Question

The correct structure of histidine in a strongly acidic solution (pH=2) is

• A.

  

• B.

  

• C.

  

• D.

  

Answer 1

The Correct Option is A

To determine the correct structure of histidine in a strongly acidic solution (pH=2), we need to understand the behavior of the amino acid in acidic environments. Histidine is an amino acid with a side chain containing an imidazole group, which can gain or lose hydrogen ions based on the pH of the solution.

In a strongly acidic solution, the carboxyl group (-COOH), the amino group (-NH₂), and the imidazole ring are all affected:

1. The carboxyl group remains protonated as -COOH because the acidic environment favors the protonated form.
2. The amino group becomes protonated and is represented as -NH₃⁺ due to the high concentration of H⁺ ions in the acid.
3. The imidazole ring also gets protonated, gaining a positive charge, making it behave like -NH₊ at certain positions depending on its structure.

Therefore, at pH=2, histidine will have both its amino group and imidazole ring protonated, and the carboxyl group remains unchanged in its protonated form. The correct structure is the one showing protonation at these sites.

This image represents the structure of histidine in a strongly acidic solution, with protonation of the amino group, imidazole ring, and retention of the proton in the carboxyl group.