JEE Main 2019 · Chemistry · Biomolecules and Polymers · medium

Lysine has pKa values 2.18 (α-COOH), 8.95 (α-NH3^+) and 10.53 (side-chain ε-NH3^+). Its isoelectric point is nearest to

Single correct option

Lysine has

\mathrm{p}K_a

values

2.18

(

\alpha

-COOH),

8.95

(

\alpha

\mathrm{NH_3^+}

) and

10.53

(side-chain

\varepsilon

\mathrm{NH_3^+}

). Its isoelectric point is nearest to

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Answer: (D) $9.74$

At the isoelectric point the net charge is zero. Lysine is a basic amino acid; its charge goes

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

with

\mathrm{p}K_a

values

2.18, 8.95, 10.53

. The neutral species exists between the second and third deprotonation steps, so

\mathrm{pI} = \tfrac12(8.95+10.53) = 9.74

. Averaging the first two values (

5.57

) is the typical mistake; it applies to a neutral or acidic amino acid, not to a basic one.

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