Figure EV4. The diprolyl peptide has a P_II‐helix geometry.

Unlike other polypeptides, polyproline peptides have a rigid fold in which proline residues lack freedom to rotate around the peptide bond. This is because cyclic structure of proline residues creates stereochemical constraints for polyproline peptides in which torsion angles ψ, φ, and θ should remain close to 150°, −75°, and 180° (for trans‐proline conformation), respectively. With these torsion angles, polyproline peptides inevitably fold into a characteristic secondary structure—P_II‐helix.

1. A Ramachandran plot, calculated for the diprolyl peptide, illustrates that the peptide has parameters of P_II‐helix. Red spot indicates torsion angles in the diprolyl peptide, labels show average values for the three most common elements of protein secondary structure—α‐helix, β structure, and P_II‐helix.
2. The diprolyl peptide structure shows positions and values of the major torsion angles. Semitransparent spheres represent van der Waals radii of the diprolyl peptide atoms.
3. Side‐by‐side structures of the diprolyl peptide, observed in this study, and an ideal P_II‐helix. Residues are numbered from the C‐ to N‐terminus.