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Figure 1
(a) Chemical drawings representing keto–enol tautomerism in cis- (lower row) and trans- (upper row) peptide bonds. Prepared with ChemDrawTM (Revvity Signals Software, Inc.). (b) Keto–enol tautomerization in PDB entry 6mu9. Shown are two peptide bonds between amino acids 298–300. Note the hydrogen-omit signal on the left peptide carbonyl, indicating that the C—O bond is protonated. 2mFo− Fc (grey) and mFo − Fc (green, positive) densities are both contoured at 3rms. Prepared with Coot version 0.98.96 (Emsley & Cowtan, 2004BB30).

IUCrJ
Volume 12| Part 3| May 2025| Pages 257-258
ISSN: 2052-2525
https://doi.org/10.1107/S2052252525003604