Alanine isomer models. Molecular models of the two optical isomers of the amino acid alanine (C3H7NO2). Alpha-alanine is on the left, and beta-alanine on the right. Amino acids are the monomers or building-blocks of the larger protein molecules. There are 20 common amino acids, which all possess a similar core structure of a carboxyl (COOH) group at one end, and an amino (NH2) group at the other. Each is distinguished by having a different side group structure. Carbon atoms are black, hydrogen are white, nitrogen is blue and oxygen is red.