Biomolecules and Chemistry in Everyday Life 1 Question 30
32. 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 $\mathrm{pH}=7.0$ ?
(2012)
| Peptide | $\boldsymbol{R}_{1}$ | |
| I | $\mathrm{H}$ | $\mathrm{R}-\mathrm{CO}-\mathrm{NH}-\mathrm{CH}-\mathrm{COO}$ |
| II | $\mathrm{H}$ | $\mathrm{R}$ |
| III | $\mathrm{CH}_{2} \mathrm{COOH}$ | $\mathrm{H}$ |
| IV | $\mathrm{CH}_{2} \mathrm{CONH}$ | $\mathrm{CH}$ |
| V | $\mathrm{CH}{2} \mathrm{CONH}{2}$ | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ |
| VI | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ | $\left(\mathrm{CH}{2} \mathrm{CONH}{2}\right){4} \mathrm{NH}{2}$ |
| VII | $\mathrm{CH}{2} \mathrm{COOH}{2}$ | $\mathrm{CH}{2} \mathrm{CONH}{2}$ |
| VIII | $\mathrm{CH}{2} \mathrm{OH}{2}$ | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ |
| IX | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ | $\mathrm{CH}_{3}$ |
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Solution:
- The amino acid remain completely in Zwitter ionic form at its isoelectric point. Amino acids with additional acidic group have their isoelectric $\mathrm{pH}$ less than 7.0 and increasing $\mathrm{pH}$ above isoelectric point makes them anionic.
On the other hand, amino acids with additional basic group have their isoelectric $\mathrm{pH}$ greater than 7.0 and decreasing $\mathrm{pH}$ below isoelectric point (by adding acid solution) makes them cationic. The given peptide with followings $R_{1}$ and $R_{2}$ are basic, will remain protonated (cationic) at $\mathrm{pH}=7.0$.
| Peptide | $\boldsymbol{R}_{\mathbf{1}}$ | $\boldsymbol{R}_{\mathbf{2}}$ |
|---|---|---|
| IV | $\mathrm{CH}{2} \mathrm{CONH}{2}$ | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{4}$ |
| VI | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{4}$ |
| VIII | $\mathrm{CH}_{2} \mathrm{OH}$ | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{4}$ |
| IX | $\left(\mathrm{CH}{2}\right){4} \mathrm{NH}_{2}$ | $\mathrm{CH}_{3}$ |
Thus, 4 is the correct integer.
