4.3. AUC

To analyse the sedimentation coefficient distribution of the SARS-CoV-2 Spike protein and the binding of the different antibodies to the trimer, sedimentation velocity type experiments were performed using a Beckman Coulter Proteomlab XL-I analytical ultracentrifuge (Brea, CA, USA) equipped with an 8 hole rotor. Samples were incubated 30 min at room temperature or 2 days at 4 °C, followed by 4 h at 37 °C and diluted in PBS (Gibco, NY, USA) to a final volume of 380 µL with nominal Spike trimer concentration of 0.53 µM and nominal concentration of 1.73 µM in the case of antibodies. Mixtures were then incubated for 30 min at room temperature before the loading in a double sector cells with sapphire windows. PBS was used as reference at a volume of 390 µL. Interference optics was applied to register the change in refractive index difference at 40,000 rpm rotational speed at a nominal temperature of 20 °C. Experiments were repeated twice using the same batch of proteins stored under the exactly same conditions.