A fibrous protein constituting most of the white fiber in the connective tissues of animals and humans, especially in the skin, muscles, and tendons. The most abundant protein in the animal kingdom, it is rich in proline and hydroxyproline. The molecule is analogous to a three-strand rope, in which each strand is a polypeptide chain. It has a molecular weight of approximately 100,000. Glue made from the collagen of animal hides and skins is still widely used as an adhesive. So-called “soluble” collagen is that first formed in the skin; upon aging it becomes increasingly cross-linked and less hygroscopic. Soluble collagen is used in the cosmetic industry as the basis for face creams, lotions, and hair preparations. Special forms of collagen have been developed for dialysis membranes. Microcrystalline collagen is used in prosthetic devices and other medical and surgical applications. Regenerated collagen, used in sausage casings, is made by neutralizing with acid collagen that has been purified by alkaline treatment. Collagen is converted to gelatin by boiling water, which causes hydrolytic cleavage of the protein to a mixture of degradation products.See Gelatin.
