Chapter 6, Problem 6.33P

Interpretation Introduction

Interpretation:

The structures of the most abundant form of arginine in solutions whose pH values are 1, 4, 8. 10, and 14 are to be drawn.

Concept introduction:

The general form of an $\text{α-amino acid}$ contains both an amino group ${\text{(NH}}_{\text{2}}\text{)}$ and a carboxyl group ${\text{(CO}}_{\text{2}}\text{H)}$ bonded to the same carbon atom. The carboxyl group is weakly acidic, and the amino group is weakly basic.

The pH of the solution determines whether the two functional groups are protonated or deprotonated, and to what extent. When the pH is one unit or more lower than the pKa of the functional group, it is (nearly) completely protonated. When the pH is higher than the pKa by one unit or more, the functional group is (nearly) completely deprotonated. When the pH is approximately the same as the pKa, the protonated and deprotonated forms of the functional group are present in nearly equal amounts.

The most acidic proton in an $\text{α-amino acid}$ is from ${\text{CO}}_{\text{2}}\text{H}$. Its pKa is around 2 whereas that of the ${\text{NH}}_{\text{4}}{}^{\text{+}}$ group is around 9 to 10.