The SAXS data were analyzed using a version of the "three shell" model. We have previously described this model in detail , and will therefore only provide a summary here, as well as complete mathematical description in SI. In this case, the vesicles are described by three concentric shells around a central core. The inner core corresponds to the inner aqueous vesicle pocket, while the three shells correspond to the inner headgrups, the hydrophobic tails and the outer headgroups, respectively, of the lipid bilayer. With absolute intensity, the lipid and peptide aggregation numbers (i.e average number of lipids and peptides per vesicle) are determined by enforcing mass balance and space-filling, as well as assuming no bulk water in the central hydrophobic shell. These are used, along with the molecular volumes to explicitly calculate the scattering length density (SLD) of each shell (See SI).