Disulfide Bonds: The Cross-Links That Make Hair Strong

Disulfide bonds are the sulfur cross-links that hold the hair's core together, and they are what bleach destroys when it weakens your hair. Each bond is a chemical link between two sulfur-containing points on the protein chains, and together they turn loose protein fibres into a strong, springy cable.

Why they are the key to strength

Hair is full of weaker, temporary bonds (the ones that let you restyle it with water and heat), but the disulfide bonds are the permanent rivets. They are why a healthy wet strand can stretch and spring back instead of snapping. The more intact disulfide bonds, the stronger the hair.

How bleach breaks them

Bleach's oxidizers attack the sulfur, converting the bond into a dead-end form called cysteic acid that cannot reconnect. Researchers have tracked this with Raman spectroscopy across repeated bleach cycles: each pass breaks more bonds, strength drops, and the change does not reverse on its own. This is the chemistry behind why bleached hair stretches, goes gummy, and snaps.

Rebuilding them

You cannot un-break a disulfide bond, but you can build new bonds in its place. Small reactive molecules that diffuse into the cortex and form fresh cross-links restore the cable. ANATOMY's molecular reconstruction does this with a reaction from the click-chemistry family (2022 Nobel Prize in Chemistry); its molecule forms two new bonds at a time, and independent testing measured a 135% strength gain on bleached hair. See Hair Porosity: The Complete Science.

Reference

• Di Foggia, M. et al. Data in Brief 38:107439 (2021). doi
• 2022 Nobel Prize in Chemistry. nobelprize.org