Polarized Raman spectra of oriented dipalmitoylphosphatidylcholine (DPPC). I. Scattering activities of skeletal stretching and methylene vibrations of hydrocarbon chains

A method of recording polarized Raman spectra of oriented phospholipids obtained from vesicle solution is described. The polarization as well as the observation of the sample between crossed polarizers shows that the molecules of DPPC are oriented nearly perpendicularly to the support. Calculation of the Raman scattering activities has been carried out by means of a simple process based on symmetry properties and additivity of elementary tensors (of C–C bonds or of CH2 groups). Since the molecules are randomly distributed around the z (chain) and Z (sample) axes, the components only depend on the tilting angle. When applied to the skeletal stretching vibrations, the method made it possible to explain the anomaly observed with DPPC (and polyethylene or other phospholipids), i.e., the νs (C–C) and νa (C–C) Raman lines are, respectively, stronger in the ZZ and ZX polarizations, contrary to what is expected when motions of the carbon atoms are considered. The scattering activities were also calculated for the methylene (stretching, rocking, twisting, wagging, and bending) vibrations and comparison with experimental intensities indicates a tilting of the chains of 15°–20°.