Question

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.

Short answer

Yes, cystine is optically active because its chirality centers remain after the reaction.

Step-by-step solution

Understand the Structure of Cysteine

Cysteine is an amino acid with a chiral center, which means it has four different groups attached to a central carbon atom. In its naturally occurring form, cysteine has the \(S\) configuration, which is important for its optical activity.

Reaction of Two Cysteines

When two cysteine molecules react, they form a disulfide bond resulting in cystine. The significant point to note is that the reaction does not involve the chirality centers; hence, each cysteine retains its \(S\) configuration.

Concept of Optical Activity

A compound is optically active if it can rotate plane-polarized light, which typically occurs if the molecule is chiral (non-superimposable on its mirror image). Each cysteine has retained its \(S\) configuration, maintaining chirality.

Determine the Symmetry of Cystine

In cystine, the two molecules of cysteine are joined via a disulfide bond. Since both chirality centers retain the same configuration (\(S\)), cystine itself does not possess any symmetry that would cause optical inactivity (like a plane of symmetry or a center of symmetry).

Conclusion on Optical Activity of Cystine

Given that cystine does not acquire any new symmetry that neutralizes its optical activity, and since the chirality centers remain intact, cystine remains optically active.

Key concepts

Cysteine

Cysteine is a vital amino acid known for containing a unique sulfur atom, distinctive among all amino acids. This sulfur atom is part of a thiol group, denoted as \(-\text{SH}\), which gives cysteine special properties. These properties are critical in forming disulfide bonds, a topic we will explore further.
Despite being non-essential since our bodies can produce it from serine, cysteine plays a crucial role in various applications. It is often found in dietary supplements and used to boost antioxidant capacity. Additionally, it acts as a building block for proteins and other important biomolecules.
In its natural configuration, the amino acid cysteine has a specific arrangement around a central carbon atom known as the chiral center, giving it unique optical properties, which we will discuss in more detail later.

Chirality Center

A chirality center, also known as a chiral center or stereocenter, is a carbon atom attached to four different groups. This arrangement allows for two non-superimposable mirror images, known as enantiomers, resembling how our left and right hands are mirrors of each other, yet not exactly the same.
This property is crucial in chemistry as it gives molecules, like cysteine, the ability to rotate plane-polarized light. Molecules with one or more chirality centers are typically optically active, meaning they can influence how light passes through them.
In the naturally occurring form of cysteine, the configuration at the chirality center is described as \(S\), following specific rules of molecular priority. This configuration not only defines the direction in which cysteine rotates light but also influences its reactivity and biological interactions.

Disulfide Bond

A disulfide bond is a strong covalent bond formed between two sulfur atoms, specifically from the thiol groups of two cysteine molecules. This type of bond is integral in the stabilization of the three-dimensional structures of proteins, particularly in enzymes and antibodies.
When two cysteine molecules undergo oxidation, they link together via a disulfide bond, resulting in a new compound called cystine. This transformation is especially important in biology because it helps maintain the structural integrity of many proteins found in the human body.
Importantly, the creation of the disulfide bond does not alter the chirality centers of the original cysteine molecules, preserving their optical activity. Understanding this transformation helps to appreciate how proteins maintain their structure and function under various conditions.

Chiral Molecules

Chiral molecules are compounds that are asymmetric, meaning they cannot be superimposed on their mirror images. This asymmetry is often due to the presence of one or more chirality centers. Chiral molecules are critical in chemistry and biochemistry because they exhibit optical activity; they can rotate plane-polarized light, which is often used to determine their purity and concentration in solutions.
The chirality of a molecule like cysteine starts from its central carbon atom, which is bonded to four different groups, giving it its chiral property. The term chirality is derived from the Greek word for hand, reflecting how a chiral object or molecule can be like left and right hands, mirror images yet not identical.
In the case of cystine, formed by two chiral cysteine molecules connected by a disulfide bond, it retains the optical activity characteristic of its components. This is because the formation of the bond does not introduce any symmetry that would otherwise negate its ability to rotate light.