Lysine has pK_a values of 2.2 (alpha-COOH), 9.0 (alpha-NH3+) and 10.5 (side-chain NH3+). Taking pI as the mean of the two pKa values that flank the neutral species, the pI of lysine is

JEE Advanced levelSingle correct optionDifficulty 3 of 3bm074

Lysine has $p K_{a}$ values of 2.2 (alpha- $COOH$ ), 9.0 (alpha- $NH X_{3} X^{+}$ ) and 10.5 (side-chain $NH X_{3} X^{+}$ ). Taking pI as the mean of the two pKa values that flank the neutral species, the pI of lysine is

Charges go

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

with increasing pH. The neutral species lies between the second and third pKa values (

9.0

and

10.5

), so pI =

(9.0 + 10.5) /2 = 9.75

. The other options average pKa values that flank charged species.