Cysteic Acid: The Chemical Fingerprint of Bleach Damage

Cysteic acid is what is left behind when bleach breaks a hair bond, and finding it in a strand is the chemical fingerprint of oxidative damage. When an oxidizer attacks a disulfide bond, the sulfur is converted, step by step, into cysteic acid: a stable, dead-end form that can never reconnect into a working bond.

Why it matters

Cysteic acid is not just inert debris. It tells you two things. First, that a bond that used to hold the strand together is permanently gone, so the hair is measurably weaker. Second, it carries a negative electrical charge, which changes how the strand behaves: it makes damaged hair more attractive to positively charged repair ingredients (useful) and to hard-water minerals like calcium and copper (not useful, and a reason porous hair stiffens).

How scientists measure it

Raman and infrared spectroscopy can see cysteic acid directly, and its signal rises with every bleach cycle. That is how the dose-dependence of bleach damage was established: more bleaching means more cysteic acid means fewer working bonds.

The repair angle

You cannot turn cysteic acid back into a disulfide bond. Repair instead builds brand-new bonds elsewhere in the cortex to take over the load, which is how molecular reconstruction restores strength to bleached, high-porosity hair. Full picture: Hair Porosity: The Complete Science.

Reference

• Di Foggia, M. et al. Data in Brief 38:107439 (2021). doi