Cysteic acid

Cysteic acid is the over-oxidized form of cysteine, where the sulfur has been converted from a thiol (–SH) or disulfide (S–S) to a sulfonic acid (–SO3H). In hair, cysteic acid is the chemical marker of irreversible bleach damage — sulfur oxidation past the point where any reconstruction can return the fiber to its original disulfide architecture.

Background

Bleaching is an oxidative process. Hydrogen peroxide and other oxidants in bleach formulas attack the sulfur atoms in disulfide bonds. The reaction proceeds in stages:

1. Cystine (the original disulfide bond, S–S, between two cysteines).
2. Cysteine (broken disulfide; two free thiol –SH groups, reversible).
3. Cysteic acid (over-oxidized; one or both sulfurs become –SO3H, irreversible).

Once cysteic acid forms, the sulfur is no longer available for disulfide bond reformation. A natural disulfide can no longer reassemble at that site; the structural element is permanently lost from the cortex's original architecture.

Implications for repair: - Sulfur-bridge crosslinkers that depend on broken disulfide sulfurs will not reattach to a cysteic acid sulfur — the chemistry has moved past that bond's accessibility. - Click-chemistry reconstruction (thiol-ene and thiol-yne) builds new covalent C–S bonds with free thiols, not cysteic acid. Damage that has progressed to cysteic acid is past the reach of click-chemistry crosslink formation at that specific site, but neighboring cysteines often remain in the free-thiol state and provide reactable sites. - Surface treatments mask the feel of cysteic-acid damage but cannot reverse it.

The presence of cysteic acid in bleached hair is well-documented in spectroscopic and chromatographic studies. Higher cumulative bleach exposure = more cysteic acid = less of the original disulfide network to recover. This is part of why bleach damage is fundamentally bounded: some sulfur is gone permanently.