Intermolecular disulfide cross-linked structural change induced by permanent wave treatment of human hair with thioglycolic acid

摘要

Permanent wave treatment for hair has been used for many decades. However, certain aspects of the treatment involved are still not well understood. Reduction of disulfide (SS) bonds and their subsequent oxidation are essential for inhibiting fiber degradation. We attempted to elucidate scission and reformation mechanisms of SS cross-links by treating them with a reducing agent, thioglycolic acid (TGA), and an oxidizing agent, sodium bromate. In order to estimate the change in the cross-link density of hair before and after treatment, a rubber elasticity theory was applied to the force-extension curve for rubber-like swollen hair fibers treated in a specific diluent mixture. A two-phase structural model was assumed that corresponds to the globular matrix of keratin-associated proteins (KAP) dispersed in a swollen network of intermediate filament (IF) proteins. Important structural parameters were obtained by fitting the force-extension curve to the theoretically derived relationship between elastic forces originating from the non-uniform network and extension ratios. The three parameters extracted are the number of intermolecular SS bonds in IF, the volume fraction of matrix protein in the fiber, and the shape of the matrix domain. It was found that TGA attack on the SS bonds between globular matrix proteins occurs preferentially as compared with those between IF proteins and induces the shape change of the matrix domain from an ellipsoidal to a near-spherical form.