Sensitivity of the C–H Stretching Band in Raman Spectra to Phospholipid Order

Phospholipid bilayers, which are a major component of cellular membranes and some drug delivery vehicles, can be in different states of order depending on the conformations of the hydrocarbon tails and their mutual arrangement. The important task of experimental characterization of phospholipid order is often addressed using Raman spectra of the C–H stretching bands. Such characterization uses some empirical relationships for apparent maxima in the spectra, although the origin of the sensitivity of the C–H band to phospholipid order and its model description remain unclear. Surely, a correct description of the sensitivity of the C–H band to phospholipid order is critical for its proper application. Here, we provide a description of the ordering sensitivity of the C–H stretching band using a polarized Raman experiment with hydrated aligned multibilayers of a saturated phospholipid. By this way, Raman contributions from symmetric and antisymmetric vibrations of CH₂ were obtained in a model-free manner. Experiments in a wide temperature range and the use of isotopic isolation helped us to consider separately the effects of conformational and lateral order of chains. The conformational sensitivity of the spectrum of antisymmetric vibrations was confirmed by DFT modeling. The outcomes of the study allowed us to provide recommendations for the use of the Raman spectrum of the C–H stretching band to characterize the conformational and lateral order of phospholipid-containing materials.