IIT-JEE 2012 · Paper 1 · Chemistry Q36

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ChemistryInteger+4Question 36 of 60

The substituents $\mathbf R_1$ and $\mathbf R_2$ for nine peptides are listed in the table given below. How many of these peptides are positively charged at pH = 7.0?

$\ce{H3N+-CH(H)-CO-NH-CH(R1)-CO-NH-CH(R2)-CO-NH-CH(H)-COO-}$

Peptide	$\mathbf R_1$	$\mathbf R_2$
I	H	H
II	H	$\ce{CH3}$
III	$\ce{CH2COOH}$	H
IV	$\ce{CH2CONH2}$	$\ce{(CH2)4NH2}$
V	$\ce{CH2CONH2}$	$\ce{CH2CONH2}$
VI	$\ce{(CH2)4NH2}$	$\ce{(CH2)4NH2}$
VII	$\ce{CH2COOH}$	$\ce{CH2CONH2}$
VIII	$\ce{CH2OH}$	$\ce{(CH2)4NH2}$
IX	$\ce{(CH2)4NH2}$	$\ce{CH3}$

Your answer

Answer and worked solution

Official answer: 4

Charges at pH 7. The terminal $\ce{NH3+}$ and $\ce{COO-}$ cancel. A lysine side chain $\ce{(CH2)4NH3+}$ adds +1; an aspartate side chain $\ce{CH2COO-}$ adds −1. H, $\ce{CH3}$ , $\ce{CH2OH}$ and $\ce{CH2CONH2}$ are neutral.

Peptide	net charge
I, II, V	0
III	−1
IV	+1
VI	+2
VII	−1
VIII	+1
IX	+1

Positively charged: IV, VI, VIII and IX.

Answer: 4

Topics:Amino acids and proteins

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IIT-JEE 2012 · Paper 1, Chemistry question 36