Anita Girelli, Maddalena Bin, Mariia Filianina, Michelle Dargasz, Nimmi Das Anthuparambil, Johannes Möller, Alexey Zozulya, Iason Andronis, Sonja Timmermann, Sharon Berkowicz, Sebastian Retzbach, Mario Reiser, Asaf Raza, Marvin Kowalski, Mohammad Sayed Akhundzadeh, Jenny Schrage, Chang Hee Woo, Maximilian D. Senft, Lara Franziska Reichart, Aliaksandr Leonau, Prabhu Rajaiah Prince, William Chèvremont, Tilo Seydel, Jörg Hallmann, Angel Rodríguez-Fernández, Jan‐Etienne Pudell, Felix Brauße, Ulrike Boesenberg, James Wrigley, Mohamed Youssef, Wei Lu, Wonhyuk Jo, Roman Shayduk, Trey Guest, Anders Madsen, Felix Lehmkühler, Michael Paulus, Fajun Zhang, Frank Schreiber, Christian Gutt, Fivos Perakis
Abstract Understanding protein motion within the cell is crucial for predicting reaction rates and macromolecular transport in the cytoplasm. A key question is how crowded environments affect protein dynamics through hydrodynamic and direct interactions at molecular length scales. Using megahertz X-ray Photon Correlation Spectroscopy (MHz-XPCS) at the European X-ray Free Electron Laser (EuXFEL), we investigate ferritin diffusion at microsecond time scales. Our results reveal anomalous diffusion, indicated by the non-exponential decay of the intensity autocorrelation function g 2 ( q , t ) at high concentrations. This behavior is consistent with the presence of cage-trapping between the short- and long-time protein diffusion regimes. Modeling with the δ γ -theory of hydrodynamically interacting colloidal spheres successfully reproduces the experimental data by including a scaling factor linked to the protein direct interactions. These findings offer insights into the complex molecular motion in crowded protein solutions, with potential applications for optimizing ferritin-based drug delivery, where protein diffusion is the rate-limiting step.