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. Raman spectroscopy showed a fourth bleaching session further reducing disulfide bonds and increasing cysteic acid compared with three (Di Foggia et al. 2021); normal bleaching breaks about 15-25% of hair's disulfide bonds, and wet strength falls with them (Robbins 2012). 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. Some treatments are designed to form fresh cross-links in their place; independent evidence for this in hair is still limited. ANATOMY's molecular reconstruction does this with a reaction from the click-chemistry family (2022 Nobel Prize in Chemistry); its formulas were independently tested at SGS proderm in Germany, 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