Coherent Diffractive Imaging of Lipid Vesicles and Synaptic Vesicles by Femtosecond X-Ray FEL Pulses Charlotte Neuhaus(1), Moritz Stammer(1), Jette Alfken(1), Karlo Komorowski (1), Markus Osterhoff (1), Richard Bean(2), Johan Bielecki(2), Raphael de Wijn(2), Romain Letrun(2), Safi Rafie-Zinedine (3), Juncheng E(2), Marcello Ganzella(4), Adrian Mancuso(2), Reinhard Jahn(4), Tim Salditt(1)* (1) Institute for X-Ray Physics, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany(2) SPB/SFX, European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany,(3) European XFEL, Holzkoppel 4, 22869 Schenefeld, Germany,(4) Dept Neurobiology, Max Planck Institute for Multidisciplinary Sciences, Am Fassberg 11, 37077 Göttingen, Germany. The structure of biological vesicles, in particular synaptic vesicles (SVs), as well as synthetic lipid model systems, in particular small unilamellar lipid vesicles (LVs), has been subject of abiding interest. Given the small size of both LVs and SVs (R≈20 nm), high spatial resolution is required to identify the distribution of lipids and protein constituents. Cryogenic electron microscopy studies of synaptic vesicles have revealed the outer and inner layer of proteins around the lipid bilayer [1]. However, the samples have to be cryogenically fixed for this technique and throughput is limited. Another standard technique for structural characterization is solution small angle x-ray scattering (SAXS), which enables the measurement of vesicles in a quasi-physiological environment combined with a high spatial resolution. However, due to the average over an extremely large polydisperse ensemble, SAXS yields information only about the average structure (size and electron density profile). The distribution function of structural parameters is not accessible, and many structural details are lost or screened by polydispersity, as well as by powder averaging [2,3]. To overcome these limitations, we now have performed coherent diffractive imaging experiments on single vesicles using single femtosecond x-ray free electron laser (XFEL) pulses. For these measurements, single vesicles surrounded by a thin buffer layer are delivered into a nano-focused XFEL beam by an aerosol injector. By the ‘diffract-before-destroy’ principle, the individual vesicles can be probed without radiation damage. This approach leads to the measurement of thousands of diffraction patterns that can now be analyzed and reconstructed. Two approaches were used for the analysis: First a classical CDI approach of iterative phase retrieval and second a classical SAXS approach of modeling the azimuthally averaged diffraction pattern. References: [1] Takamori, S. et al., (2006). Molecular Anatomy of a Trafficking Organelle. Cell, 127 (831-846)[2] Castorph, S. et al., (2010). Synaptic Vesicles Studied by SAXS: Derivation and Validation of a Model Form Factor. J Phys Conf Ser, 247[3] Komorowski, K. et al., (2022). Neurotransmitter uptake of synaptic vesicles studied by X-ray diffraction. Eur Biophys J, 51 (465-482)