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 $\mathrm{pH}=7.0$ ? $\begin{array}{|c|c|c|} \hline ext{Peptide} & \mathbf{R}_{1} & \mathbf{R}_{2} \ \hline ext{I} & \mathrm{H} & \mathrm{H} \…

Question

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 $\mathrm{pH}=7.0$ ?

$\begin{array}{|c|c|c|} \hline \text{Peptide} & \mathbf{R}_{1} & \mathbf{R}_{2} \\ \hline \text{I} & \mathrm{H} & \mathrm{H} \\ \hline \text{II} & \mathrm{H} & \mathrm{CH}_{3} \\ \hline \text{III} & \mathrm{CH}_{2} \mathrm{COOH} & \mathrm{H} \\ \hline \text{IV} & \mathrm{CH}_{2} \mathrm{CONH}_{2} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text{V} & \mathrm{CH}_{2} \mathrm{CONH}_{2} & \mathrm{CH}_{2} \mathrm{CONH}_{2} \\ \hline \text{VI} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text{VII} & \mathrm{CH}_{2} \mathrm{COOH} & \mathrm{CH}_{2} \mathrm{CONH}_{2} \\ \hline \text{VIII} & \mathrm{CH}_{2} \mathrm{OH} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} \\ \hline \text{IX} & \left(\mathrm{CH}_{2}\right)_{4} \mathrm{NH}_{2} & \mathrm{CH}_{3} \\ \hline \end{array}$

Aicharya · Accepted Answer

Peptides with isoelectric point (pI) more than 7 would exist as cation in neutral solution $(\mathrm{pH}=7)$ which means the given polypeptide is of basic nature, so it must contain two or more amino groups. Hence IV, VI, VIII and IX are correct options.

The substituents $R_{1}$ and $R_{2}$ for nine peptides are listed in the table given below. How many of…

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