Effects of chain packing and chain mobility on the Raman spectra of biomembranes.

The degree of lateral crystal-like order between hydrocarbon chains in biomembrane systems can be estimated from Raman measurements in the C-H stretching region. Observations of the temperature dependence of the Raman spectra of crystalline n-C16H34 and the urea clathrate of n-C16H34 have enabled us to separate to some extent the overlapping effects of chain packing and chain mobility, effects that are normally not distinguished in considering lateral order. The mobility is associated with the freedom of an extended chain to rotate and twist about its long axis. A high degree of such motion must be ascribed to n-C16H34 in a urea clathrate in order to explain the unusual temperature behavior observed for Raman bands at 2885 and 1174 cm-1. Comparison of the temperature behavior of the Raman spectra of the clathrate with that of crystalline n-C16H34 permits the effects due to packing and to mobility to be distinguished. The same effects can be expected to be present in the Raman spectra of biomembranes.