Stretch Vibrations of CH2 as a Measure of Conformational and Lateral Orders in Fatty Acid and Phospholipid Layers

The intensity of the line of antisymmetric vibrations of CH2 in the Raman spectrum of molecular layers of fatty acids and phospholipids is sensitive both to the molecular order in the layer (lateral order) and to the conformational order of the hydrocarbon chain of the molecule. Though the intensity of this line is widely used as an indicator of order in molecular layers containing hydrocarbon tails, the nature of sensitivity of the line of antisymmetric vibrations to the order is still disputable. This issue has been experimentally studied, and it has been demonstrated that disordering leads to redistribution of vibrational modes and to changes in polarizability of antisymmetric vibrations. A hypothesis is proposed that the intensity of the line of the Raman scattering of antisymmetric vibrations of CH2 is determined to a large extent by their interaction with electron excitation of the hydrocarbon chain.