Reproduction Mechanism of SS Cross-Links in Permed Hair by the Washing after Reduction with Thioglycolic Acid

Abstract

: 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 crosslinks 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.

Disulfide Cross-Linked Network Structure of Intermediate Filament and Matrix in Hair and Wool Cortices

“…7). This is because, as already suggested [11,13], some of the intermolecular SS crosslinks between the subcomponent molecules converted to asymmetrical disulfide, namely mix disulfide (K-SS-CH2 COOH) and cysteic acid (KSO3H) groups and rendered the intramolecular disulfide bonds on the surface of regularly assembled KAP molecules mechanically ineffective. Consequently, no reformation of the regularly arranged structure of KAP molecules takes place and this may lead to the disordering of the KAP molecules around the IF assembly.…”

“…On the contrary, thioglycolic acid (TGA) reduces easily the SS bonds between KAP molecules, while the SS bond scission of IF protein is limited under usual perming conditions [13]. These findings on the selectivity for the reaction of SS bonds in microstructure of wool and hair were profitable to establish our model.…”

“…SS bonds except for SSE-KAP and SSKAP-KAP types tend to reform during oxidation step in permanent wave treatment. The observed recovery level is due to the almost perfect recovery of the SS cross-links within the IF molecule [11,13]. An important finding is that the total number of intermolecular cross-links on the IF chain (138 µmol/g, see hair sample B in Table 1) decreases to 85 µmol/g under the strongest reducing conditions examined (1.5 M TGA concentration) and then recovers up to 135 µmol/g after oxidation (see Figure 5 in Ref.…”

“…The effect of the time of washing after reduction with TGA is one of the central problems of permanent wave industries. The reformation mechanism of SS cross-links during the washing was severely discussed on the basis of structural parameters determined by fitting the experimental data of stress-strain curves of swollen fibers to a theoretically derived equation including several variables related to the structure of SS cross-links in IF and KAP components of the hair cortex [13]. Important conclusions were obtained that (i) TGA attacks preferentially the SS cross-links between KAP molecules, which results deaggregation of KAP component proteins and leads to spherical shape from ellipsoidal form of KAP molecular aggregates, and that (ii) the reproduction of SS cross-links between KAP molecules by the washing after TGA-reduction results a significant recovery of mechanical stress and strain properties of hair, which is due to the reverse reaction of reduction induced by removing reductant molecules from equilibrium system by their diffusions.…”

Structure and Its Extensional Properties of Straightened Hair Prepared by Heat Treatment with Glyoxylic Acid

“…Relationships between equatorial SAXS intensity and scattering vector S for untreated, control, and GA/heat-treated hair samples: (1) untreated,(2) control, (3) G1, (4) G3.…”