(A) Sodium dodecyl sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) total protein stain of RBD variants and angiotensin-converting enzyme-2 (ACE-2). (B) Size-exclusion chromatography (SEC) profiles of the purified proteins run in a BioSep-SEC-S3000 column. Purity was determined by peak integration with the Empower software. (C) Thermal denaturation curves of the RBD wild-type (wt), N439K, and N501Y variants. Data are represented as the first derivative of the intrinsic fluorescence ratio 350:330 nm of the mean of three replicates. Vertical dashed lines represent the inflection temperatures (Ti). Biolayer interferometry (BLI) sensorgrams of RBD wt (D), N439K (E), and N501Y (F) binding to ACE-2-Fc immobilized in anti-human Fc capture (AHC) sensors. ACE-2-immobilized sensors were dipped into 7- to 11-point dilution series of RBD for 500 s, followed by dissociation for another 500 s.