Polymers of Tripeptides as Collagen Models

The structures of four polyhexapeptide collagen models have been investigated by X-ray diffraction. It has been found that (Gly-Ala-Pro-Gly-Pro-Pro),, (Gly-Pro-Ala-Gly-Pro-Pro),, (Gly-Ala-Pro-Gly-Pro-Ala), and (Gly-Ala-Ala-GlyPro-Pro),, all show typical collagen-like diffraction patterns, although there is some variation among the polymers in modes of lateral packing and in axial repeat distances. A systematic investigation of sterically allowed collagen-like structures has been carried out for each of the four polyhexapeptides. The intensity ratios of the equatorial reflections were calculated for the various sterically possible structures and compared with the observed values. In this way, it has been shown that none of the polyhexapeptides adopts the conformation of the two-bonded model for collagen, even though, from their amino acid sequences, three of the polymers might be expected to do so. On the other hand, for all four polyhexapeptides, the analyses have revealed sterically acceptable one-bonded structures which are consistent with the intensity data. These structures all closely resemble that found for (Pro-Gly-Pro),, as described in Yonath & Traub (1969). It is suggested that collagen itself probably has the same one-bonded conformation as these synthetic model compounds.