Role of cystine in the structure of the fibrous protein, wool

Abstract

A number of the important physical and chemical properties of wool are closely associated with the presence of disulfi de cross-linkages between the polypeptide chains of the protein. This conclusion results from a study of the behavior of wool before and after alteration of the mode of linkage of the sulfur by a series of highly specific reactions.The cystine in wool can be readily reduced to cysteine with thioglycolic acid. Strongly alkaline solutions of the reagent have been shown by previous investigators t o dissolve the protein and destroy its fibro us structure. It is now shown that wool can be reduced with thioglycolic acid over a wide range of pH, and that when the reduction is carried out in neutral or acid solution, the fib rous structure of the wool is not destroyed when the disulfide groups are reduced to sulfhydryl groups. T he sulfhydryl groups of fibers reduced in this way react readily with alkyl halides to form thioether groups.Thus reaction of reduced wool with alkyl monohalides, such as methyl iodide, results in permanent rupture of disulfide linkages, and the fibers are greatly increased in extensibility and decreased in strength. Alkylation with aliphatic dihalides, such as methylene iodide or trilnethylene dibromide, 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.Wool in which the disulfide linkages have been broken by reduction, or by reduction and alkylation with alkyl monohalides, possesses much higher alkalisolubility t han untreated wool, while wool in which the disulfide cross-linkages have been replaced by new covalent cross-linkages through reduction followed by alkylation with dihalides possesses m uch lower alkali-solubility. Since th!' susceptibility of wool to degradation by alkalies is one of its greatest disadvantages, p rocesses that would make it stable toward alkalies should also enhance its durability. CONTENTS

Studies on Reduced Wool I. The Extent of Reduotion of Wool with Inoreasing Conoentrations of Thiol, and the Extraotion of Proteins from Reduoed and Alkylated Wool

“…Mercaptoethanol and thioglycollate are very similar in their extent of reduction of wool at low concentrations of thiol but the neutral thiol is more effective as a reducing agent at high concentrations. In contrast with the earlier findings of Patterson et al (1941) the reduction was driven towards completion with increasing concentrations of thiol as would normally be expected for a reversible reaction. With 4)1[ mercaptoethanol 4% of the disulphide bonds were intact in the reduced wool but the extracted proteins contained no disulphide groups.…”

“…In general the results show that mercapto-ethanol is a more effective reducing agent than potassium thioglycollate. With potassium thioglycollate, although the experiments were not all carried out in parallel, it appears that equilibrium is reached more slowly than would be expected from the findings of Patterson et al (1941). The differences between the two thiols were most marked at high concentrations where the ionized carboxyl group of potassium thioglycollate gives the solution a higher ionic strength than is the case with mercaptoethanol.…”

“…More of the reduced and alkylated proteins are eluted at low salt concentrations. V. DISCUSSION Patterson et al (1941) made an extensive study of the variables pH, temperature, concentration of reagent, and time of reaction on the reduction of the disulphide bonds of wool by sodium thioglycollate. Our results do not support their conclusion that concentrations of thioglycollate above 1M are ineffective in producing greater reduction of the disulphide bonds of wool.…”

“…In the early studies on the reduction of the disulphide bonds in wool (Goddard and Michaelis 1935;Patterson et al 1941), thioglycollate solutions at various pH values were used. Although reduction occurs at all pH values, Patterson et al showed that there was a marked increase in the level of reduction at pH values above 6.…”

Studies on the Depilatory Activity of Sodium Sulphide and Some Related Compounds

“…A possible reason is that swelling of the wool root is only an early stage in the weakening of the root that is necessary for depilation, whilst the more drastic treatment required for wool fibre damage may be due to the resistant nature of this material as compared with the newly formed keratin of the wool root shaft. Below pH 10 depilation is difficult to achieve, even with concentrated solutions of reducing ageJ;lts, and this is probably associated with a corresponding difficulty in completely reducing -S-S-bonds below this pH value (Patterson et al 1941). Inhibition of depilation by high concentrations of sodium sulphide or by sodium sulphide plus sodium chloride was observed on many occasions.…”