Glutamic acid has three ionisable groups: alpha-COOH (pK_a = 2.2), side-chain COOH (pK_a = 4.3) and alpha-NH3+ (pK_a = 9.7). Taking pI as the mean of the two pKa values that flank the electrically neutral species, the pI is

JEE Advanced levelSingle correct optionDifficulty 3 of 3bm073

Glutamic acid has three ionisable groups: alpha- $COOH$ ( $p K_{a} = 2.2$ ), side-chain $COOH$ ( $p K_{a} = 4.3$ ) and alpha- $NH X_{3} X^{+}$ ( $p K_{a} = 9.7$ ). Taking pI as the mean of the two pKa values that flank the electrically neutral species, the pI is

As pH rises the charges go

+ 1 \to 0 \to - 1 \to - 2

, and the neutral zwitterion exists between

p K_{a} = 2.2

and

p K_{a} = 4.3

. So pI =

(2.2 + 4.3) /2 = 3.25

. Using the amino pKa instead gives the values in the other options, which refer to charged species.