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The Collagen Molecule

Every third amino acid is glycine. Proline and hydroxyproline follow each other relatively frequently, and the (gly, pro, hyp) sequence makes up about 10% of the molecule. This triple helical structure generates a symmetrical pattern of three left-handed helical chains that are, in turn, slightly displaced to the right, superimposing an additional “supercoil” with a pitch of approximately 8.6 nanometers (nm).

These chains, known as α-chains, have a molecular weight of around 100 k Daltons and contain approximately 1,000 amino acids for the interstitial collagen Types I, II, and III. The amino acids within each chain are displaced by a distance h = 0.201 nm with a relative twist of 100 degrees, making the number of residues per turn 3.27, and the distance between each third glycine 0.87 nm. The individual residues are nearly fully extended in the collagen structure, since the maximum displacement within a fully stretched chain would be approximately 0.36 nm.

This separation is such that it will not allow intrachain bonds to form (as does occur in the alpha helix), and only interchain hydrogen bonds are possible. The exact number of hydrogen bonds that stabilize the triple helical structure has not been determined. One model describes two hydrogen bonds for every three amino acids, whereas another assumes one.

In addition to these intramolecular conformational patterns, there seems to exist a supermolecular coiling. Microfibrils, possibly representing intermediate stages of packing, have been described.

A process of self assembly causes the collagen molecules to organize into fibers. The thermodynamics of such a system involve changes in the state of the water molecules, many of which are associated with nonpolar regions of the collagen molecule.

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