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JEE Advanced · 2020 · 27 Sep 2020 · Paper 2 · Biomolecules

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The structure of a peptide is given below

If the absolute values of the net charge of the peptide at , and are and , respectively, then what is ?

Answer: 5

Solution

The peptide contains three amino acids: Tyrosine-Glutamic acid-Lysine.

Ionizable groups:
  • 2 groups (N-terminal and Lysine side chain)
  • 2 groups (C-terminal and Glutamic acid side chain)
  • 1 phenolic (Tyrosine side chain)

At pH = 2 (strongly acidic):
  • Both groups are protonated: (+2 charge)
  • All remain protonated (neutral)
  • Net charge = +2, so

At pH = 6 (near neutral):
  • Lysine is protonated (+1)
  • Glutamic acid is deprotonated (-1)
  • Zwitterionic form, net charge = 0, so

At pH = 11 (strongly basic):
  • Both are deprotonated: (-2)
  • Phenolic is deprotonated: (-1)
  • Net charge = -3, so

JEE Advanced · 2020 · 27 Sep 2020 · Paper 2 · Integer answer

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