Chapter 11 371 The Effect of the Reductive Agents The disulphide bond is also attacked by reductive agents. This reaction is, probably, the most investigated one,11-12 and references therein as it is fundamental, not only for permanent waving of hair, but also for many biological processes, including folding of proteins or activity of enzymes.13, 14 The overall reaction of cystine with thiols as reductive agents, is given below: Ker–S–S–Ker + 2R–S–H à 2Ker–S–H + R–S–S–R cystine cysteine The sequence of pathways behind the overall reaction is assumed to comprise two nucleophilic displacement reactions, (b) and (c) below:8, 10 R-SH + H 2 O à R-S- + H 3 O+ (a) R-S- + Ker-S-S-Ker à R-S-S-Ker + Ker-S- (b) R-S-S-Ker + R-S- à R-S-S-R + Ker-S- (c) Ker-S- + H 3 O+ à Ker-SH + H 2 O (d) A pathway was also proposed to account for the formation of lanthionine that is found under some circumstances (at pH usually 6) during reductive reactions with hair:9 R-S-S-R R-SO-S-R R-SO2-S-R [R-SO2-SO-R] R-SO2-SO2-R 2R-SO3H R-SH + R-SO2H-R-SO2H + R-SOH R-SO3H H2O H2O H2O H2O R-SO2H R-SO3H R-SO2H R-SOH + + Figure 4.