Cysteine has pK_a values of 1.96 (COOH), 8.18 (side-chain SH) and 10.28 (NH3+). Taking pI as the mean of the two pKa values flanking the neutral species, the pI of cysteine is

JEE Advanced levelSingle correct optionDifficulty 3 of 3bm080

Cysteine has $p K_{a}$ values of 1.96 (COOH), 8.18 (side-chain $SH$ ) and 10.28 ( $NH X_{3} X^{+}$ ). Taking pI as the mean of the two pKa values flanking the neutral species, the pI of cysteine is

Charges go

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

as pH rises, because the thiol ionises (pKa 8.18) before the ammonium (pKa 10.28). The neutral species lies between 1.96 and 8.18, so pI =

(1.96 + 8.18) /2 = 5.07