Chemical Mystery Article: A Story That Will Curl Your Hair

Chemical Mystery Article: A Story That Will Curl Your Hair

Since ancient times people have experimented with ways to change their hair. Today, getting a permanent wave is a routine procedure that can be done either in a hairdresser shop or at home. Changing straight hair to curly hair is a practical application of protein denaturation and renaturation.

Hair contains a special class of proteins called keratins, which are also present in wool, nails, hoofs, and horns. X-ray studies show that keratins are made of α—helices coiled to form a superhelix. The disulfide bonds (—S—S—) linking the α—helices together are largely responsible for the shape of the hair. The figure on p. Shows the basic steps involved in a permanent wave process. Starting with straight hair, the disulfide bonds are first reduced to the sulfhydryl groups (—SH)

2HS—CH₃COO^—+O—S—O ⤍ ^—OOCCH₂—S—S—CH₂COO^—+2 O—SH

Where the red spheres represent different protein molecules joined by the disulfide bonds and thioglycolate (HS—CH2COO^—) is the common reducing agent. The reduced hair is then wrapped around curlers and set in the desired pattern. Next, the hair is treated with an oxidizing agent to reform the disulfide bonds. Because the S—S linkages are now formed between different positions on the polypeptide chains, the result is a new hairdo of wavy hairs.

This process involves the denaturation and renaturation of keratins. Although disulfide bonds are formed at different positions in the renatured proteins, there is no biological consequence because keratins in hair do not have any specific functions. The word “permanent” applies only to the portion of hair treated with the reducing and oxidizing agents, and the wave lasts until new and untreated keratins replace it.

Chemical Clues

1. Describe the bonding in the —S—S— linkage.

2. What are the oxidation numbers of S in the disulfide bond and in the sulfhydryl group?

3. In addition to the disulfide bonds, the α helices are joined together by hydrogen bonds. Based on this information, explain why hair swells a bit when it is wet.

4. Hair grows at the approximate rate of 6 in per year. Given that the vertical distance for a complete turn of an α helix is 5.4 Å (1 Å = 10^-8 cm), how many turns are spun off every second?

5. In the 1980s an English heiress died from a long illness. Autopsy showed that the cause of death was arsenic poisoning. The police suspected that her husband had administered the poison. The year prior to her death the heiress had taken three 1-month trips to America to visit friends on her own. Discuss how forensic analysis eventually helped the law enforcement build their case against her husband. [Hint: Arsenic poisoning was discussed in another chemical mystery. Studies show that within hours of ingesting as little as 3 mg of arsenic trioxide (As2O3), arsenic enters in the blood and becomes trapped and carried up the follicle in the growing hair. At the time of her death, the heiress had shoulder-length hair.]

Reference:
Chang, Raymond. 2010. Chemistry 10th Edition. New York: McGraw-Hill