Deciphering the orientation of lipid molecules by principal component analysis of Raman mapping data

The orientation of lipid molecules is an essential characteristic of supported phospholipid layers, synthetic lipid structures, and biological specimens. Here, we perform Raman spectroscopy to analyze the orientation order in lipid structures. For this purpose, we studied dry oriented planar DMPC samples and multilamellar DPPC vesicles in water using Raman mapping. Principal component analysis (PCA) was applied to extract the information about the orientational order of lipid molecules. Using PCA, we revealed the features observed in the phospholipid spectra that are sensitive to hydrocarbon chain orientation relative to the polarization of laser radiation. These spectral features include Raman peaks corresponding to stretching C-C, twisting CH2, rocking and stretching CH3 modes. We suggest to use them as markers of hydrocarbon chain orientation along with light polarization. The proposed Raman analysis can be used to study samples with different levels of hydration.