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: Reactions occurring during permanent waving (perm) treatment of hair reduction and subsequent oxidation were studied by using FT-IR technique and a polarographic method to estimate the contents of cysteic acid, intermediate cystine oxides, thiol, and disulfide (SS) groups in the perm hairs. The reducing agents used were thioglycolic acid (TGA) and L-cysteine (CyS). The perm hair samples were prepared by repeating from one to five times of reduction-oxidation treatment. The difference in the fluorescence activity, SS bond reduction, FT-IR spectra, waving efficiency, and stress-strain properties between the samples obtained by the reduction systems with TGA and CyS has been discussed. It was found that the SS contents of the samples from both systems were decreased in approximately similar order with the increase of the number of times of treatment and the decrease in the SS groups was due to the conversion wholly into cysteic acid via the cleavage of SS bond through the S-S fission mechanism and partly into free cysteine groups. It was presumed, further, that TGA attacks the cross-links between the intermediate filament (IF) and the matrix protein (KAP), and between KAP-KAP component proteins, whereas CyS attacks the SS cross-links associated with the non-helical segments of IF acting as a KAP component, and that the reduction sites of SS cross-links in microstructure of hair relate the waving efficiency of perm hair.

A curing method for permanent hair straightening using thioglycolic acid (TGA) has been widely used for decade. On the basis of the chemical reactions occurring during the curing treatment and the conformational changes of-helical chains, the straightening mechanism has been proposed. However, the changes of the hair structure in supermolecular level have not been clarified so far. In this report, the effects of reduction and heat treatments on the morphological changes of hair were studied by transmission electron microscopy (TEM). The ultrathin cross sections of the untreated and cured hair samples stained with uranyl acetate and followed by lead citrate were subjected to TEM observations. It was found that the intact cuticle and cortex morphologies of the untreated hair were confirmed from the fact that the A-layer, exocuticle, and endocuticle with high TEM contrast were clearly observed in each layer of the cuticle and the ill-defined cortex morphology with a low contrast was observed due to weak reactions of the staining reagents to the cortex. On the other hand, it was revealed for the stained cured hair, that (i) the decrease in TEM contrast toward the cortex center reflects the decrease of reduction reaction of the cortex components resulting from the extent of diffusion of TGA into the fiber, (ii) many voids were produced on the cuticle layers due elution of the proteins contained therein, (iii) a fine morphology with a high contrast of the structure of the whorl-like macrofibril is resulted from the reactions of the staining reagents with carboxyl groups produced by the disruption of-crystallites, and (iv) deformation, disruption and melting of the-crystallites occur on the arrangements of microfibril + matrix structural units near the cell membrane complex (CMC).

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Permanent waving and straightening hair treatments are important processes to set hair configuration for the maintenance of wanted hairstyle. There have been some unresolved problems related to hair damage arising from the use of thioglycolic acid as the reducing agent of hair. The most important one is the formation of s-carboxymethyl-3alanyl disulfide (CMAD) groups, the so-called mixed disulfide groups, since the CMAD side chain groups on the protein backbone chains lead to a decrease of the number of disulfide cross-links in the treatment hairs and result in a considerable damage or lowering in the mechanical performance of the treated hairs. At present, we have no reliable quantitative analytical method for the CMAD groups formed in the hairs during permanent treatments. In this paper, it has been proposed that the content of the mixed disulfide groups could be estimated simply by using a High Pressure Differential Scanning Calorimetry (PDSC). The plots of the enthalpy of evolution of heat, ΔHevol, evaluated from the exothermic peak area against the content of CMAD groups estimated by the calculation from the amino acid analytical data of the reduced hair samples showed a good linear relationship, which enables to an accurate measurement of the content of CMAD groups. Furthermore, the exothermal phenomenon observed on the PDSC curves for both permanent wave and straight hairs was considered to be closely related to hydrothermal reactions including an exothermic reaction of the side chain groups of sulfenic acid with the thiol groups of cysteine residues to form disulfide cross-links in hair. To elucidate the heat-induced reaction mechanism in the PDSC pans, CMAD wool keratin (CMADK) proteins with average molecular weight of 45,000 were synthesized as a model compound of the permanent treatment hair. The PDSC response of the CMADK proteins showed a similar pattern to those of the treated hairs suggesting the validity of our interpretation for the hydrothermal reactions in the hair.