Molecular Imaging of Cholesterol and Lipid Distributions in Model Membranes

Over recent decades, lipid membranes have become standard models for examining the biophysics and biochemistry of cell membranes. Interrogation of lipid domains within biomembranes is generally done with fluorescence microscopy via exogenous chemical probes. However, fluorophores have limited partitioning tunability, with the majority segregating into the liquid-disordered phase, and fluorescence only strictly reports on the small percentage of tagged lipids. We present simple, label-free imaging of domain formation in lipid monolayers, with chemical selectivity in unraveling lipid and cholesterol composition in different domain types. Exploiting conventional vibrational contrast in spontaneous Raman imaging, combined with chemometrics analysis, allows for examination of ternary systems containing saturated lipids, unsaturated lipids, and cholesterol. We confirm features commonly observed by fluorescence microscopy and provide a quantitative thermodynamic analysis of cholesterol distribution at the single-monolayer level.