AbstractThe influence of inserting certain residues (X) into a polytripeptide sequence conformed into a poly‐L‐proline II helix is examined theoretically. It is found that for sequences such as ‐Gly‐Pro‐X‐ and ‐Gly‐X‐Pro‐, the introduction of glycyl,L‐alanyl orL‐seryl residues in the X position destabilizes the helix so that it is no longer the most stable intramolecular form. On the other hand,L‐prolyl andL‐hydroxyprolyl residues cause the PP II helix to be most stable. Of the many stable intramolecular forms, the majority will not pack efficiently to form fiber or solid‐state structures. The Rich‐Crick and Ramachandran collagen model structures were examined in terms of a Gly‐Pro‐Ala sequence, the Ramachandran, one‐hydrogen‐bond structure, being the most stable. However, another triple‐strand structure for (Gly‐Pro‐Ala)n, is much more energetically favorable. Hence, it may be concluded that none of the aforementioned is an entirely satisfactory collagen model. The new triple helix conformation proposed by Traub, Yonath, and Segal for (Gly‐Pro‐Pro) is found to give a more favorable intramolecular conformation for (Gly‐Pro‐Ala)nthan those derived from other collagen models. It is concluded that the collagen molecule derives its stability from interchain interactions in proline‐sparse r