The general formula of the 20 naturally occurring amino...

The general formula of the 20 naturally occurring amino acids is drawn below. In glycine, $\mathbf{R}=\mathbf{H},$ in cysteine, $\mathbf{R}=\mathbf{C H}_{2} \mathbf{S} \mathbf{H},$ in aspartic acid, $\mathrm{R}=\mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H},$ and in threonine, $\mathbf{R}=\mathbf{C H}(\mathbf{O} \mathbf{H}) \mathbf{M e} .$ (a) Which of these amino acids are chiral? (b) For each chiral molecule, draw a representation of the $(R)$ -enantiomer. (c) What can you say about the chiral or achiral nature of the remaining 16 naturally occurring amino acids?

The general formula of the 20 naturally occurring amino acids is drawn below.
In glycine, $\mathbf{R}=\mathbf{H},$ in cysteine, $\mathbf{R}=\mathbf{C H}_{2} \mathbf{S} \mathbf{H},$ in
aspartic acid, $\mathrm{R}=\mathrm{CH}_{2} \mathrm{CO}_{2} \mathrm{H},$ and in threonine, $\mathbf{R}=\mathbf{C H}(\mathbf{O} \mathbf{H}) \mathbf{M e} .$ (a) Which of these amino acids
are chiral? (b) For each chiral molecule, draw a representation of the $(R)$ -enantiomer. (c) What can you say about the chiral or achiral nature of the remaining 16 naturally occurring amino acids?

Key Concepts

Enantiomers

Enantiomers are pairs of molecules that are mirror images of each other but are not superimposable. They are key in stereochemistry because their different spatial arrangements often result in different interactions with chiral environments, like those found in enzymes, affecting their functions and biological activity.

Stereochemistry of Amino Acids

The stereochemistry of amino acids is crucial for protein structure and function. Since nearly all naturally occurring amino acids (except glycine) have a chiral ??carbon, the specific three?dimensional arrangement (as seen in R or S configurations) determines the way amino acids interact during protein folding and biochemical processes.

Chirality

Chirality is a fundamental property in chemistry where a molecule cannot be superimposed on its mirror image. In organic molecules such as amino acids, chirality arises when a carbon atom (typically the ??carbon) is bonded to four distinct substituents, leading to the existence of two non?interchangeable forms called enantiomers.

Chiral Center

A chiral center, often the ??carbon in amino acids, is an atom that is connected to four different groups. This unique bonding pattern is responsible for a molecule’s handedness, causing it to have two distinct stereoisomeric forms (commonly designated as R and S) which can have different biological activities.

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