What is changed back to the cystine state during the oxidation and neutralization process?

The correct answer is cysteine. During the oxidation and neutralization process, cysteine, which is an amino acid with a thiol side chain, can undergo oxidation to form disulfide bonds. When these disulfide bonds are broken, typically during a reduction process, cysteine can be reverted back to its original state. This is particularly relevant in the context of hair and other keratinous structures, where the formation and breaking of sulfur bonds influence the texture and shape of the hair.

The other substances mentioned, such as keratin, collagen, and elastin, are proteins that are primarily composed of various amino acids, including cysteine, but they do not revert back to a cystine state during oxidation. Instead, keratin is structured from many amino acids, with disulfide bridges between cysteine residues contributing to its resilience and structural integrity. Collagen and elastin are structural proteins found in various connective tissues, and while they contain different amino acids, they do not interchangeably convert between forms like cysteine and cystine during the described processes. Understanding the role of cysteine in disulfide bond formation is fundamental in processes like wave-setting or straightening hair, where oxidation plays a key role.