抄録
Permanent waving is important process to set hair configuration for the maintenance of wanted hairstyle. This process consists of scission of disulfide (SS) cross-links in hair keratin by reduction, washing with water to remove reductants, and subsequent reformation by oxidation. It is important to assess the type, number and location of SS cross-links to understand setting mechanism which has been still unknown. Even though the reduction of SS bonds with thiol is equilibrium reaction, there are few researches focusing on washing step which may decrease drastically the concentration of reductants inside or on the surface of the hair fiber. In a permanent waving system consisting of three step processes of reduction with thioglycolic acid (TGA), washing with water and oxidation by sodium bromate, we attempted to quantify the amount of reformation of SS cross-links during washing, and clarify the effect of washing time on the reproduction of the SS cross-links in microstructures of hair. In order to estimate the change in the cross-link density of hair, the force-extension curve for rubber-like swollen hair fibers treated in a specific diluents mixture was analyzed by using a rubber elasticity theory derived from a two-phase model comprising of the globular matrix of keratin-associated proteins (KAP) dispersed in a swollen network of the intermediate filament (IF) proteins. It was found that the integrity of the SS cross-linked structure of IF is retained, while the intermolecular SS cross-links between KAP molecules cleaved by the reduction are regenerated by the reverse reaction of the equilibrium reactions occurring during washing, and this leads to increase of the extension modulus and breaking stress of the hair fibers in water.
