Final answer:
Ammonium thioglycolate is the most common reducing agent in permanent wave solutions, used to break disulfide bonds in hair before reforming them with an oxidizing agent to set the hair's new shape.
Step-by-step explanation:
The most common reducing agent in permanent wave solutions is ammonium thioglycolate. In the process of creating a permanent wave, ammonium thioglycolate is applied to the hair to reduce and break the disulfide bonds. This action allows the hair's alpha-helices to shift positions relative to each other when put on curlers or rollers. After the hair is set in the desired shape, an oxidizing agent such as hydrogen peroxide is used to reform the disulfide bonds and set the hair in its new shape.