The naturally occurring form of the amino acid cysteine...

The naturally occurring form of the amino acid cysteine (Problem 5.63) has the $S$ configuration at its chirality center. On treatment with a mild oxidizing agent, two cysteines join to give cystine, a disulfide. Assuming that the chirality center is not affected by the reaction, is cystine optically active? Explain.

Key Concepts

Disulfide Bond Formation

Disulfide bond formation is a process in which two thiol (-SH) groups oxidize to form a disulfide (-S-S-) linkage. In biochemistry, this is an important reaction, for example, in stabilizing protein structures. When two amino acids like cysteine form a disulfide bond, the configuration at the original chirality centers can be maintained, but the overall molecule may acquire new symmetry features that affect its optical properties.

Chirality Centers

A chirality center in a molecule is an atom—usually carbon—that has four different substituents, leading to non-superimposable mirror images (enantiomers). The configuration of these centers (R or S) determines the molecule’s handedness, which in turn affects its interactions with other chiral substances and its ability to rotate plane-polarized light.

Optical Activity

Optical activity refers to the ability of chiral substances to rotate the plane of polarized light. A compound with one or more chirality centers typically shows optical activity because its mirror images (enantiomers) interact differently with polarized light. However, if a molecule possesses an internal plane of symmetry or is a meso compound, it may be optically inactive despite containing chiral centers.

Meso Compounds

Meso compounds contain multiple chirality centers but are overall achiral because they possess an internal plane of symmetry. This symmetry causes the optical activities of the individual chiral centers to cancel each other out. In cases where the formation of new bonds (such as a disulfide bond) creates this internal symmetry, the resulting compound becomes optically inactive even though the original monomers were optically active.

Please give Ace some feedback