Chapter 4 149 Disulphide Bond Breakage—cuticle and cortical protein changes: During oxidative dyeing, the disulfide bonds of cystine are oxidized with the consequence that the disulfide bond is eventually broken and sulfonate groups are formed (Eq. 13). The detailed mechanism is not known, but it is thought that the mechanism involves the perhydroxyl anion333 as the key oxidant species responsible for bond breakage. It has also been shown that oxidation of disulfide bonds can occur via a free radical mechanism if there are redox active metals such as copper and iron in the hair. S S SO3- - O3S Eq. 13 The major changes in hair structure caused by this chemistry are observed in the high sulfur regions such as the cuticle A-layer and the exo-cuticle, as well as in the cortical proteins. The disulfide bonds of cystine play a key role in cross-linking the proteins, giving hair its strength. Thus, the breaking of these bonds can significantly decrease wet tensile strength334 and increase hair swelling in water. This additional swelling can cause increased dye uptake at the tips of hair where the level of oxidative damage is higher and also cause more rapid wash-fade of dyes from the tips. For the consumer, this can lead to uneven color between the roots and tips. Free Radical Reactions—cuticle changes One of the consequences of creating cysteic acid groups, either via removal of the F-Layer or oxidation of the cystine groups, is that the cysteic acid acts in a similar manner to an ion-exchange column when the hair is washed in tap water containing metal ions such as calcium, magnesium, copper and iron.35 The acidic functional groups complex metal ions from the water (Figure 5) and thus, consumers who frequently color their hair will have higher levels of these metals in their hair. Figure 5 Association of the metals from tap water to the hair