The correct structure of histidine in a strongly acidic solution is

Question

The correct structure of histidine in a strongly acidic solution  is

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Ionization of histidine at pH 2:

Histidine has three ionizable groups with pKa values:
| Group | pKa |
|---|---|
| –COOH (carboxyl) | ~1.8 |
| Imidazole (side chain –NH) | ~6.0 |
| –NH₃⁺ (amino) | ~9.2 |

At pH 2 (strongly acidic):

pH 2 is below all pKa values, so ALL ionizable groups are in their protonated (acidic) form:

- –COOH: pH < pKa(1.8)? Actually pH 2  pKa 1.8, so –COOH is partially deprotonated. But at pH 2, the predominant form has –COOH (since pKa ≈ 1.8, at pH 2 about 60% is –COO⁻

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