In collagen, the collagen helix is a major shape in secondary structure. It consists of a triple helix made of the repetitious amino acid sequence glycine - proline - hydroxyproline. Each of the three chains is stabilized by the steric repulsion due to the pyrrolidone rings of proline and hydroxyproline residues . The pyrrolidone rings keep out of each other’s way when the polypeptide chain assumes this extended helical form, which is much more open than the tightly coiled form of the alpha helix. The three chains are hydrogen bonded to each other. The hydrogen bond donors are the peptide NH groups of glycine residues. The hydrogen bond acceptors are the CO groups of residues on the other chains. The OH group of hydroxyproline also participates in hydrogen bonding. The rise of the collagen helix (superhelix) is 290 picometres (0.29 nm) per residue.

See also : tertiary structure -- α helix -- β sheet