Permanent hair treatments break and later reform the covalent crosslinks that lock keratin's shape in place. Which functional group supplies those crosslinks?

Core Concept

Each cysteine carries a terminal sulfhydryl, written SH. Oxidation couples two of these groups into a covalent sulfur-sulfur bond, and networks of such disulfide bridges fix keratin fibers into particular shapes. Straightening or perming first reduces existing bridges to free sulfhydryls, resets the fiber's shape, then reoxidizes new pairings to freeze the new geometry. Hydroxyl, carboxyl, and phosphate groups take part in polarity, acidity, and signaling chemistry but never build these crosslinks.