The primary structure is the amino acid sequence that makes up the collagen polypeptide chain; the collagen molecule is composed of 3 left-handed helices (secondary structure) of the polypeptide chains, which are intertwined to form a right-handed helix (tertiary structure) around the central molecular axis ; Intact collagen molecules are about 280 nm in length and 1.5 nm in diameter; in the two-dimensional structure of type I collagen fibrils (small-angle X-ray diffraction patterns and transmission electron micrographs), collagen molecules pass one or more 4D distances Interleaved with another collagen molecule, D represents the basic repeat distance seen in the small angle X-ray diffraction pattern, or the repeat distance seen in the electron micrograph. Because the length of collagen molecules is about 4.4D, the interleaving of collagen molecules results in a folded region of about 0.4D and a defect region of about 0.6D.
