Effect of disulfide bonds in human hair fibers on the melting behavior of their crystalline structure

In order to reveal the role of disulfide (S-S) bonds in the fine structure of human hair fibers, especially in the complex between intermediate filaments (IFs) and intermediate filament-associated proteins (IFAP), we investigated the relationship between melting behavior of (-crystallites on thermal analysis and structural orientation of the fibers when S-S bonds were oxidized using 1.5 wt% peracetic acid (PAA) solution. It was revealed that oxidative treatment of human hair fibers immersed in PAA solution made cystine residues dissociate to cysteic acids and that the (-crystalline structure of the oxidized hair fibers was maintained despite the fission of S-S bonds. Their thermographs in water on high pressure differential scanning calorimetry (HPD-SC) showed that the endothermic peak assigned as the melting of (-crystallites became separated into two endothermic peaks with progression of the treatment time, and the one started melting at a lower temperature while the other had a higher melting temperature than the melting temperature of untreated hair. Furthermore, the sum of the two endothermic peak areas decreased. Polarized light microscopic observation indicated that the orientation of IFAP still existed at the temperature at which the orientation of IFs disappeared. Therefore, these results suggested that some S-S linkages at the interface between IFs and IFAP dissociated with the PAA treatment and that there was not only some region of IFs stabilized by S-S bonds but also another region of IFs stabilized by hydrophobic interaction for their thermostability. Keywords:  human hair fiber, peracetic acid, disulfide bonds, intermediate filament, (-crystallites, thermostability, high pressure differential scanning calorimetry, polarized light microscopy, X-ray diffraction, Fourier transform infrared spectroscopy