Question

All amino acids have a chiral centre, except

• A. Lysine
• B. Arginine
• C. Histidine
• D. Glycine

Hint:

Glycine is the simplest amino acid. Its side chain is just a single hydrogen atom. Because the central carbon is now attached to two hydrogens, it's not chiral. All others are.

Answer 1

The Correct Option is D

Step 1: Define a chiral center. A chiral center (or asymmetric carbon) is a carbon atom that is attached to four different types of atoms or groups of atoms.
Step 2: Examine the general structure of an amino acid. An amino acid has a central carbon atom (the \(\alpha\)-carbon) bonded to an amino group (-NH\(_2\)), a carboxyl group (-COOH), a hydrogen atom (-H), and a variable side chain (-R group).
Step 3: Analyze the condition for chirality. For the \(\alpha\)-carbon to be chiral, the four groups attached to it must be different. This means the R group must be different from -H, -NH\(_2\), and -COOH.
Step 4: Identify the exception. For the amino acid Glycine, the R group is simply another hydrogen atom (-H). This means the \(\alpha\)-carbon is bonded to two hydrogen atoms, which are identical. Therefore, the \(\alpha\)-carbon in glycine is not a chiral center, and glycine is an achiral molecule. All other standard amino acids have more complex R groups, making their \(\alpha\)-carbons chiral.