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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3bm260

For the amino acid shown, $p K_{a}$ values are 1.82 (COOH), 6.00 (side chain) and 9.17 ( $α$ - $NH X_{3} X^{+}$ ). Which of the following is/are correct?

AThe side chain contains an aromatic heterocyclic ring.
BThe net charge at pH 1 is +2.
CThe isoelectric point lies between 7 and 8.
DThe net charge at pH 4 is +2.

Answer: A, B, C

The imidazole side chain is aromatic. At pH 1 all three groups are protonated: COOH (0),

α

-

NH X_{3} X^{+}

(+1) and imidazolium (+1), net +2. The pI is

(6.00 + 9.17) /2 = 7.585

, between 7 and 8. At pH 4 the COOH is ionised (-1), giving +1, not +2. The amino acid is histidine.

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