Action of Thioglycolic Acid and L-Cysteine on Disulfide Cross-Links in Hair Fibers During Permanent Waving Treatment

The study from 2008 examined how thioglycolic acid (TGA) and L-cysteine (CyS) affect disulfide bonds in hair fibers during permanent waving treatments. Using Fourier Transform Infrared spectroscopy and polarographic methods, the researchers found that both TGA and CyS reduce disulfide cross-links, but they target different molecular structures within the hair. TGA primarily affects cross-links between intermediate filament and matrix proteins, while CyS targets non-helical segments of intermediate filaments. After five treatment cycles, there was an 11-12% loss of disulfide groups and a 75% increase in thiol content compared to untreated hair. The study also noted that TGA and CyS have different efficiencies in waving hair, which is influenced by the specific sites they reduce within the hair's microstructure and their permeability into the cortex. Despite similar reductions in disulfide bonds, the agents differed in their impact on hair's mechanical and physical properties due to the location of the reduction reactions.