Changing the Shape of Hair 166 that hair should not be washed immediately after a permanent wave. Eventually, the free thiol groups will be oxidized to cysteic acid by exposure to air and will no longer be available to catalyze set loss during shampooing. Sodium bisulfite also breaks S-S bonds in hair. The reaction occurs through the formation of a Bunte salt (Eq.4): Eq. 4 The optimum pH for the reaction with bisulfite ranges about pH 3.0–6.0 but at most 50% of the hair disulfide bonds can be broken even after treatment for 72 hours with a large excess of bisulfite under optimum conditions.25 The equilibrium constant for the reaction is only ~0.001 compared to ~1.0 for reduction with TGA, so reduction is much less efficient with bisulfite. Kinetic studies of hair reduction: The kinetics of the reduction reaction can be followed by reacting the hair with iodoacetate to convert reduced cysteine to S-carboxymethyl cysteine (SCMC) prior to digestion of hair and amino acid analysis for SCMC.17,40 If the hair is digested in the presence of dithiopropionic acid (DTDPA), unreacted cystine is converted to cysteine mercaptopropionic acid Figure 7 Sulfide disulfide interchange. Stresses in the hair, shown by the vertical arrows, relax in each step as new bonds are formed as described by Feughelman. This requires the presence of free –S- groups to initiate the process.