JEE Main · 2019 · 12 Jan 2019 · Shift 2 · Biomolecules

Histidine carries three ionisable groups $-$ the carboxyl ($p{K}_a\approx 2$), the $\alpha$-amino ($p{K}_a\approx 9$) and the imidazole side chain ($p{K}_a\approx 6$). At $\mathrm{pH}=2$, well below the amino and imidazole values, both nitrogen centres are protonated, as $-\mathrm{NH}{}_3{}^{+}$ and a protonated imidazole ring, while the carboxyl is largely held as $-\mathrm{COOH}$. The structure with $-\mathrm{COOH}$, $-\mathrm{NH}{}_3{}^{+}$ and a protonated imidazole is the correct one.