Amino acids: ionisation and acid-base properties

Amino acids are amphotheric compounds, which means they simultaneously display acidic and basic properties. Here you can see the generic amino acid on its three ionisation forms: protonated, double ion (“zwitterion”) and deprotonated:


The simplest way to study these ionisation equilibria is to perform an acid-base titration. A way to do it is by dissolving the amino acid at a very low pH (e.g., pH=1) and then adding small volumes of a strong base, while the pH is being measured. In this way, a titration curve like this is obtained:

If you click on the different areas of this curve, you will see which is the predominant form of the amino acid at each one.

Predominant form:

Quiz 1:

Which is the form displayed?

Quiz 2:

Which is this amino acid?