To determine the correct structure of histidine in a strongly acidic solution (pH = 2), we need to understand its acid-base properties and protonation states. Histidine is an amino acid with three ionizable groups: a carboxylic acid group (-COOH), an amino group (-NH₂), and an imidazole side chain (which has a nitrogen that can be protonated).

The pKa values for histidine are approximately:
- pKa₁ (carboxylic acid) ≈ 1.8
- pKa₂ (imidazole) ≈ 6.0
- pKa₃ (amino group) ≈ 9.2

At a very low pH of 2, which is strongly acidic, the solution has a high concentration of H⁺ ions. This causes protonation of basic groups. Comparing the pH to the pKa values:

• Since pH (2) is slightly above pKa₁ (1.8), the carboxylic acid group is mostly protonated (-COOH).
• Since pH (2) is much below pKa₂ (6.0), the imidazole group is fully protonated (positively charged).
• Since pH (2) is much below pKa₃ (9.2), the amino group is fully protonated (-NH₃⁺).

Therefore, at pH = 2, all three groups are protonated. The structure should have:
- COOH (uncharged)
- Imidazole ring with N protonated (positive charge)
- NH₃⁺ (positive charge)

This gives a net charge of +2. The correct structure is the one where the imidazole nitrogen is protonated (forming -NH⁺- in the ring) and the amino group is -NH₃⁺, while the carboxylic acid is -COOH.

Among the options, the structure with both the amino group and the imidazole group protonated (and carboxylic acid group not deprotonated) is correct. Typically, this is represented with positive charges on the amino nitrogen and the imidazole nitrogen (often the nitrogen not involved in the double bond in the ring).

Final Answer: The structure where the imidazole side chain is protonated (has a positive charge) and the amino group is also protonated (-NH₃⁺), while the carboxylic acid is in its neutral form (-COOH). This corresponds to the option showing $H_{3}N+-CH-COOH$ with the imidazole ring having a protonated nitrogen (positive charge).

Related Topics

Amino Acid Protonation: Amino acids have multiple ionizable groups. The protonation state depends on pH relative to pKa. At low pH, all basic groups are protonated; at high pH, all acidic groups are deprotonated.

Isoelectric Point (pI): The pH at which an amino acid has no net charge. For histidine, pI is around 7.6, but at pH=2, it is far from pI and highly protonated.

Formulae

The Henderson-Hasselbalch equation: $pH=p{K}_a+\log \left(\frac{[A-]}{\left[HA\right]}\right)$ helps predict the protonation state. If pH < pKa, the group is protonated; if pH > pKa, it is deprotonated.