Structural investigation on damaged hair keratin treated with α,β-unsaturated Michael acceptors used as repairing agents

Many restoring formulations for damaged hair keratin have been developed. Some patents claim that the hair repair occurs through the reconstruction of disulfide bridges of keratin, through α,β-unsaturated Michael acceptors, such as shikimic acid and bis-aminopropyl diglycol dimaleate. To gain more insights into the possible repairing mechanism, this study is aimed at assessing, by IR and Raman spectroscopies coupled to scanning electron microscopy (SEM), the structural changes induced in keratin from bleached hair by the treatment with commercial reconstructive agents as well as shikimic acid and dimethyl maleate, chosen as model compounds. Vibrational spectroscopy revealed that shikimic acid- and maleate-based restoring agents interacted with hair fibers modifying both their cortex and cuticle regions. None of the investigated treatments induced an increase in the S[sbnd]S disulfide bridges content of the hair cortex, although it cannot be excluded that this phenomenon could have occurred in the cuticle. S[sbnd]S rearrangements were found to occur. None of our results can be interpreted as direct evidence of the sulfa-Michael reaction/cross-linking. From a morphological point of view, beneficial effects of the restoring agents were observed by SEM analyses, in terms of a more regular hair surface and more imbricated scales.