Raman study of temperature-induced hydrocarbon chain disorder in saturated phosphatidylcholines

Raman spectra of hydrated bilayers of 1,2-dilauroyl-sn-glycero-3-phosphocholine (12:0 PC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (14:0 PC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (18:0 PC) were studied in a wide temperature range to characterize the temperature-dependent hydrocarbon chain disorder of the saturated phosphatidylcholines. Temperature dependencies of the Raman line intensities were analyzed for the antisymmetric CH₂ stretching and CC stretching vibrations, which are sensitive to the lipid order. It was found that chain disordering processes occur significantly below the gel-fluid transition. The chain conformational order characterized by the CC stretching line intensity can be well described by a model involving the excitation of the ordered conformational state to the kinked and highly disordered, fluid-like state. A relation between the excitation energy to the disordered state and the enthalpy of the gel-fluid transition is discussed, including also data for other phosphatidylcholines studied before. Temperature behavior of the antisymmetric CH₂ stretching line indicates that non-conformational degrees of freedom are released above ∼ 200 K. Experimental findings concerning the hydrocarbon chain length dependence of the Raman polarizability of antisymmetric CH₂ stretching vibrations and a low-temperature solid-solid phase transition, identified in 12:0 PC at heating, are also discussed in the work.