Proline residues substituent effects

In the context of the collagen triple helix it can be concluded that the Cf-exo pucker of the proline in Yaa position is favored by stereoelectronic effects of a (4R)-OH substituent which also stabilizes the traus-Xaa-(4R)-Hyp peptide bond, thus preorganizing this residue in a conformation that best befits a triple helix. Conversely, in the Xaa position the Pro residue is preferred in the Cy-endo pucker the related dihedral angles are reported in Table 11.3. 

---