Figure 4. Backrubs at aromatic residues in antiparallel β-sheet.

Crystal structure ensembles for Phe/Tyr across from Gly (light blue) vs. anything else (light green) are related by a backrub. Lowest-energy BRDEE conformations for 1z84 Phe171 across from Gln188 (visually truncated at C_β for clarity) (dark green) vs. Gln188→Gly (dark blue) have a similar relationship. Aromatic C_α and C_β displacements for both average crystal structures (lighter, in parentheses) and low-energy BRDEE conformations (darker) evoke a hinge-like backrub operation. Ensemble mainchain-mainchain H-bonds are illustrated with “pillows” of green all-atom contact dots [44].