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 | $R_1$ | $R_2$ |
|---|---|---|
| $I$ | $H$ | $H$ |
| $II$ | $H$ | $CH_3$ |
| $III$ | $CH_2COOH$ | $H$ |
| $IV$ | $CH_2CONH_2$ | $(CH_2)_4NH_2$ |
| $V$ | $CH_2CONH_2$ | $CH_2CONH_2$ |
| $VI$ | $(CH_2)_4NH_2$ | $(CH_2)_4NH_2$ |
| $VII$ | $CH_2COOH$ | $CH_2COMH_2$ |
| $VIII$ | $CH_2 OH$ | $(CH_2)_4NH_2$ |
| $IX$ | $(CH_2)_4NH_2$ | $CH_3$ |
Show Answer
Answer:
Correct Answer: 32. $(4)$
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.
BETA
