AbstractThe properties of collagen are affected by the replacement of Pro by imino acid analogues. The structural effect of the low‐level local substitution ofL‐azetidine‐2‐carboxylic acid (Aze) has been analyzed by computing the energy of CH3CO‐(Gly‐Pro‐Pro)4‐NHCH3triple helices in which a single residue of one strand has been replaced by Aze. When Aze is in position Y of a (Gly‐X‐Y) unit, low‐energy local deformations are introduced in the triple helix, i.e., it becomes more flexible. On the other hand, the flexibility of the triple helix is not increased with Aze in position X. The energy of the triple helix to coil transition is not changed significantly by this amount of substitution. In an earlier study, we have demonstrated that the regular substitution of Aze in every tripeptide distorts or destabilizes the triple helix to a large extent [A. Zagari, G. Némethy,&H. A. Scheraga (1990)Biopolymers, Vol. 30, pp. 967–974 ]. Thus, it appears that a high level of substitution is required to cause the observed chemical and biological effects of Aze on collagen. ©