X-ray scattering from unilamellar lipid vesicles

An improved small-angle X-ray scattering (SAXS) method for determining asymmetric lipid bilayer structure in unilamellar vesicles is presented. From scattering theory, analytic expressions are derived for the bilayer form factor over flat and spherical geometries, assuming the lipid bilayer electron density to be composed of a series of Gaussian shells. This is in contrast to both classic diffraction and Guinier hard-shell SAXS methods which, respectively, are capable only of ascertaining symmetric bilayer structure and limited-resolution asymmetric structure. Using model fitting and direct calculation of the form factor, using only one equation, an asymmetric electron density profile of the lipid vesicle is obtained with high accuracy, as well as the average radius. The analysis suggests that the inner leaflet of a unilamellar lipid vesicle is 'rougher' than the outer one.