J. Cosmet. Sci., 63, 177–196 (May/June 2012) 177 Intermolecular disulfi de cross-linked structural change induced by permanent wave treatment of human hair with thioglycolic acid KAZUYUKI SUZUTA, SATOSHI OGAWA, YASUFUMI TAKEDA, KATSUMI KANEYAMA, and KOZO ARAI, Central Research Institute, Milbon Co. Ltd., 2-3-35 Zengenji, Miyakojima, Osaka 534-0015 (K.S., S.O., Y.T., K.K.), and KRA Wool Research Laboratory, 1-498-13 Aioi, Kiryu, Gunma 376-0011 (K.A.), Japan. Accepted for publication November 15, 2011. Synopsis 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 disulfi de (SS) bonds and their subsequent oxidation are essential for inhibiting fi ber 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, so- dium 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 fi bers treated in a specifi c 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 fi lament (IF) proteins. Important structural parameters were obtained by fi tting the force–extension curve to the theoretically de- rived 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 fi ber, and the shape of the matrix domain. It was found that TGA attack on the SS bonds be- tween 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. INTRODUCTION The setting process for permanent waving and straightening of hair is important in the cos- metic industry. Although setting reactions including reduction and oxidation have been stud- ied for many years by various researchers, certain aspects of the setting process are still not well understood. Various mechanisms have been proposed to explain permanent waving achieved by chemical treatment (1–9) and a combination of chemical and heat treatments (10). This paper was presented in part at the 24th Congress of the International Federation of Societies of Cosmetic Chemists, Osaka, 2006, and in proceedings of the IFSCC Congress as “Application of Rubber Elasticity Theory to Swollen Hair for Elucidating the Disulfi de Cross-linked Structure of Cosmetically Treated Hair.”

JOURNAL OF COSMETIC SCIENCE 178 Human hair consists of two major morphological components: the cuticle and the cortex. The cortex is a complicated disulfi de (SS) cross-linked system comprised of intermediate fi lament (IF) proteins and globular matrix proteins (KAP). The stabilization of curled conformation in permanent waving is accomplished through the cleavage of the cystine SS bonds by reduction and their subsequent recombination by oxidation. In order to under- stand the change in the hair structure induced by permanent wave treatment, it is impor- tant to assess the number, type, and location of SS cross-links in the hair cortex. A few reports assessing the type of cross-links in keratins have been published to date (11,12). Concerning the nature of cross-links in hair and wool, Naito and Arai (13) found that fi - bers pretreated with a concentrated aqueous LiBr solution containing N-ethylmaleimide, which serves as a blocking agent for free thiol (SH) groups, show typical rubber elasticity in a mixed solution of aqueous 8 M LiBr and diethylene glycol mono-n-butyl ether. Therefore, they proposed a semi-quantitative method for determining the SS cross-link density of various keratin fi bers. They attempted to evaluate the number of SS cross-links by using a method of Gaussian chain statistics for untreated and thioglycolic acid (TGA)- reduced hair samples in various degrees of reduction and concluded that SS linkages are divided into two groups: the intermolecular linkages group (SS1 and SS2) and the intra- molecular linkages group (SS3). SS1 linkages may be located in water-accessible terminal regions of IF proteins, and SS2 linkages are located on the surface region of the KAP do- main, while SS3 linkages exist in the hydrophobic inner region of the KAP domain. A quantitative method for analyzing the number, type, and location of SS cross-links in the keratin cortex has been developed by applying a non-Gaussian elastic equation of state to the stress–strain curve for swollen fi bers (14–16). The difference in the reactivity of SS bonds in keratin structures was determined by comparing the analytical results obtained from both chemical and mechanical tests. The difference in the reactivity of SS bonds under fi ber extension and non-extension states was utilized for the assignment of bonds existing in water-accessible or inaccessible regions, allowing detailed cross-linked structures in microdomains to be determined. It was found that 13.8% and 86.2% of all SS cross-links of hair (i.e., SS content in hair was 627 μmol/g) were located in IF and KAP components of hair proteins, respectively. In addition, the percentage ratios of intermolecular and intramolecular cross-links were found to be 66:34 for IF and 12:88 for the KAP component. Qualitative distribution of SS1, SS2, and SS3 in the hair cortex structures showing different reactivities with TGA were quantifi ed using this method. Important information concerning the cross-links within and between KAP domains was also obtained, i.e., the number of SS cross-links within a KAP molecule (average molecular weight of 20,000) was evaluated as 17 moles and the number of SS cross-links between KAP molecules was 2.3 moles, corresponding to approximately only 12% of the SS cross-links in the KAP molecule (15). An important conclusion was that the cross-linking sites of 4.6 moles were distributed on the surface region of an individual globular matrix protein. The objectives of the present study are as follows: (i) to clarify the number, type, and loca- tion of cross-links in the microstructure of keratin fi bers by using data from previous studies, (ii) to elucidate the network structure change on IF and KAP induced by reduc- tion and oxidation treatments, (iii) to clarify the number and type of reactive SS cross- links existing in the region accessible to the reducing agent, and (iv) to better understand the mechanical properties relating to intermolecular SS cross-links in the microstructure.