Question

Arrange the following amino acids in order of their ${\mathrm{pK}}_{\mathrm{a}}$ order.

• A. $\mathrm{Lys}>\mathrm{Arg}>\mathrm{Gly}>\mathrm{Asp}$
• B. $\mathrm{Arg}>\mathrm{Lys}>\mathrm{Asp}>\mathrm{Gly}$
• C. $\mathrm{Gly}>\mathrm{Asp}>\mathrm{Arg}>\mathrm{Lys}$
• D. $\mathrm{Arg}>\mathrm{Lys}>\mathrm{Gly}>\mathrm{Asp}$

Answer 1

The correct option is D

$\mathrm{Arg}>\mathrm{Lys}>\mathrm{Gly}>\mathrm{Asp}$

Explanation for the correct option:

(D) we know,

Acidity $\mathrm{\alpha }$ ${\mathrm{K}}_{\mathrm{a}}$

Acidity $\mathrm{\alpha }$$\frac{1}{P_{K_a}}$

1. According to above formula, the compounds which are most acidic have least ${\mathrm{pK}}_{\mathrm{a}}$
2. In the case of given amino acids, glycine is a neutral amino acid while Aspartet is acidic amino acid due to the extra -COOH group.
3. Arginine has an extra $3$ lone pairs donating side situated on nitrogen so it is more basic compared to Lysine.
4. So this is the reason to have a least $P_{K_a}$ value of Arginine.

Therefore, correct order: $\mathrm{Arg}>\mathrm{Lys}>\mathrm{Gly}>\mathrm{Asp}$

Hence, option (D) is correct.