Fig. 6. Ball and stick figures for 4 examples of inter amino acid interaction in RBD: (a) Gly504 with Asp405, Val503, and Tyr505. Gly504 has the highest total AABP value of 1.513 e⁻, which originates from exceptionally strong C–N bonds with its NN Val503 and Tyr505. The contribution of 0.070 e⁻ from off-diagonal bonding to Asp405 is a single O⋯H HB. (b) Interaction of Lys386 with Ser383, Asp389, Leu387, and Thr385. Lys386 has a large AABP value of 1.171 e⁻ and the larger contribution from off-diagonal part of 0.289 e⁻ (0.256 e⁻ + 0.019 e⁻ + 0.015 e⁻). (c) Interaction of Asp389 with Lys386, Leu390, and Asn317. Asp38 has the large off-diagonal AABP of 0.270 e⁻. (d) Interaction of Cys336 with Leu335, Pro337, Phe338, Cys361 and Val362. Cys336 is bonded to another Cys (Cys361) with an S–S bond and a large off-diagonal AABP of 0.254 e⁻. An interesting observation is the relatively strong S–S bond with a BO value of 0.173 e⁻ between Cys336 and Cys361 that contribute to the large off-diagonal AAB. In the above, the bond order for the bonds are marked. The dashed lines represent the HB. Some of them have regular covalent O–H bond such as between Lys386 and Asp389 in (b) and (c) with bond order of 0.256 e⁻ and bond length of 1.030 Å.