Structure and polydispersity of single lipid vesicles by small-angle X-ray scattering at European XFEL

Structure analysis of supra-molecular assemblies is of considerable interest in a wide range of research fields, but also extremely challenging. Biomolecular colloidal particles, such as biological vesicles or small unilamellar lipid vesicles, for example, measure only a few tens of nanometers, and need to be probed in hydrated and functionalized environments. Small-angle X-ray scattering (SAXS) is a well established technique offering high resolution in solution and at room temperature. However, due to the average over an extremely large ensemble, SAXS yields information only about the average structure (size and electron density profile). Many structural details are lost or screened by polydispersity, as well as by powder averaging. To overcome these limitations, we study single vesicles sequentially by coherent diffractive X-ray imaging using femtosecond X-ray free-electron laser (XFEL) pulses. For these experiments, single vesicles surrounded by a thin water layer are delivered into a nano-focused XFEL beam by an aerosol injector. Following the ‘diffract-before-destroy’ principle, the individual vesicles are probed on time scales where radiation damage has not yet set in. This approach leads to the measurement of thousands of diffraction patterns that can now be analyzed without limits associated with ensemble averaging.