Small-angle X-ray scattering from lipid bilayers is well described by modified Caille theory but not by paracrystalline theory

X-ray scattering data at high instrumental resolution are reported for multilamellar vesicles of L(alpha) phase lipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidyl at 50 degrees C under varying osmotic pressure. The data are fitted to two theories that account for noncrystalline disorder, paracrystalline theory (PT) and modified Caille theory (MCT). The MCT provides good fits to the data, much better than the PT fits. The particularly important characteristic of MCT is the long power law tails in the scattering. PT fits (as well as ordinary integration with no attempt to account for the noncrystalline disorder) increasingly underestimate this scattering intensity as the order h increases, thereby underestimating the form factors used to obtain electron density profiles.