Biomolecules and Chemistry in Everyday Life 1 Question 9
9. The correct structure of histidine in a strongly acidic solution $(\mathrm{pH}=2)$ is
(2019 Main, 12 Jan II)
(a)
(c)
(b)
(d)
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Solution:
- Histidine has following structure in
At highly acidic $\mathrm{pH}$, i.e. 2 both the nitrogens with lone pairs will accept one $\mathrm{H}^{+}$each and $-\mathrm{C}-\mathrm{OH}$ will not loose its $\mathrm{H}^{+}$. Thus,
the final structure of histidine at $\mathrm{pH}=2$ will be
Thus, option (d) is the correct answer.
Note Amino acids have following generalise structure:
They have the tendency to loose $\mathrm{H}^{+}$of their $-\mathrm{COOH}$ group at alkaline (higher) $\mathrm{pH}$ while the $-\mathrm{NH}_{2}$ group present in them have the tendency to gain $\mathrm{H}^{+}$at acidic (lower) $\mathrm{pH}$.
