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 $C H_{3}$

III $- C H_{2} C O O H$ H

IV $- C H_{2} C O N H_{2}$ $(C H_{2})_{4} N H_{2}$

V $- C H_{2} C O N H_{2}$ $- C H_{2} C O N H_{2}$

VI $(C H_{2})_{4} N H_{2}$ $(C H_{2})_{4} N H_{2}$

VII $- C H_{2} C O O H$ $- C H_{2} C O N H_{2}$

VIII $- C H_{2} O H$ $(C H_{2})_{4} N H_{2}$

IX $(C H_{2})_{4} N H_{2}$ $C H_{3}$

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Solution

In the given general structure of amino acid we have $R_{1}$ and $R_{2}$ group.
At pH- 7, the carboxylic acid will exist in carboxylate form since it is a strong acid. So for make the amino acid positively charge it should have more than one strong basic group. Thus, these basic group will get protonated at pH-7 and they becomes positively charge.
Hence, among $R_{1}$ and $R_{2}$ atleat one should be have a strong basic group to get positively charged peptide.
Amide are less basic than amine because their lone pair involved in resonance. Amine are basic enough to get protonated at pH-7
Hence, peptide (IV), (VI), (VIII) and (IX) are positively charged at pH-7.

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