Studies of short-wavelength collective molecular motions in lipid bilayers using high resolution inelastic X-ray scattering.

We summarize a series of experimental results made with the newly developed high resolution X-ray scattering (IXS) instrument on two pure lipid bilayers, including dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) in both gel and liquid crystal phases, and lipid bilayers containing cholesterol. By analyzing the IXS data based on the generalized three effective eigenmode model (GTEE), we obtain dispersion relations of the high frequency density oscillations (phonons) of lipid molecules in these bilayers. We then compare the dispersion relations of pure lipid bilayers of different chain lengths among themselves and the dispersion relations of pure lipid bilayers with those of the cholesterol containing bilayers. We also compare our experimental results with collective dynamics data generated by computer molecular dynamics (MD) simulations for dipalmitoylphosphatidylcholine (DPPC) in gel phase and DMPC in liquid crystal phase.