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

Abstract

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possesses much higher alkali-solubilities than untreated wool, while wools in which the disulfide cross-linkages have been replaced by new cross-linkages through reduction followed by re action with dihalides possess much lower alkali-solubilities.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.

The action of sulphites on the cystine disulphide linkages of wool

“…Goddard & Michaelis. [1934; 1935] found that wool was dissolved by strongly alkaline solutions of thiolacetic acid owing to the complete reduction ofthe disulphide-S. Patterson, Geiger, Mizell & Harris [1941] have studied the reduction of the disulphide-S of wool with solutions of thiolacetic acid of low acidities and it is of interest to compare the extent of disulphide reduction which they obtained' with our results. These workers reduced wool in buffer solutions of thiolacetic acid.…”

“…It thus appeared probable that thiolacetic acid reduced about 2*0 % disulphide-S under the conditions used by Patterson et al [1941]. In order to find out whether the disulphide-S reduced belonged to fractions A and B, a sample of our wool was immersed for 18 hr.…”

Further Studies on the Digestion of Wool Keratin by Papain-Urea: the Effect of Adding Compounds that Rupture Disulphide Bonds in Alkaline Solution

“…In the cysteine solutions slight softening of the fibres was detected at pH 10.0 and 10.5. The form of the pH-thioglycollate reduction curve is similar to that reported by Patterson et al (1941) although, because of the milder conditions employed, the extent of reduction is less. The formation of sulphydryl groups in the presence of potassium cyanide has been confirmed by increasing the concentration of this reagent to 1.0M, adjusting the pH value to 10.5, and incubating as before for 1 hr.…”

Nature of the reaction of wool with alkali

“…However, it was subsequently shown [6] that few if any free sulfhydryl groups are present in the alkali-treated wool even prior to hydrolysis. In addition, this hypothesis, when applied to wool, would result in the rupture of the disulfide cross-links, a process which would be expected to yield a material of very low strength and extremely high alkalisolubility [7]. Yet samples of wool, treated with alkali by methods described elsewhere, exhibited relatively high strengths and low alkali-solubilities (see table 1).…”

“…A sodium nitroprusside test, used for the qualitative detection of sulfhydryl groups, was negative on both samples. Immediately after the washing treatment, small portions of the wools were treated with benzyl chloride [7] in order to combine any cysteine residues that might be present.…”