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JEE Advance - Chemistry (2012 - Paper 1 Offline - No. 19)

The substituents R1 and R2 for nine peptides are listed in the table given below. How many of these peptides are positively charged at pH = 7.0 ?

IIT-JEE 2012 Paper 1 Offline Chemistry - Biomolecules Question 10 English

Peptide $${R_1}$$ $${R_2}$$
I H H
II H $$C{H_3}$$
III $$C{H_2}COOH$$ H
IV $$C{H_2}CON{H_2}$$ $${(C{H_2})_4}N{H_2}$$
V $$C{H_2}CON{H_2}$$ $$C{H_2}CON{H_2}$$
VI $${(C{H_2})_4}N{H_2}$$ $${(C{H_2})_4}N{H_2}$$
VII $$C{H_2}COOH$$ $$C{H_2}CON{H_2}$$
VIII $$C{H_2}OH$$ $${(C{H_2})_4}N{H_2}$$
IX $${(C{H_2})_4}N{H_2}$$ $$C{H_3}$$

Answer
4

Explanation

For basic amino acids with pH > 7, peptides will exist as cations. For example, when the substituents are basic, that is R1 = CH2CONH2 and R2 = (CH2)4NH2 or when R1 = (CH2)4NH2 and R2 = (CH2)4NH2 or when R1 = CH2OH and R2 = (CH2)4NH2 or when R1 = (CH2)4NH2 and R2 = CH3.

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