An amino acid is an organic compound containing both amine (-NH₂) and carboxylic acid (-COOH) functional groups.

The general structure of an α-amino acid is RCH(NH₂)COOH.

The displayed formula of an α-amino acid is:

Amino acids differ by the side chain (R) attached to the α-carbon atom.

A zwitterion is a neutral compound that contains both a positively charged part and a negatively charged part within the same molecule.

NH₃⁺CH₂COO^- is an example of a zwitterion.

The isoelectric point is the pH at which the zwitterion is formed i.e. the pH where the amino acid has no overall charge.

Each amino acid has its own unique isoelectric point.

When the pH of a solution is below the isoelectric point of an amino acid, the amino acid exists as a positively charged ion. This occurs because the amine group (-NH₂) acts as a base and accepts a proton (H⁺) from the acidic environment, forming a protonated amine group (-NH₃⁺). The carboxyl group (-COOH) remains unchanged.

The general structure of an amino acid in acidic solutions is shown below (R = alkyl or aryl group):

When the pH of a solution is above the isoelectric point of an amino acid, the amino acid exists as a negatively charged ion. This occurs because the carboxyl group (-COOH) acts as an acid and donates a proton (H⁺) to the alkaline environment, forming a deprotonated carboxylate group (-COO^-). The protonated amine group (-NH₃⁺) loses its extra proton, reverting to its neutral form (-NH₂).

The general structure of an amino acid in alkaline solutions is shown below (R = alkyl or aryl group):

Amino acids exist as zwitterions because they contain a positive NH₃⁺ group and a negative COO^- group, formed by the internal transfer of H⁺.

The zwitterion of glycine is shown below.