Chapter 11 369 of keratin,4 the volume of hydrogen bonds cross-linking the keratin surpasses those of disulphide bonds. On the other hand, the strength of a hydrogen bond, at 3-5 kcal/mol, is about 10 to 15 times smaller than that of a disulphide bond,5 indicating that while small effects can be related to hydrogen bonds, the major changes are dominated by the behavior of cystine. To sum up, the behavior of hair is inherently related to the state of disulphide and hydrogen bonds. The Effect of pH Three distinct mechanisms have been proposed for describing the effect of pH on the S-S bonds in proteins: hydrolysis, alpha- elimination and beta-elimination, of which beta-elimination occurs most frequently.6-8 Figure 2 provides examples of all three mechanisms in question. HC–CH2–S–S–CH2–CH+2OH- HC–CH2–S-+-OS–CH2–CH+H2O HC–CH2–S–S–CH2–CH+OH- HC–CH2–S–S–CH–CH+H2O 2CH–CH2–SO-S- HC–CH2–S-+HC–HC–CH2–SO2- Thiol Sulphenic acid Sulphinic acid Hydrolysis: a-Elimination: HC–CH2–S–S–CH–CH HC–CH2–S-+S–CH–CH Thiol Thioaldehyde HC–CH–S+OH- HC–CH–O+HS- Aldehyde b-Elimination: HC–CH2–S–S–CH2–CH+OH- HC–CH2–S–S–CH2–C-+H2O HC–CH2–S–S–CH2–C- HC–CH2–S–S-+CH2–C Persulphide Dehydroalanyl residue HC–CH2–S–S- HC–CH2–S-+S Thiol Figure 2. Mechanisms for describing the effect of pH on the S-S bonds in proteins