55—EFFECTS OF CHEMICAL MODIFICATIONS ON THE PHYSICAL PROPERTIES OF WOOL : A MODEL OF THE WOOL FIBRE

Abstract

Conversion of the disulphide bonds of wool to S-methyl groups accelerates supercontraction in solutions of LiBr, but has no appreciable effect on the level of the equilibrium contraction. Evidence is presented that the rate of supercontraction is partly determined by interchange reactions between sulphydryl and disulphide groups and that inhibition of supercontraction when free bromine is present in the solutions of LiBr is due to oxidation of the sulphydryl groups. It is concluded that the contractile structures of the fibre are not cross-linked by disulphide bonds, but are embedded in material containing inter- and intra-molecular disulphide bonds (the matrix), the rate of plastic deformation of the latter structure being affected by sulphydryl interchange with disulphide bonds. Nitration, iodination, mild oxidative treatments, and treatments with acids or alkalis accelerate the first stage of supercontraction in 6M LiBr at pH 6, but retard the second stage. This retardation is not observed with contractions in 4M LiBr/IN HCl, suggesting that ionic interactions are responsible for the retardation. Ionic forces between the anionic low-sulphur proteins of the wool and the cationic high-sulphur proteins may be involved. The data are accomrnodated by a model which assigns the low-sulphur proteins to the microfibrils, the half-cystine residues lying in adjacent positions on the peptide chain. The high-sulphur protein is assigned to the intermicrofibrillar matrix. It is suggested that the distribution of half-cystine residues along the chain at intervals of five to seven residues with intermediate proline residues would facilitate exchange reactions between sulphydryl and disulphide groups. The model has been tested in part by comparing the proximity of half-cystine residues in the low-sulphur and high-sulphur proteins with predictions from the model. Other predictions concerning changes in the relative values of the wet and dry breaking loads of fibres treated with anhydrous and aqueous sulphuric acids have also been checked.