The shape of hair is determined in part by the pattern of disulfide bonds in keratin, its major protein. From first to last, arrange the five steps needed to induce permanent curls in hair.

Answers:

1. Keratin's a-helices convert to B-sheets.

2. Keratin's disulfide bonds break.

3. Physically curl the hair.

4. Apply an oxidizing agent.

5. Keratin's disulfide bonds reform to hold shape.

6. Apply a thiol-containing reagent and gentle heat.

7. Keratin's amino acids rearrange into the curled conformation.

Apply a thiol-containing reagent and gentle heat. Keratin's disulfide bonds break. Physically curl the hair. Apply an oxidizing agent. Keratin's disulfide bonds reform to hold shape.

Explanation:

The hair is mostly made up of keratin (a protein contains the amino acid cysteine) molecules that are arranged in straight bundles. The disulfide bonds hold these bundles together and give shape and strength to the hair. One sulfur atom on a polypeptide chain of cysteine amino acids forms a disulfide bond with another sulfur atom on a neighboring polypeptide chain.

While inducing permanent curls, the hair is treated with a thiol-containing reagent such as ammonium thioglycolate which acts as a reducing agent and then applies gentle heat. The thiol group breaks the disulfide bonds by replacing sulfur atoms in the disulfide bond with hydrogen atoms. When the disulfide bonds break, the keratin bundles come apart, and the polypeptide chains are free to move around. As a result, the hair is weakened and the keratin molecules can adjust to their new shape. The hair is then curled. In order to make this curling permanent, a lotion containing an oxidizing agent or neutralizer like hydrogen peroxide is applied after thoroughly rinsing the hair. As a result, the hydrogen atoms are removed and the broken disulfide bonds are reformed, and the keratin molecules hold the shape of the curls.