Flexibility of phospholipids with saturated and unsaturated chains studied by Raman scattering: the effect of cholesterol on dynamical and phase transitions.

Raman scattering spectra were obtained at 25-320 K for bilayers prepared from saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mono-unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) phospholipids, with and without cholesterol. Raman intensities were measured at modes sensitive to lipid inter-chain interactions and/or intra-chain torsional motion (asymmetric CH2 stretching at 2880 cm(-1)) and to the conformational state of lipids (C-C stretching at 1130 cm(-1)). These intensities decreased with temperature, which could be ascribed to increased lipid flexibility. For cholesterol-free and cholesterol-containing DPPC bilayers, the decrease of Raman intensities observed above ∼200 K could be related to the phenomenon of dynamical transition known for biological systems near these temperatures. For a cholesterol-free POPC bilayer, the decrease of intensity for the asymmetric CH2 stretching mode started at a lower temperature, above 100 K, while the addition of cholesterol shifted this starting temperature to a more normal ∼200 K value. The low-temperature lipid flexibility in the case of POPC was related to the abundance of free-volume holes, which disappeared in presence of cholesterol. Near gel-fluid phase transitions, Raman intensities for cholesterol-free bilayers dropped sharply, while for cholesterol-containing bilayers, they changed smoothly.