Evaluation of Cysteic Acid in Bleached Hair Using Infrared Spectroscopy

In hair-bleaching treatment, it is well-known that disulfide bond in keratin fiber is oxidatively cleaved to generate cysteic acid. In the present study, we examined cysteic acid generation behavior in human hair fiber based on infrared spectroscopy. It was suggested from a result of attenuated total reflection (ATR) method that the amount of cysteic acid increases linearly with square root of bleaching time, and the cysteic acid generation in the hair is a diffusion-controlled reaction. As the analysis part of the hair by the ATR method is cuticles, this diffusion-controlled behavior seems to occur mainly in the A-layer and exocuticle with higher disulfide crosslink density and slower diffusion rate of hydrogen peroxide. In addition, it was found out that cysteic acid generation in cortex is also diffusion-controlled because the amount of cysteic acid obtained by KBr method increases linearly with square root of bleaching time similarly to the results of ATR method. In an IR microscopic measurement using synchrotron radiation source, it was revealed that the amount of cysteic acid generated by bleaching is even within the cortex. From this result, it was suggested that the diffusion-controlled behavior for cysteic acid generation in the cortex is caused by controlling the diffusion of hydrogen peroxide from cell membrane complex into cortical cell.