Conformational Properties of Polypeptide Models of Collagen

Individual collagen chains exist as Polypro II helices because of their large imino content and their super-coiling into the collagen triple helix is facilitated by Gly in every third position. Because of this, collagen may be considered as being made up of Gly-led triplets. One fourth of such triplets in collagen have the sequence Gly-Pro-X and another one fifth, Gly-X-Hyp, where X is an α-amino residue. The stereochemical properties of the imino peptide bond, the position of the imino residue in the sequence and its interactions with neighboring residues, determine the conformation. Synthetic polytripeptides of sequence (Gly-Pro-X)n generate collaqen-like conformations in aqueous solutions whereas (Gly-X-Pro)n sequences usually do not. This difference has been attributed to different H-bonding properties of X-NH in the two sequences. Stabilizing interactions are said to occur between the side chain of X and Pro ring atoms in X-Pro but not in Pro-X. We investigated this question using (Gly-Pro-Sar)n (I) and (Gly-Sar-Pro)n (II). (I) generated collagen-like conformations in aqueous solutions but (II) did not although some order was elicited in helix promoting solvents. Since NH-H bonds are not possible at Sar residues, they may not play a final role in stabilizing the collagen helix. It appears likely that non-bonding interactions of the imino residue with the residue on its C-terminal may play a significant role in stabilizing collagen-like conformations.