Waving of hair

The loss of structure by a protein is called denaturation. This structural change may be a loss of quaternary, tertiary, or secondary structure it may also be degradation of the primary structure by cleavage of the peptide bonds. Even mild heating can cause irreversible denaturation. When we cook an egg, the protein called albumen denatures into a white mass. The permanent waving of hair, which consists primarily of long a helices of the protein keratin, is a result of partial denaturation. 

A practical application of the chemistry of disulfide bonds is permanent waving of hair, as summarized in Fig. 22.26. The S—S linkages in the protein of hair are broken by treatment with a reducing agent. The hair is then set in curlers to change the tertiary protein structure to the desired shape. Then treatment with an oxidizing agent causes new S—S bonds to form, which allow the hair protein to retain the new structure. 

USE In cold-waving of hair as depilatory in bacteriology for the prepn of thioglycolate media as analytical reagent, see Thioglycolic Acid. 

Problem 20.67. Permanent waving of hair involves the treatment of the hair with a reducing agent, placing the hair in curlers, and treating it with an oxidizing agent. Explain the chemistry involved. 

The sodium and anunonium salts of thioglycolic acid are used in cold waving of hair. These compounds work by breaking the disulfide bonds of hair proteins, which maintain the overall structure of hair (Section 27.6Q.The disulfide bonds are subsequently reformed by oxidation (Section 10.9G) in a second step. The calcium salt of thioglycolic acid is used as a depilatory that is, it is used to remove body hair. 

Describe how the "permanent waving" of hair is based on the chemistry of the amino acid cysteine and how it influences the tertiary structure of the proteins in the hair. 

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