Chapter 25.2, Problem 6ATS

Interpretation Introduction

Interpretation:

The difference in acid-base properties of the hydroxyl groups present in serine and tyrosine need to be explained.

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.

The deprotonated forms or protonated forms are more stable when there is a resonance.  As due to resonance the charge is delocalized more leading to a more stable ion.

To explain: why hydroxyl of tyrosine is more acidic than the hydroxyl group in serine