The substituents $\mathbf{R}_{\mathbf{1}}$ and $\mathbf{R}_{\mathbf{2}}$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at $\mathrm{pH}=7.0$ ? | Peptide | $\mathrm{R}_{1}$ | $\mathrm{R}_{2}$ | | --- | --- | --- | | I | H | H | | II | H | $\mathrm{CH}_{3}$ | | III | $\mathrm{CH}_{2} \mathrm{COOH}$ | H | | IV | $\mathrm{CH}_{2} \mathrm{CONH}_{2}$ | $(\mathrm{CH}_{2})_{4} \mathrm{NH}_{2}$ | | V | $\mathrm{CH}_{2} \mathrm{CONH}_{2}$ | $\mathrm{CH}_{2} \mathrm{CONH}_{2}$ | | VI | $(\mathrm{CH}_{2})_{4} \mathrm{NH}_{2}$ | $(\mathrm{CH}_{2})_{4} \mathrm{NH}_{2}$ | | VII | $\mathrm{CH}_{2} \mathrm{COOH}$ | $\mathrm{CH}_{2} \mathrm{CONH}_{2}$ | | VIII | $\mathrm{CH}_{2} \mathrm{OH}$ | $(\mathrm{CH}_{2})_{4} \mathrm{NH}_{2}$ | | IX | $(\mathrm{CH}_{2})_{4} \mathrm{NH}_{2}$ | $\mathrm{CH}_{3}$ |

JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · Biomolecules

The substituents

R_{1}

and

R_{2}

for nine peptides are listed in the table given below. How many of these peptides are positively charged at

pH = 7.0

Peptide	$R_{1}$	$R_{2}$
I	H	H
II	H	$CH_{3}$
III	$CH_{2} COOH$	H
IV	$CH_{2} CONH_{2}$	$(CH_{2})_{4} NH_{2}$
V	$CH_{2} CONH_{2}$	$CH_{2} CONH_{2}$
VI	$(CH_{2})_{4} NH_{2}$	$(CH_{2})_{4} NH_{2}$
VII	$CH_{2} COOH$	$CH_{2} CONH_{2}$
VIII	$CH_{2} OH$	$(CH_{2})_{4} NH_{2}$
IX	$(CH_{2})_{4} NH_{2}$	$CH_{3}$

Answer: 4

Solution

A peptide is positively charged at pH 7.0 when its isoelectric point (pI) > 7. This occurs when the peptide has more basic groups than acidic groups. The peptide structure shown has one N-terminal

NH_{3}^{+}

and one C-terminal

COO^{-}

at pH 7. Additional basic groups (

(CH_{2})_{4} NH_{2}

from lysine side chains) will be protonated at pH 7, while acidic groups (

CH_{2} COOH

from aspartate/glutamate) will be deprotonated.

Peptides with net positive charge at pH 7:

IV: has one basic side chain ( $(CH_{2})_{4} NH_{2}$ ) → net positive
VI: has two basic side chains → net positive
VIII: has one basic side chain → net positive
IX: has one basic side chain → net positive

Total: 4 peptides are positively charged at pH 7.0.

JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · Integer answer

Practise more: all JEE Advanced PYQs on Biomolecules →

JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · Biomolecules

Solution

More JEE Advanced questions on Biomolecules