Phase behavior of palmitoyl and egg sphingomyelin

Despite the biological significance of sphingomyelins (SMs), there is far less structural information available for SMs compared to glycerophospholipids. Considerable confusion exists in the literature regarding even the phase behavior of SM bilayers. This work studies both palmitoyl (PSM) and egg sphingomyelin (ESM) in the temperature regime from 3 degrees C to 55 degrees C using X-ray diffraction and X-ray diffuse scattering on hydrated, oriented thick bilayer stacks. We observe clear evidence for a ripple phase for ESM in a large temperature range from 3 degrees C to the main phase transition temperature (T-M) of similar to 38 degrees C. This unusual stability of the ripple phase was not observed for PSM, which was in a gel phase at 3 degrees C, with a gel-to-ripple transition at similar to 24 degrees C and a ripple-to-fluid transition at similar to 41 degrees C. We also report structural results for all phases. In the gel phase at 3 degrees C, PSM has chains tilted by similar to 30 degrees with an area/lipid similar to 45 angstrom(2) as determined by wide angle X-ray scattering. The ripple phases for both PSM and ESM have temperature dependent ripple wavelengths that are similar to 145 angstrom near 30 degrees C. In the fluid phase, our electron density profiles combined with volume measurements allow calculation of area/lipid to be similar to 64 angstrom(2) for both PSM and ESM, which is larger than that from most of the previous molecular dynamics simulations and experimental studies. Our study demonstrates that oriented lipid films are particularly well-suited to characterize ripple phases since the scattering pattern is much better resolved than in unoriented samples.