X-ray crystallographic determination of a collagen-like peptide with the repeating sequence (Pro-Pro-Gly)

The crystal structure of the triple-helical peptide (Pro-Pro-Gly)10 has been re-determined to obtain a more accurate description for this widely studied collagen model and to provide a comparison with the recent high-resolution crystal structure of a collagen-like peptide containing Pro-Hyp-Gly regions. This structure demonstrated that hydroxyproline participates extensively in a repetitive hydrogen-bonded assembly between the peptide and the solvent molecules. Two separate structural studies of the peptide (Pro-Pro-Gly)10 were performed with different crystallization conditions, data collection temperatures, and X-ray sources. The polymer-like structure of one triple-helical repeat of Pro-Pro-Gly has been determined to 2.0 Å resolution in one case and 1.7 Å resolution in the other. The solvent structures of the two peptides were independently determined specifically for validation purposes. The two structures display a reverse chain trace compared with the original structure determination. In comparison with the Hyp-containing peptide, the two Pro-Pro-Gly structures demonstrate very similar molecular conformation and analogous hydration patterns involving carbonyl groups, but have different crystal packing. This difference in crystal packing indicates that the involvement of hydroxyproline in an extended hydration network is critical for the lateral assembly and supermolecular structure of collagen.