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JEE Main 2022 · Chemistry · Biomolecules and Polymers · easy

Aspartic acid has pKa values 2.1 (α-carboxyl), 3.9 (side-chain COOH) and 9.8 (α-ammonium group). Its isoelectric point is

Single correct option

Aspartic acid has pKa\mathrm{p}K_a values 2.12.1 (α\alpha-carboxyl), 3.93.9 (side-chain COOH) and 9.89.8 (α\alpha-ammonium group). Its isoelectric point is
  1. A. 3.03.0
  2. B. 6.06.0
  3. C. 5.955.95
  4. D. 2.12.1
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Answer: (A) 3.03.0

Aspartic acid is an acidic amino acid. Its net charge is +1+1 at very low pH, 00 after the first ionisation, −1-1 after the second and −2-2 after the third. The neutral zwitterion exists between the first two pKa\mathrm{p}K_a values, so pI=12(2.1+3.9)=3.0\mathrm{p}I=\tfrac12(2.1+3.9)=3.0. Averaging the wrong pair (3.93.9 and 9.89.8) gives 6.856.85, and averaging all three gives 5.275.27.
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