Chemistry - Biomolecules Question with Solution | TestHub

Question Explanation:

A given peptide structure contains several ionizable functional groups. The question asks for the absolute value of the net charge of the peptide at pH = 2, 6, and 11, and then to calculate the sum $|z_1|+|z_2|+|z_3|$.

Concept:

This question is based on the isoelectric behaviour of peptides, acid–base properties of amino acids, and ionizable groups in peptides.

Solution:

From the structure, the peptide contains the following ionizable groups:

* N-terminal –NH₂ (basic, +1 when protonated)

* C-terminal –COOH (acidic, −1 when deprotonated)

* Aspartic acid side chain –COOH (acidic, −1 when deprotonated)

* Lysine side chain –NH₂ (basic, +1 when protonated)

* Tyrosine phenolic –OH (weakly acidic, ionizes only at high pH $\approx$ 10)

At pH = 2 (strongly acidic):

* N-terminal –NH₃⁺ → +1

* Lysine side chain –NH₃⁺ → +1

* All –COOH groups protonated → 0

* Tyrosine –OH unionized

Net charge: $z_1+2⟹|z_1|=2$

At pH = 6 (near neutral):

* N-terminal –NH₃ ⁺→ +1

* Lysine side chain –NH₃⁺ → +1

* C-terminal –COO⁻ → −1

* Asp side chain –COO⁻ → −1

* Tyrosine –OH unionized

Net charge: $z_2=0⟹|z_2|=0$

At pH = 11 (strongly basic):

* N-terminal –NH₂ → 0

* Lysine side chain –NH₂ → 0

* C-terminal –COO⁻ → −1

* Asp side chain –COO⁻ → −1

* Tyrosine –O⁻ → −1

Net charge: $z_3=-3⟹|z_3|=3$

Thus,

$|z_1|+|z_2|+|z_3|=2+0+3=5$.