Electron microscopy of the collagen fibril

Collagen is identified by those properties that stem from the predominantly triple-chain helical structure of its molecules. A prerequisite for the formation of this triple helix is a Gly-X-Y repeating tripeptide unit in the amino acid sequence of the three chains, where X and Y can be any amino acids but are often the imino acids proline and hydroxyproline. This sequence, with glycine in every third position and with an unusual abundance of hydroxyproline, forms the basis for the chemical identification of collagen (for review, see 1). An unambiguous physical identification is provided by X-ray diffraction; the helix parameters established by high-angle X-ray scattering are unique to collagen (2).