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JEE Main 2017 · Chemistry · Biomolecules and Polymers · medium

An amino acid has pKa values 2.0 (α-COOH), 4.0 (side chain COOH) and 9.8 (α-NH3^+). Its isoelectric point and the direction of migration in an…

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An amino acid has pKa\mathrm{p}K_a values 2.02.0 (α-COOH\alpha\text{-COOH}), 4.04.0 (side chain COOH\mathrm{COOH}) and 9.89.8 (α-NH3+\alpha\text{-NH}_3^+). Its isoelectric point and the direction of migration in an electric field at pH=7\mathrm{pH}=7 are respectively
  1. A. 3.03.0, towards the anode
  2. B. 5.95.9, towards the anode
  3. C. 3.03.0, towards the cathode
  4. D. 6.96.9, towards the cathode
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Answer: (A) 3.03.0, towards the anode

The amino acid is acidic (extra COOH\mathrm{COOH}). The zwitterionic neutral form lies between the two lowest pKa\mathrm{p}K_a values, so pI=(2.0+4.0)/2=3.0\mathrm{pI}=(2.0+4.0)/2=3.0 (not (2.0+9.8)/2=5.9(2.0+9.8)/2=5.9). At pH=7>pI\mathrm{pH}=7>\mathrm{pI} the molecule carries a net negative charge (both carboxyls deprotonated, ammonium still protonated) and therefore moves towards the anode.
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