Biomolecules and Chemistry in Everyday Life - Result Question 31
####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 $pH=7.0$ ?
(2012)
| Peptide | $\boldsymbol{R} _1$ | |
| I | $H$ | $R-CO-NH-CH-COO$ |
| II | $H$ | $R$ |
| III | $CH _2 COOH$ | $H$ |
| IV | $CH _2 CONH$ | $CH$ |
| V | $CH _2 CONH _2$ | $\left(CH _2\right) _4 NH _2$ |
| VI | $\left(CH _2\right) _4 NH _2$ | $\left(CH _2 CONH _2\right) _4 NH _2$ |
| VII | $CH _2 COOH _2$ | $CH _2 CONH _2$ |
| VIII | $CH _2 OH _2$ | $\left(CH _2\right) _4 NH _2$ |
| IX | $\left(CH _2\right) _4 NH _2$ | $CH _3$ |
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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 $pH$ less than 7.0 and increasing $pH$ above isoelectric point makes them anionic.
On the other hand, amino acids with additional basic group have their isoelectric $pH$ greater than 7.0 and decreasing $pH$ 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 $pH=7.0$.
| Peptide | $\boldsymbol{R} _{\mathbf{1}}$ | $\boldsymbol{R} _{\mathbf{2}}$ |
|---|---|---|
| IV | $CH _2 CONH _2$ | $\left(CH _2\right) _4 NH _4$ |
| VI | $\left(CH _2\right) _4 NH _2$ | $\left(CH _2\right) _4 NH _4$ |
| VIII | $CH _2 OH$ | $\left(CH _2\right) _4 NH _4$ |
| IX | $\left(CH _2\right) _4 NH _2$ | $CH _3$ |
Thus, 4 is the correct integer.
