JEE PYQ: Biomolecules And Polymers Question 62
Question 62 - 2019 (12 Jan Shift 1)
The correct structure of histidine in a strongly acidic solution (pH $= 2$) is:
(1) $\text{H}_3\overset{+}{\text{N}}-\text{CH}-\text{COOH}$ with $\text{NH}_2$ on imidazole
(2) $\text{H}_3\overset{+}{\text{N}}-\text{CH}-\text{COO}^-$ with NH on imidazole
(3) $\text{H}_3\overset{+}{\text{N}}-\text{CH}-\text{COOH}$ with $\overset{+}{\text{NH}}$ on imidazole
(4) $\text{H}_3\overset{+}{\text{N}}-\text{CH}-\text{COO}^-$ with $\text{NH}_2$
Show Answer
Answer: (3)
Solution
At pH = 2 (strongly acidic), both the amino group and the imidazole ring nitrogen are protonated, and the carboxyl group remains protonated ($-\text{COOH}$).
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