Fourier-transform infrared and Raman characterization of bilayer membranes of the phospholipid SOPC and its mixtures with cholesterol

Abstract

One of the primary roles of cholesterol (Chol) in the biological cell is to modulate the physical properties of the bilayer phospholipid membrane. Moreover, the effect of cholesterol on lipid bilayers is strongly dependent on the concentration, hence it can easily adapt to the changes in the cell temperature. Incorporation of cholesterol in membranes induces diverse changes in the bilayer properties, including variation of the bilayer thicknesses and changing the lipid order. Taking into consideration these physical and structural characteristics of the lipid membranes with cholesterol, as well as their optical birefringence, we apply the typical structural methods for studying these complex biological systems. We have used Fourier transform infrared (FTIR) and micro-Raman spectroscopy, aiming on study of the specific physical characteristics of the lipid membrane of the type 1-stearoyl-2-oleoyl-sn-glycerol-3-phosphocoline (SOPC). The analysis of the FTIR and Raman fingerprint spectral range, including band deconvolution, indicated that hydrogen bonds (HBs) exist between the hydroxyl groups of cholesterol and the carbonyl ester groups of the polar–apolar interface of the bilayer membrane. Upon insertion into the bilayer Chol actively participates in H-bonding at the CO sites, facilitates H-bonding of water to the PO2-site and relaxes the “improper H-bonding” of H2O molecules to the choline moiety. We also establish an overall ordering effect of Chol on the lipid bilayer. The interplay of cholesterol and water in realization of HB with the phospholipid moieties, in dependence on the Chol concentration, was analyzed.