Which of the twenty standard biological amino acids is optically inactive (achiral) because its α-carbon is bonded to two identical hydrogen atoms?
Answer and explanation
Correct answer: B. Glycine
- Step 1: An -amino acid has the general formula .
- Step 2: Optical activity requires a chiral center, a carbon atom bonded to four chemically distinct substituents.
- Step 3: In glycine, the side chain is simply a hydrogen atom (): .
- Step 4: Because the -carbon is attached to two identical hydrogen atoms, it lacks a chiral center (it possesses a plane of symmetry), making glycine the only optically inactive standard amino acid.
Answer
Glycine
Why not the others: 'Alanine' (), 'Valine' (), and 'Proline' (cyclic secondary amine) all possess chiral centers and are optically active.
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