An amino acid with a neutral side chain has pK_a1 = 2.20 (COOH) and pK_a2 = 9.60 (NH3+). Taking the isoelectric point as the mean of the two pKa values, its pI is

JEE Main levelSingle correct optionDifficulty 2 of 3bm064

An amino acid with a neutral side chain has $p K_{a 1} = 2.20$ (COOH) and $p K_{a 2} = 9.60$ ( $NH X_{3} X^{+}$ ). Taking the isoelectric point as the mean of the two pKa values, its pI is

For a neutral amino acid, pI =

(p K_{a 1} + p K_{a 2}) /2 = (2.20 + 9.60) /2 = 5.90

. The other values come from adding, or subtracting, the pKa values incorrectly.