Quantifying the Molar Percentages of Cholesterol in Supported Lipid Membranes by Time-of-Flight Secondary Ion Mass Spectrometry and Multivariate Analysis

The uneven cholesterol distribution among organelles and within the plasma membrane is postulated to be critical for proper cellular function. To study how interactions between cholesterol and specific lipid species contribute to the uneven cholesterol distribution between and within cellular membranes, model lipid membranes are frequently employed. Although the cholesterol distributions within membranes can be directly imaged without labels by using time-of-flight secondary ion mass spectrometry (TOF-SIMS), quantifying the cholesterol abundance at specific membrane locations in a label-free manner remains a challenge. Here, partial least-squares regression (PLSR) of TOF-SIMS data is used to quantitatively measure the local molar percentage (mol %) of cholesterol within supported lipid membranes. With the use of TOF-SIMS data from lipid membranes of known composition, a PLSR model was constructed that correlated the spectral variation to the mol % cholesterol in the membrane. The PLSR model was then used to measure the mol % cholesterol in test membranes and to measure cholesterol exchange between vesicles and supported lipid membranes. The accuracy of these measurements was assessed by comparison to the mol % cholesterol measured with conventional assays. By using this TOF-SIMS/PLSR approach to quantify the mol % cholesterol with location specificity, a better understanding of how the regional lipid composition influences cholesterol abundance and exchange in membranes may be obtained.