Disulfide bond

A disulfide bond is a covalent S–S linkage between the sulfur atoms of two cysteine amino-acid residues in a protein. In hair, disulfide bonds connect adjacent keratin chains in the cortex and account for most of the fiber's mechanical strength.

Background

Hair is a keratin protein. Keratin chains run lengthwise inside the cortex — the structural core of the fiber. Disulfide bonds link those chains laterally, the way rungs connect the two sides of a ladder. The density and integrity of those rungs determines how the fiber responds to stretch, bend, and break.

Bleach, perm chemistry, and severe heat oxidize disulfide bonds (S–S → S–O–H or S–O–O–H), breaking the lateral linkage. Once broken, the cortex loses lateral cohesion. The fiber feels weaker, stretches further before breaking, and recovers less when released.

Repair claims fall into three categories by mechanism: - Sulfur-bridge crosslinkers (e.g. Olaplex's bis-aminopropyl diglycol dimaleate) try to bridge the broken sulfur sites with a single intermediate molecule. - Keratin-mimetic peptides (e.g. K18) try to bind back into damaged keratin sequences. - Click-chemistry C–S formation (e.g. ANATOMY's Pro-amino X and Aminalyl S) forms new covalent C–S bonds between the patented molecule and free thiol groups on cysteine residues, building new crosslinks rather than restoring the original ones.

Disulfide bonds are not the only structural element of the cortex (hydrogen bonds and salt bridges also matter), but they are the dominant covalent crosslink and the primary site of irreversible damage.