Each amino acid has at least one amine and one acid functional group as the name implies. The different properties result from variations in the structures of different R groups. The R group is often referred to as the amino acid side chain. Amino acids have special common names, however, a three letter abbreviation for the name is used most of the time. A second abbreviation, single letter, is used in long protein structures. Consult the table on the left for structure, names, and abbreviations of 20 amino acids.