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Collagen triple helix 2026-11-05
A 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.[ref]
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.
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 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.[ref]
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.
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.
G. Kartha 2026-11-05
G. Kartha worked with G. N. Ramachandran on the structure of collagen. Their 1955 paper developed a model in which three polypeptide chains wind around one another, contributing to the explanation of the collagen triple helix. The proposed arrangement had to accommodate the amino acid composition, hydrogen bonding and the pattern of experimental X-ray diffraction.[ref]
Structure of collagen by G. N. Ramachandran and G. Kartha. Nature 176, 593–595, 1955; original paper reprinted in National Medical Journal of India, Classics in Indian Medicine
The history illustrates how a structural model is tested. A geometrically plausible arrangement can still require revision if its predicted repeat disagrees with measured X-ray diffraction. The value of the model lies in connecting molecular packing with several independent observations, while leaving its detailed geometry open to correction. A proposed protein structure and a later experimentally refined atomic structure represent different stages of that process.[ref]
Structure of collagen by G. N. Ramachandran and G. Kartha. Opening discussion of the revised 1955 model and disagreement of the earlier proposal with diffraction data