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JEE Main 2018 · Chemistry · Biomolecules and Polymers · medium

Lysine has pKa values 2.2 (α-COOH), 9.0 (α-NH3^+) and 10.5 (ε-NH3^+). Its isoelectric point and the direction in which it migrates in an electric…

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Lysine has pKapK_a values 2.22.2 (α-COOH\alpha\text{-COOH}), 9.09.0 (α-NH3+\alpha\text{-NH}_3^+) and 10.510.5 (ε-NH3+\varepsilon\text{-NH}_3^+). Its isoelectric point and the direction in which it migrates in an electric field at pH 77 are, respectively,
  1. A. 9.759.75, towards the cathode
  2. B. 9.759.75, towards the anode
  3. C. 5.65.6, towards the cathode
  4. D. 6.356.35, towards the anode
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Answer: (A) 9.759.75, towards the cathode

The neutral (zwitterionic) form of lysine lies between the second and third dissociation steps, since the net charge goes +2,+1,0,−1+2,+1,0,-1. So pI=9.0+10.52=9.75pI=\dfrac{9.0+10.5}{2}=9.75. At pH 7<pI7<pI, the molecule carries a net positive charge (+1+1) and therefore migrates towards the cathode (negative electrode). Averaging the first two pKapK_a values (as for neutral amino acids) is the common mistake and gives 5.65.6.
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