Side-chain chemistry: which amino acid contains an imidazole/imidazolium ring? Identify the amino acid whose side chain features the imidazole ring capable of protonation to form an imidazolium.

Introduction / Context:
Side-chain functional groups determine enzyme catalysis and buffering near physiological pH. Histidine’s imidazole ring has a pKa near 6, enabling it to donate or accept protons in active sites.

Given Data / Assumptions:

• We focus on canonical amino acids.
• Physiological pH is approximately 7.2–7.4.

Concept / Approach:
The imidazole ring (neutral) can be protonated to the imidazolium cation. Histidine uniquely bears this ring, making it prevalent in catalytic triads and metal-binding motifs.

Step-by-Step Solution:
Scan residues: alanine/valine/leucine are aliphatic; cysteine contains a thiol but no imidazole.Identify histidine as the only residue with an imidazole side chain.Conclude histidine is correct.

Verification / Alternative check:
Active sites of many proteases (for example, serine proteases) require histidine’s proton-transfer capacity.

Why Other Options Are Wrong:
They lack an imidazole ring and do not exhibit the same acid–base behavior near neutral pH.

Common Pitfalls:
Confusing histidine with histamine (a decarboxylation product) or with aromatic residues tyrosine/tryptophan.

Final Answer:
Histidine.