Keratin

Keratin is the structural protein that makes up hair, nails, and the outer layer of skin. Hair keratin is rich in cysteine (the amino acid that contains a sulfur atom), which is why disulfide bonds — the covalent S–S crosslinks between cysteine residues — dominate hair's mechanical architecture.

Background

Hair keratin is classified as a hard alpha-keratin: long polypeptide chains that fold into alpha-helices and assemble into intermediate filaments. Those filaments pack into macrofibrils, which pack into the cortex — the load-bearing structural core of each hair fiber.

Keratin's cysteine content (~14% by mass in human hair, much higher than most other body proteins) is what makes hair both strong and chemically vulnerable. Cysteine residues form disulfide bonds with neighboring cysteine residues in adjacent chains. Those disulfide bonds are responsible for most of hair's tensile strength.

Damage mechanisms: - Bleach oxidizes disulfides (S–S → cysteic acid, S-O3H), breaking the crosslinks. - Heat can denature the alpha-helix structure above ~150°C; cumulative exposure permanently disorders the fiber. - Mechanical stress (brushing, friction) can break weakened fibers at points where crosslinks have already been compromised.

Repair claims operate at different points in this architecture. Surface treatments and conditioners coat the cuticle (the outer scale layer) without changing the cortex. Reconstruction claims operate inside the cortex, where the keratin lives.