Discriminating and Imaging Different Phosphatidylcholine Species within Phase-Separated Model Membranes by Principal Component Analysis of TOF-Secondary Ion Mass Spectrometry Images

摘要

Time-of-flight secondary ion mass spectrometry (TOFSIMS)nenables chemically imaging the distributions ofnvarious lipid species in model membranes. However,ndiscriminating the TOF-SIMS data of structurally similarnlipids is very difficult because the high intensity, lownmass fragment ions needed to achieve submicrometernlateral resolution are common to multiple lipid species.nHere, we demonstrate that principal component analysisn(PCA) can discriminate the TOF-SIMS spectra ofnfour unlabeled saturated phosphatidylcholine species,n1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-ndimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-ndipalmitoyl-sn-glycero-3-phosphocholine (DPPC), andn1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) accordingnto variations in the intensities of their low massnfragment ions (m/z e 200). PCA of TOF-SIMS imagesnof phase-separated DSPC/DLPC and DPPC/DLPC membranesnenabled visualizing the distributions of eachnphosphatidylcholine species with higher contrast andnspecificity than that of individual TOF-SIMS ion images.nComparison of the principal component (PC) scoresnimages to atomic force microscopy (AFM) imagesnacquired at the same membrane location before TOFSIMSnanalysis confirmed that the PC scores imagesnreveal the phase-separated membrane domains. Thenlipid composition within these domains was identifiednby projection of their TOF-SIMS spectra onto PC modelsndeveloped using pure lipid standards. This approachnmay enable the identification and chemical imaging ofnstructurally similar lipid species within more complexnmembranes.