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DSC measurements carried out at different heating rates were used for the kinetic analysis of the endothermic process assigned to the denaturation of the helical material from human hair in water excess. We found that the kinetic mechanism is autocatalytic and that the value of the activation energy is close to disulphide bond scission rather than to protein denaturation. This allowed us to propose a multistep mechanism for the thermal denaturation of hard alpha-keratins in water excess that relies on the 3-phase model which describes their structure. The limiting step of the thermal denaturation process is then the scission of S-S bonds between the main morphological components, namely IF and matrix (IFAP). The theoretical proposed model shows a good agreement with the experimental recorded data.

The morphology and molecular mobility changes of the side chains for hard alpha-keratin due to oxidative and reductive/oxidative treatments for temperatures around the DSC denaturation peak were investigated by (1)H, (13)C, and (129)Xe NMR spectroscopy and (1)H spin diffusion. Proton wide-line spectra were used to obtain the phase composition (rigid, interface, and amorphous fractions) and molecular dynamics of each phase. Proton spin diffusion experiments using a double-quantum filter and initial rate approximation were employed to obtain the dependence of the rigid domain sizes on chemical treatments and denaturation temperatures. A drastic reduction in the rigid domain thickness takes place for the reductive/oxidative treatment. The keratin mobility gradient in the interfacial region at different denaturation temperatures was measured for hard alpha-keratin from (1)H spin diffusion data. (13)C CPMAS spectra were used to provide a detailed examination of the effects of the chemical treatments especially on the disulfide bonds. Thermally polarized (129)Xe spectra suggest the existence of voids in the hard alpha-keratin induced by the reductive and oxidative treatment. The surface of the hard alpha-keratin fiber surface is probed by the laser hyperpolarized (129)Xe. A qualitative model describing the changes induced in hard alpha-keratin protein by chemical transformation was developed and could be correlated with the changes in domain thickness, phase composition, and molecular dynamics.

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