Question

Arrange the following amino acids in order of their pKa order.
Lysine, Aspartic acid, Arginine, Glycine.
a) lys > Arg > Gly > Asp
b) Arg > Lys > Asp > Gly
c) Gly > Asp > Arg > Lys
d) Arg > Lys > Gly > Asp

Answer 1

Hint: The pKa value is used to indicate the strength of an acid. The pKa is the negative log of the Ka (acid dissociation constant) value. In other words, we can say that lower value of pKa indicates a stronger acid.

Complete Step by Step Solution:
In order to solve this question, we need to study the structures of each given amino acid and find out the acidic and basic group present in each of them.
A. Lysine:

Image caption – Structure of lysine
It has two basic groups in the form of \[ - N{H_2}\] and one acidic group in the form of \[ - COOH\].
Hence it is very slightly basic.

B. Aspartic acid:

Image caption – Structure of Aspartic acid
Similarly, this amino acid has two acidic groups and a basic group. Hence, it is acidic in nature.

C. Arginine:
 
Image caption – Structure of Arginine
Here three basic groups and an acidic group are found. Hence, it is the least acidic among all other options.

 D. Glycine:

Image caption – Structure of Glycine
Here, there is a single acidic and a single basic group. Hence, it is neutral in nature.
As already mentioned above, by arranging the amino acids according to their acidic order, we get: Aspartic acid> Glycine> Lysine> Arginine
As already mentioned earlier, pKa is the negative log of Ka hence,
pKa order of these amino acids are, Arginine> Lysine> Glycine> Aspartic acid
Hence, the correct answer is option (D).

Note: The lower the value of pKa, the higher its acidity. The quantitative behavior of acids and bases in solution can be understood only if their pKa values are known. In particular, the pH of a solution can be predicted when the analytical concentration and pKa values of all acids and bases are known; conversely, it is possible to calculate the equilibrium concentration of the acids and bases in solution when the pH is known.