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Extended Data Fig. 6 | Measurement of the binding aff inities between the
RBDs and ACE2 by SPR assay using Biacore. Purified recombinant RBDs were
covalently immobilized on the sensor chip through their amine groups and
purified recombinant ACE2 f lowed over the RBDs. ACE2 was diluted to
different concentrations (from 5 to 80 nM for SARS-CoV-2 RBD and chimeric
RBD, and from 20 to 320 nM for SARS-CoV RBD) before being injected. The
resulting data were fit to a 1:1 binding model. Each experiment was repeated
independently twice with similar results. Each time, five different protein
concentrations were used to calculate the Kd values.