Question

Given that the pKas of the α-amino group and the α-carboxylic acid group in an amino acid are about 9 and 2 respectively, explain why the zwitterion is the predominant form of an amino acid at pH 7.

Answer 1

Zwitterion comes from the German word for ‘two’ (zwei) or ‘double’ (zwitter) and the word ‘ion’. While neutral, the zwitterion form of an amino acid will have a positive and a negative charge.
This is the zwitterion form of an amino acid

The zwitterion is said to be dominant form at neutral pH of 7, as it attains isoelectric point at neutral pH, The isoelectric point or pI of an amino acid is the pH at which an amino acid has a net charge of zero.

At physiological pH or isoelectric point, amino acids will exist with a net charge of zero. That’s not because they have no charge: it’s because their charges balance or cancel out, as shown in the figure above.

The standard structure contains both a carboxyl and an amine in the backbone. Acidic and basic amino acids may have additional groups in their side chains.

Zwitterions are also electrically neutral and so do not migrate in an electric field, Zwitterions provide strong hydration through electrostatic interaction with water molecules and provide a physical and energetic barrier against protein adsorption. These are all the main contributions of Zwitterions play as predominant form at neutral pH of 7.