Q: Which one of the following is an achiral amino acid?
(a) Alanine
(b) Glycine
(c) Phenylalanine
(d) Proline

Which Amino Acid Is Achiral? Understanding Chirality in Amino Acids

Chirality is a fundamental concept in organic and biological chemistry, especially when studying the structure of amino acids. While most amino acids are chiral, one of them stands out for being achiral.

Q: Which one of the following is an achiral amino acid?
(a) Alanine
(b) Glycine
(c) Phenylalanine
(d) Proline

✅ Correct Answer:

(b) Glycine

🔬 Explanation:

Chirality in molecules refers to the presence of an asymmetric carbon atom — a carbon attached to four different groups. This leads to two non-superimposable mirror images, called enantiomers. In amino acids, chirality is determined by the central (alpha) carbon.

✔️ Why Glycine is Achiral:

• Structure of Glycine:
  H₂N–CH₂–COOH

• Glycine’s side chain is a hydrogen atom, making its alpha carbon bonded to two hydrogen atoms, along with –NH₂ and –COOH groups.

• Because it does not have four different groups, glycine is not chiral — it is achiral.

❌ Why the Other Options Are Chiral:

• Alanine: Side chain is –CH₃

• Phenylalanine: Side chain is –CH₂–C₆H₅ (benzyl group)

• Proline: Side chain loops back to form a ring with the amino group, but still forms a chiral center
  All of them have four different groups around the alpha carbon, making them chiral.

🧬 Importance of Chirality in Amino Acids:

• Chirality affects how amino acids interact with enzymes, receptors, and drugs.

• In proteins, only the L-isomers of amino acids are used in ribosomal synthesis.

• An achiral amino acid like glycine adds flexibility in protein structures, especially in turns and loops of polypeptide chains.

Conclusion:

Glycine is the only achiral amino acid because its side chain is a hydrogen atom, resulting in a symmetrical alpha carbon. Understanding chirality is essential for students and researchers in biology, chemistry, and medicine.