Question

All amino acids have two ionizable functional groups: an α-amino group (average pKa of 9.4) and an α-carboxylic acid group (average pKa of 2.2). Arginine also has an ionizable side chain (R group) with a pKa of ~12.5. One of the possible ionization states of arginine is shown below.

At what pH would the above structure be the predominant ionization state? Note: Consider the ionization state of all three of the functional groups.

The protonated form of the R group of arginine is shown in the structure above. The ratio of the charged (protonated) form to the deprotonated form depends on the pKa of the R group and the pH of the solution. Select all the pH values at which the charged form of the R group would predominate.

3.9

7.4

12.5

12.8

I don't know how to upload the image but if it is needed please help me upload it. thank you!

*I know PH is 1.5 but i keep getting the second part wrong**

Answer 1

The amino acid arginine can be protonated either at the imine nitrogen (C=NH) or the amine nitrogen (C-NH2). But due to the higher pKa of the imine, it is what is protonated and that is considered as the protonated form of arginine generally. Being a basic side-chain, at pH lower than its pKa and an acidic pH, the amino acid's side chain is protonated. Hence, at pH of 3.9, the protonated arginine is predominant and at a pH of 7.4, the amino acid deprotonated at the carboxylic acid end due to its low pKa. However, at the precise pH of 12.5 which is considered here as the pKa of the guanidyl group, there exists an equilibrium containing the same quantity of both the protonated and the deprotonated form. Last of all, at the pH of 12.8, all the basic groups remain pristine and while the acidic group, which is the carboxylic acid on the alpha carbon here, will be deprotonated. Now, considering the pKa of all the acidic and basic groups in arginine, these will be the predominant species at the given pH values.

P.S:- I don't know what pH of 1.5 you keep getting wrong. If that is also a pH in the question, then the protonated arginine species predominant at pH 3.9 is again predominant at pH of 1.5.