Chapter 25, Problem 46PP

Interpretation Introduction

Interpretation:

At physiological pH which nitrogen in the side chain of histidine will be protonated need to be identified.

Concept introduction:

Amino acid has both carboxylic acid and amine functional group.  In presence of acid or base either one form will predominate which depends upon the pK_a value.  Usually the carboxylic acid group is deprotonated first and then the amino group is deprotonated.  At higher pH the carboxylic acid gets deprotonated and at lower pH the amino group gets protonated.

• The carboxylic acid form will predominate over carboxylate form if ${\text{pH<pK}}_{\text{a}}$ and vice-versa.
• The protonated form of $\alpha$ amino will predominate over the  $\alpha$ amino form if ${\text{pH<pK}}_{\text{a}}$ and vice-versa.
• If side chain is present the same rule as above applies.

To identify : To identify the nitrogen that will be protonated at physiological pH in the side chain of histidine.