POLYMERS OF TRIPEPTIDES AS COLLAGEN MODELS .V. AN X-RAY STUDY OF POLY(L-PROLYL-GLYCYL-GLYCINE)

The structure of (Pro-Gly-Gly)n has been determined from an X-ray diffraction study. The polymer forms left-handed helices, with each tripeptide corresponding to an axial translation of 9.3 Å and a rotation of 360 °, approximately equally divided between the three amino acid residues. (Pro-Gly-Gly)n thus has essentially the same conformation as polyproline II and several other related polymers. The chains are held together by two NH ... O hydrogen bonds per tripeptide to form double-layered sheets, with the two layers anti-parallel. All the pyrrolidine rings of the proline residues are on the outsides of the sheets, as are the carbonyl groups (one per tripeptide) which are not involved in interchain hydrogen bonding. A closely related structure has been derived for a complex of (Pro-Gly-Gly)n with formic acid. The conformation of the chains and the hydrogen bonding between them is apparently the same as in the dry form, but the sheets of polypeptide chains are further apart, providing space for the incorporation of formic acid. © 1969.