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-3-alanyl 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, △H_(evol), 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.
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