Disulfide Bonds: The Cross-Links That Make Hair Strong

Bleach does not simply break a disulfide bond and leave two matching sulfur ends waiting to be joined again. Oxidation changes the sulfur chemistry. When cystine is converted to cysteic acid, the original disulfide linkage cannot be restored under ordinary hair-care conditions.

The exact link

Cysteine is an amino-acid residue with a sulfur-containing side chain. Two cysteine residues can be oxidized together to form cystine, joined by a covalent sulfur–sulfur bond: a disulfide bond. This is a specific chemical connection, not a general name for any force holding hair together.

Those links occur at different levels of the keratin assembly. Some stabilize regions within a protein structure; others connect neighboring keratins or keratin-associated proteins. The resulting network helps stabilize the intermediate filaments and matrix of the cortex. The cuticle also contains extensively cross-linked protein, so disulfide chemistry is not confined to the hair’s core.

Location matters. Detailed work in wool, a related keratin fiber, shows that tensile stress does not affect every disulfide bond equally. The bonds implicated under strain vary with their protein location and with whether the fiber is wet or dry. A simple count of “bonds broken” would miss that mechanical complexity even if it could be measured at home.

What bleach changes

Lightening formulations are designed to oxidize melanin, but their chemistry is not confined to pigment. Cystine can pass through several oxidized sulfur species on the way to cysteic acid. Other amino acids and lipids can also oxidize. The result is broader than a row of neatly severed cross-links.

Di Foggia and colleagues documented part of this progression in brown human hair treated repeatedly with commercial persulfate and peroxide formulations. Moving from three bleaching treatments to a fourth, Raman signals associated with disulfide bridges decreased while signals associated with cysteic acid increased in the cortical region. Infrared measurements also tracked oxidation products nearer the surface.

That experiment supports the oxidation pathway; it does not supply a universal percentage of bonds lost in one salon service. Starting color, formulation, exposure, prior condition, and measurement method all matter. More recent work on protein carbonylation also shows why cysteic acid is a valuable marker without being the whole map of oxidative damage.

Water changes a different part of the system

Wet hair does not become more extensible because water dissolves its disulfide bonds. Water disrupts many hydrogen bonds and softens the protein matrix, changing how force is distributed through the structure. The disulfide network remains important, but its mechanical role is expressed against a different background.

This separates two events that are often collapsed into one. Wetting produces a reversible change in mechanical state. Oxidative bleaching produces chemical changes that do not disappear when the strand dries.

Check the history before the stretch

A strand test cannot count disulfide bonds or identify their oxidation products. A more useful self-check starts with the exposure history.

• Note where lightener, oxidative color, a perm, or a relaxer was applied, and whether the same lengths were treated more than once.
• Record changes that appeared afterward: early breakage during gentle care, altered wet recovery, or a clear difference between exposed and unexposed zones.
• Do not keep stretching hair to make it feel “gummy.” Feel alone cannot distinguish sulfur oxidation from the many other changes that accompany chemical processing.

If chemical work was followed by a persistent change in handling or breakage, the free Damage Profile consultation offers a more useful next step than another strand-pulling test. It takes about a minute and asks a few questions about color, heat, washing, and how your hair behaves. You will be asked for your email before the Damage Profile is shown with a suggested order of care.

What to do after oxidative damage

1. Prevent unnecessary overlap. Make previous lightening and chemical services visible in any new professional consultation, including which lengths were treated.
2. Reduce the mechanical work imposed on altered hair. Use conditioning for slip, avoid forceful detangling, and stop any home test that produces stretching without recovery or breakage.
3. Read product evidence at the right level. Ask what was measured, on which hair substrate, against which control, and under what conditions. A reaction name or a plausible diagram is not itself evidence of a particular chemical event inside hair.

ANATOMY is built on click chemistry, a reaction class recognized by the 2022 Nobel Prize in Chemistry. It is patented and made in Switzerland, and was independently tested at SGS proderm in Germany, on bleached hair. That test setting should not be read as evidence that cysteic acid can be converted back into cystine.

References

• Di Foggia M, Boga C, Micheletti G, Nocentini B, Taddei P. Vibrational Raman and IR data on brown hair subjected to bleaching. Data Brief. 2021;38:107439. Full text.
• Grosjacques C, Babiel S, Hodes J, Bobier A, Cavagnino A, Baraibar M. Carbonylation of hair proteins: A robust biomarker of molecular and structural oxidative damage in hair fibres. Int J Cosmet Sci. 2025;47(4):604-625. Full text.
• Harland DP, Popescu C, Richena M, et al. The susceptibility of disulfide bonds to modification in keratin fibers undergoing tensile stress. Biophys J. 2022;121(11):2168-2179. Full text.
• The Nobel Prize in Chemistry 2022. NobelPrize.org.