A new method for describing the helical conformation of collagen: dependence of the triple helical twist on amino acid sequence.
Collagen fibre diffraction patterns are typically interpreted assuming a monotonous, average triple helical conformation for the collagen molecule. Two different helical symmetries have been proposed: seven residues in two turns versus 10 residues in three turns. Collagen model peptides show predominantly the 7-fold symmetry but provide evidence for local changes in the helical twist, which are related to some extent to the local sequence of the peptides but also to the lattice interactions in the crystal. Thus, it is difficult to determine precisely to what degree the amino acid sequence dictates the fine details of collagen conformation. A new method is presented here in which an internal triple helical twist is defined. This method takes into consideration all three chains simultaneously, and facilitates investigating the sequence dependence of helical twist variation, the conformational consequences of collagen interruptions, and the effects on collagen conformation introduced upon receptor or ligand-binding. Analysis of the crystal structures of model peptides suggests that collagen varies gradually and continuously its helical twist according to the local distribution of imino acid residues, with the 7-fold and 10-fold symmetries representing the limits of this variation for the cases of imino acid saturation or absence, respectively.