Determination of size and structure of lipid IV_A vesicles by quasi-elastic light scattering and small-angle X-ray scattering

Heavy sonication of lipid IV_A in different buffer solutions at pH values around 7.4 produces mostly unilamellar vesicles, which are stable over several weeks. For short sonication times these vesicles are very large, about 250 nm in terms of an average hydrodynamic radius with slight polydispersity, as determined by quasi-elastic light scattering. Therefore, small-angle X-ray scattering curves obtained with a conventional instrument do not contain enough information to determine the overall size of the lipid IV_A aggregates. However, the pair distance distribution function (PDDF) of the scattering curve is in agreement with the assumption of large vesicular particles. The thickness of the double layer is very small compared to the overall diameter. It can be evaluated assuming a lamellar particle. The corresponding PDDF provides a thickness of about 5 nm. The electron density distribution within the double layer can be calculated directly from the PDDF by a convolution square-root operation. Head groups and hydrocarbon chains are represented by two regions of different electron densities.