Cysteine

Cysteine is one of the 20 standard amino acids and the only one that contains a sulfur atom in a thiol (–SH) functional group. In hair keratin, cysteine residues form disulfide bonds (S–S) with other cysteines, creating the covalent crosslinks that give hair most of its mechanical strength.

Background

Cysteine's chemical formula is HSCH2-CH(NH2)-COOH. The thiol group is the reactive part; the rest is standard amino-acid backbone.

In hair, cysteine accounts for ~14% of keratin by mass — much more than in most body proteins. This concentration is why hair is uniquely cross-linked. Two cysteine thiol groups oxidize together to form one disulfide bond, two water molecules being the only byproduct.

When hair is damaged: - Bleach oxidizes disulfides further, converting cysteine sulfurs to cysteic acid (S-O3H) — a one-way reaction. - Reduction (perm chemistry, certain straighteners) breaks disulfides into free thiols (–SH), reversibly. - Mechanical and thermal stress can fracture chains at weak points; fragments can recombine or remain as free thiol groups.

Free thiols on cysteine residues are the targets for click-chemistry reconstruction. Pro-amino X (alkene-bearing) and Aminalyl S (alkyne-bearing) react with those free thiols via thiol-ene and thiol-yne click reactions to form new C–S covalent bonds. The reaction site is, at the chemistry level, the cysteine sulfur.

A note on related amino acids: cystine is the disulfide-bonded dimer of two cysteines (cys-S-S-cys). Cysteic acid is the over-oxidized version (cys-S-O3H), which marks irreversible damage.