Comment on the carbon-hydrogen stretching region of vibrational Raman spectra of phospholipids.

The broad, environmentally sensitive 2935-2910 cm-1 feature in the vibrational Raman spectra of phospholipids originates primarily from methylene symmetric carbon-hydrogen (C-H) stretching modes. For ordered acyl chains exhibiting local C2h symmetry at low temperatures, the infrared active methylene C-H asymmetric stretching modes become increasingly apparent at higher temperatures in the Raman spectrum as intramolecular chain disorder (trans-gauche isomerization) leads to a loss of chain symmetry. The 2935 cm-1 shoulder, assigned to the acyl chain methyl group C-H symmetric stretching vibration, is difficult to distinguish as a separate spectral transition in the chain disordered state. An ordering of lipid fluidity for several phospholipids is presented in terms of the intensities of features in the 3000-2800 cm-1 region with particular emphasis upon the 2935-2910 cm-1 contour.