Temperature-Dependent Hydrocarbon Chain Disorder in Phosphatidylcholine Bilayers Studied by Raman Spectroscopy

Raman scattering of five phosphatidylcholines [1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dilinoleoyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dilignoceroyl-sn-glycero-3-phosphocholine (PC24)] was studied in a wide temperature range. These phospholipid bilayers differ in the temperature of the gel–fluid transition T m (from −57 to +80 °C) and in the number of unsaturated bonds. For all lipids, the temperature dependence of both asymmetrical methylene stretching and C–C stretching bands provides evidence that the disordering processes occur significantly below T m. Temperature onset of the decrease of Raman intensity of the asymmetrical methylene stretching band is the same for unsaturated lipids (DLPC, POPC, and DOPC), which was interpreted as the importance of packing defects in the bilayers of these phospholipids. The chain conformational order was characterized by Raman intensity of the high-frequency C–C stretching mode. An approach was used where the Raman intensity of this mode serves as a measure of hydrocarbon chains in the ground conformational state. Temperature dependence of the chains in the ground conformational state was well described by a simple model with the ground and two excited states: the kinked state and the highly disordered, fluidlike state.