= Collagen triple helix
A \b[collagen triple helix] is a <protein structure> in which three <polypeptide> chains wind together into a right-handed superhelix; each individual chain has a left-handed, polyproline-II-like conformation. It is characteristic of <collagen> and differs from a single <alpha helix>. The close-packed structure connects <protein primary structure> to a three-chain assembly: glycine commonly occurs at every third residue in the triple-helical region, conventionally written Gly–X–Y.<Structure of collagen by G. N. Ramachandran and G. Kartha>[1955 revised triple-chain model; the contemporary 1956 clarification describes the refinement from the 1954 proposal.]{ref}<Crystal and molecular structure of a collagen-like peptide at 1.9 angstrom resolution by J. Bella and others>[Abstract: supercoiled polyproline-II helices and interchain hydrogen bonding.]{ref}
<Glycine>'s small side group permits close approach near the central axis. <Proline> and <hydroxyproline> are frequent at the other positions, while interchain <hydrogen bonds> and interactions with water help organize the assembly. A model segment with ten Gly–X–Y repeats has 30 residues per chain and 90 across three chains; the three-chain count is distinct from the three-residue sequence period.
The structural picture developed from fiber-diffraction constraints in work by <G. N. Ramachandran>, <G. Kartha> and other investigators, rather than from directly seeing every atom in an intact tissue. Later, a synthetic collagen-like peptide provided a sharper test. Its 1994 <X-ray crystallography> structure, reported at $1.9$ angstrom resolution, confirmed triple-helical packing and interchain bonding. A deliberately introduced glycine-to-alanine substitution caused local untwisting and replaced direct interchain contacts near the substitution with water-mediated bridges.<Crystal and molecular structure of a collagen-like peptide at 1.9 angstrom resolution by J. Bella and others>[Abstract: synthetic peptide, Gly-to-Ala substitution, hydration cylinder and altered local bonding.]{ref}
The experiment illustrates why sequence changes can affect a material: even a small side-group change can disturb local packing. The triple-helical molecule is one level of organization; the larger <collagen> fiber includes additional assembly beyond these three chains.
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