[Two types of tripeptide conformation in collagen. Calculation of the structure of (Gly-Pro-Ser)n and (Gly-Val-Hyp)n polytripeptides].

Conformational analysis of polypeptides (Gly-Pro-Ser)n and (Gly-Val-Hyp)n was carried out for collagen-like triple helical complexes (coiled coils with screw symmetry). The lowest energy structure of the first polymer (helical parameters t 52,8, h 0,282 nm) is very close to that of (Gly-Pro-Hyp)n. The hydroxyl group of a serine residue does not form any intramolecular hydrogen bonds in this structure. (Gly-Val-Hyp)n triple complex is shown to unwind to t 7,7, h 0,297 nm as a result of optimization procedure. These findings confirm the assumption, made earlier on the basis of conformational analysis of (Gly-Pro-Hyp)n, (Gly-Pro-Ala)n, (Gly-Ala-Hyp)n, (Gly-Ala-Ala)n, that the collagen triple helix contains stable wound triplets with proline in the second position, while the absence of imino acid in the 2nd position facilitates the unwinding of the triple helix. Thus, a collagen helix appears to have different parameters for the sites differing in the amino acid sequence. The values measured in the X-ray experiments (h 0,29 nm, t' 36) should be considered as a result of averaging. The model allows to reconcile the X-ray data for collagen and crystalline (Gly-Pro-Pro)10 oligomer.