Geralized Three Effective Eigenmode Theory is an appropriate theory for analyses of IXS spectra from lipid bilayers. It allows one to extract: the dynamic structure factor, the phonon dispersion relation, the phonon damping, and thermal diffusion coefficients of the bilayers.
The phonon dispersion relation of a bilayer shows a deep minimum at the peak of the structure factor. This minimum frequency is lower in the LC phase than that in the gel phase.
Given a chain length, the high-frequency sound speed is higher in the gel phase than in the LC phase. For DMPC it is 2230 m/s vs 1936 m/s.
SUMMARY – (2)
The phonon frequency at the peak of the dispersion relation increases with the chain length while that at the valley decreases with the chain length.
Addition of cholesterol to DMPC, originally in the LC phase, has an effect of changing the dynamic structure factor and the phonon dispersion relation closer to that of the gel phase. This can be interpreted to mean that an addition of cholesterol to bilayer at LC phase temperature has a tendency to increase the order of lipid molecules in the bilayer and makes the bilayer more solid-like.
Addition of cholesterol to a bilayer hardly changes the thermal diffusion coefficient DT of the bilayer.
