chemical Ruler", Frank R. Moss, III and Steven G. Boxer, Journal of the American Chemical Society, 138, 16737-16744 (2016). [pdf]
[309] "Dynamic Reorganization and Correlation Among Lipid Raft Components", Mónica M Lozano, Jennifer S. Hovis, Frank R. Moss III, and Steven G. Boxer, Journal of the American Chemical Society, 138, 9996-10001 (2016). [pdf]