Hair Damage 368 provides the grounding for most of its physical and chemical properties.2 The disulphide bond differs from the others within folded proteins, such as hydrogen bonds and electrostatic and van der Waals interactions, in being covalent and in its stability can be impacted by the environment.3 A classification of possible disulphide linkages in alpha-keratins, based on structural features, is given in Figure 1.2 While IFAP-IFAP, IF-IF, and IF-IFAP disulphide linkages are highly probable to exist, intracoil disulphide bridges (i.e. within a single alpha-helix) can be excluded, and intrarope disulphide bonds (i.e. between two alpha- helices in the same rope) are unlikely to be present due to stearic constraints. From these considerations, it appears that most of the damage incurred by hair is, in one way or another, related to the chemistry of the cystine. Consequently, the interaction of cystine with various chemical or physical agents provides understanding with regard to how the hair damage may occur. In addition to disulphide bonds, hydrogen bonds also play an important role. The chains of amino acids may easily bridge hydrogen bonds between the oxygen atoms of carbonyls and the amide groups from the neighboring chain. The side carboxyl groups of various amino acids (e.g. asparagyl, glutamic) may also interact via hydrogen bonds with the amino group of lysine. Overall, with an estimated one hydrogen bond and 1/9 disulphide bond per 122 mw Figure 1. Classification of disulphide linkages in alpha-keratin based on structural features IF = intermediate filament chains IFAP = intermediate filament associated proteins or matrix chains linker = rod domain segments L1, L2 or L12 coiled coil refers to a component chain of the two-strand rope