Analysis of heterogeneous reaction between reducing agents and keratin fibers using Raman spectroscopy and microspectrophotometry

In order to investigate the heterogeneous reaction between reducing agents (thioglycolic acid (TG) and l-cysteine (CYS)) and keratin of on keratin fibers at the molecular level, the disconnected –SS– content of cross-sections at various depths of virgin white human hair treated with reducing agents was directly analyzed using Raman spectroscopy and microspectrophotometry. The reducing section of –SS– groups existing in the cortex region by performing the permanent waving treatment was clearly the matrix protein (IFAP). The TG relative concentration and the disconnected relative concentration of disulfide (–SS–) groups at various depths of the hair samples with pH 9.0 were in good agreement, indicating that the disconnected –SS– pattern of TG into virgin human hair was Fickian type characteristics. We demonstrated that diffusion of TG and the disconnected of –SS– groups proceeded together, gradually, beyond the cuticle region, and toward the inside of the cortex region for virgin hair samples at pH 9.0. While, the disconnected relative concentration of –SS– groups at various depths of the hair samples with pH 9.0 was less than the CYS relative concentration, indicating that the reaction rate (the disconnection of –SS– groups) was slower than the diffusion rate of CYS into human hair. Consequently, CYS hardly penetrated into the cortex region of the virgin human hair at pH 9.0, since the free amino groups of CYS electrostatically interacted with the anionic ions of the fiber surface.From these experiments, we concluded that the virgin human hair treated with CYS was less damaged as compared with the hair treated with TG, since CYS hardly penetrated into the cortex region of the virgin human hair at pH 9.0, and the disconnected ability of –SS– groups of CYS was low (60% below).

► The reaction between reducing agents and human hair was analyzed using Raman spectroscopy.
► The diffusion behavior of reducing agents into human hair was analyzed using microspectrophotometry.
► The reducing section of –SS– groups by performing the permanent waving treatment was the matrix.
► The disconnected –SS– pattern of thiaglycolic acid was Fikian type characteristics.
► The disconnected –SS– pattern of l-cysteine was non-Fikian type characteristics.