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ChemistryInteger+3 / -1Question 23 of 54

JEE Advanced 2020 · Paper 2, Chemistry question 5

The structure of a peptide is given below.

If the absolute values of the net charge of the peptide at pH = 2, pH = 6, and pH = 11 are ∣z1∣|z_1|, ∣z2∣|z_2|, and ∣z3∣|z_3|, respectively, then what is ∣z1∣+∣z2∣+∣z3∣|z_1| + |z_2| + |z_3|?

The tripeptide Tyr-Asp-Lys (all L): H₂N-CH(CH₂C₆H₄OH)-CO-NH-CH(CH₂CO₂H)-CO-NH-CH((CH₂)₄NH₂)-CO₂H, drawn as a zigzag with wedged/dashed side chains.
Answer and worked solution

Official answer: 5

The peptide is Tyr-Asp-Lys. Its ionisable groups, with approximate pKa:

  • α-COOH, about 2-3;
  • Asp side-chain COOH, about 3.9;
  • α-NH₃⁺, about 9;
  • Tyr phenol, about 10;
  • Lys ε-NH₃⁺, about 10.5.

pH 2. Both amino groups are protonated and the carboxyls are essentially un-ionised: net +2+2.

pH 6. Both amines are ++ and both carboxyls are −-: net 00.

pH 11. The amines are (essentially) neutral. The two carboxylates and the phenolate are −-: net −3-3.

∣z1∣+∣z2∣+∣z3∣=2+0+3=5.|z_1|+|z_2|+|z_3|=2+0+3=5.
Answer: 5

Topics:Amino acids and proteinspH and ionisation
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