Q.13 Among the following, the structure representing histidine is

Correct answer

The correct structure representing histidine is option (C), which shows an imidazole ring attached via a –CH₂– group to the α‑carbon bearing the protonated amino group and the deprotonated carboxylate group, i.e., the usual zwitterionic form of histidine at physiological pH.

Introduction

Multiple‑choice questions on amino acid structures often test recognition of side chains and ionic states at physiological pH, and histidine is a favourite because of its imidazole ring and ability to exist in several protonation states. Understanding the histidine structure representing histidine in zwitterionic form helps in biochemistry, enzyme mechanism questions, and CSIR‑NET level examinations.

Core features of histidine structure

• Histidine has the side chain –CH₂–imidazole, meaning the imidazole ring is separated from the α‑carbon by one methylene carbon.
• At physiological pH, histidine is usually drawn as a zwitterion: H₃N⁺–CH(COO⁻)–CH₂–imidazole, where the amino group is protonated and the carboxyl group is deprotonated.
• The imidazole ring contains two nitrogens (one pyridine‑like, one pyrrole‑like), and in the neutral side chain only one N is protonated; extra protonation gives an imidazolium cation but that is not the common zwitterionic form used for standard amino acid structures.

Detailed analysis of each option

Option A

In option A, the α‑carbon correctly carries the NH₃⁺ and COO⁻ groups, but the side chain ring appears directly attached to the α‑carbon without the intervening –CH₂– group.

This eliminates A, because histidine must have a β‑carbon (–CH₂–) between the α‑carbon and the imidazole ring, whereas option A shows the ring bonded straight to the α‑carbon.

Option B

Option B again shows the amino acid in zwitterionic form, but the imidazole ring appears incorrectly oriented so that both ring nitrogens are in positions inconsistent with the standard histidine connectivity relative to the side‑chain carbon.

More importantly, the side chain seems to lack the proper –CH₂– linkage or misplaces it within the ring, so this structure no longer matches the known histidine skeleton and must be rejected.

Option C (correct)

Option C displays H₃N⁺–CH(COO⁻)–CH₂– followed by an imidazole ring in which one nitrogen is protonated and the other is neutral, matching the classic textbook structure of histidine at physiological pH.

The presence of a single –CH₂– group between the α‑carbon and the imidazole ring, together with correct arrangement of the two ring nitrogens, makes C the only structure accurately representing histidine.

Option D

Option D shows an imidazole ring further substituted so that the side chain length or point of attachment is incorrect, effectively changing the amino acid into a different imidazole‑containing derivative rather than histidine.

In addition, the pattern of protonation on the ring nitrogens does not correspond to the major histidine zwitterion normally depicted, so D cannot be accepted as the structure representing histidine.

Key takeaways for similar MCQs

• Always check the correct backbone: NH₃⁺–CH(COO⁻)–.
• Verify the correct side chain length (how many methylenes before the functional group).
• Confirm the correct functional group and typical protonation state at physiological pH (for histidine, an imidazole ring with one –CH₂– spacer).

Using these checkpoints quickly identifies option (C) as the correct histidine structure in such MCQs.