Which amino acid is unique for having a secondary nitrogen in its backbone and often introduces kinks in polypeptide chains?

Question

BlackTom AI · Accepted Answer

1. Relevant knowledge points and concepts:
Proline is unique among the 20 standard amino acids because its side chain forms a covalent bond with the backbone nitrogen, creating a five-membered pyrrolidine ring. That makes the backbone nitrogen a secondary amine (bound to two carbons instead of one), so proline is often called an imino acid. This cyclic structure fixes the phi dihedral angle and prevents the backbone N from donating a hydrogen for backbone hydrogen bonding.

2. Step-by-step reasoning:
- The question asks which amino acid has a secondary nitrogen in its backbone and frequently introduces kinks.
- Secondary nitrogen in the backbone implies the backbone nitrogen is bonded to two carbon atoms (one from the α-carbon and one from the side chain).
- Among the choices, only proline has its side chain bonded to the backbone nitrogen, forming a ring and converting that nitrogen into a secondary amine.
- The ring restricts φ (phi) angles and prevents the N–H hydrogen from participating in regular peptide hydrogen bonds, disrupting regular secondary structures.

3. Intermediate structural details (no numerical calculation needed):
- In a typical amino acid the backbone amide nitrogen is —NH— (part of peptide bond) where the side chain is attached to the α-carbon only.
- In proline the side chain (γ, δ carbons) connects back to the backbone amide nitrogen, so the N is bonded to two carbons: the α-carbon and the side-chain carbon — a secondary nitrogen.

4. Why the correct answer is correct:
Proline’s cyclic structure creates the secondary backbone nitrogen and rigid geometry that introduces bends or “kinks” in polypeptide chains and often terminates or distorts α-helices and appears in turns.

5. Why the other options are incorrect:
- Methionine: contains a thioether side chain (sulfur), not a ring to the backbone N.
- Glycine: has only a hydrogen side chain (very flexible) and a primary backbone N, not secondary.
- Serine: has a hydroxyl-containing side chain attached to the α-carbon, backbone N remains typical.
- Tyrosine: has a bulky aromatic side chain with a phenolic OH, backbone N is not secondary.

6. Conclusion:
The correct answer is Proline, which matches the provided answer.

Which amino acid is unique for having a secondary nitrogen in its backbone and often introduces kinks in polypeptide chains?单项选择题

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