Figure 3. Backbone amide distances measured in solution by NMR are sensitive measures of a flipped peptide bond.

A, Stick representations of the backbone conformations in the sugar-free and sugar-bound crystal structure (PDB code 3S5V and 3S5X, ^[3] respectively). A short distance (2.2 Å) between the backbone amide protons (H^N) of W77 and G78 in site 2 (dashed red arrows) in the sugar-free conformation is longer (4.4 Å) in the sugar bound conformation. Binding site 1 displays the sugar-bound conformation in the sugar-free crystal structure due to protein-protein contacts within the crystal. B, NOE cross peaks corresponding to the distances depicted in A for different temperatures (298 K, top; 277 K, bottom) between H^N W77 and H^N G78 (red) and between H^N G11/G78 and H^N G12/G79 (green). The intensity ratio between these NOE cross peaks indicates that both the sugar-free and the sugar-bound conformations are present. C, The different backbone conformations sampled by OAA in solution (magenta) match the sugar-free (cyan) and sugar-bound (yellow) conformations seen in the X-ray structures in both binding sites.