Fig. 2. (a) The ribbon structure of SD1. Ball and stick figure showing (b) SD1 with all 24 AA marked, (c) interaction between the first three amino acids Ser305, Phe306, and Thr307, and (d) interaction among four AAs Glu309, Lys310, Gly311, and Tyr313. Both shows two NNs forming covalent bonds (solid lines) and one off-diagonal HB (dashed line). (e) The ribbon structure of RBD.