The Rôle of Cystine in the Structure of the Fibrous Protein, Wool

Wool is remarkable among textile materials in that it possesses high extensibility and the ability to return to its original length or shape after being stretched or distorted. These properties are of advantage in such products as clothing, blankets, and carpets. It is shown that a number of the important physical properties as well as some of the chemical properties of wool are dependent upon a unique molecular structure, provided by the presence of disulfide or cystine cross-linkages between the main molecular chains of the fiber. As a result, wool fibers possess a three-dimensional molecular net-work while most other textile fibers are composed of bundles of chain-like molecules, arranged more or less parallel to the axes of the fibers. The rôle of these disulfide cross-linkages is made clear by a study of the behavior of wool before and after alteration of the mode of linkage of the sulfur. For example, the disulfide cross- linkages are readily broken to form sulfhydryl groups by the re ducing agent, thioglycolic acid. The sulfhydryl groups of the reduced fibers readily react with alkyl halides to form thioether groups with two possible results. Thus, the reaction of reduced wool with alkyl monohalides results in permanent rupture of di sulfide linkages and greatly increases the extensibility and decreases the strength of the fibers. The reaction with aliphatic dihalides introduces hydrocarbon chains between pairs of sulfur atoms of cystine molecules in the fibers. Such fibers are very similar to untreated fibers in physical properties. Wools in which the disulfide linkages have been broken by reduction, or by reduction followed by treatment with alkyl mono halides possesses much higher alkali-solubilities than untreated wool, while wools in which the disulfide cross-linkages have been replaced by new cross-linkages through reduction followed by re action with dihalides possess much lower alkali-solubilities. Since the susceptibility of wool to degradation by alkalies is one of its greatest disadvantages, practical processes that would make it stable at its cross-linkages should also enhance its durability.