Chapter 6, Problem 6.71P

Interpretation Introduction

Interpretation:

The structure of the most abundant form of histidine at given pH values is to be drawn.

Concept introduction:

The general form of an $\text{α-amino}$ acid has both an amino group and a carboxylic group. In the aqueous solution, the amino group is weakly basic, and the carboxylic group is weakly acidic. Under highly acidic conditions, the N atom in the amino group is protonated. As the pH of the solution increases, the most acidic proton can be removed from the carboxylic group. As the solution more basic, a proton from the side chain group is lost and the dominant form will be formed. As the solution becomes more basic, the proton of ${\text{NH}}_{\text{3}}{}^{\text{+}}$ group is deprotonated. The strongest acid has the lowest $\text{pKa}$ value, and strongest base has a higher $\text{pKa}$ value.