Reactions of the Mercaptan Group

The previous section described various reagents that have been used for the reduction of the disulfide bond in keratin fibers. Most of these reactions produce cysteinyl residues, or mercaptan groups in the fibers. 

The mercaptan group is one of the most reactive functional groups in all organic chemistry, and it readily undergoes oxidation, nucleophilic displacement, nucleophilic addition, and free-radical addition and displacement reactions. This section discusses some of the chemical literature pertaining to these types of reaction in reduced keratin fibers and illustrates the potential reactivity of the mercaptan group in hair. 

The oxidation of the mercaptan group can occur by two distinct pathways— the S—S fission route (pathway in the presence of most chemical oxidants), and the C—S fission route (pathway for radiation-induced cleavage of the disulfide bond). Only the S—S fission route will be discussed in this section because it is the most relevant pathway in relation to permanent waves and reducing agents. For a more complete discussion of both of these mechanistic schemes, see Chapter 4. 

The oxidation of the mercaptan group can occur in several stages  

The mercaptan group is an extremely powerful nucleophile and readily undergoes nucleophilic displacement reactions. This property is the basis of several quantitative tests for cysteine and/or cystine, including the Sullivan test, which involves nucleophilic displacement by mercaptide ion on iodoacetate [86]. 

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