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.

In acidic solutions, amino acids exist as positively charged ions. 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):

In alkaline solutions, amino acids exist as negatively charged ions. 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.