Any of a group of twenty hydrocarbon molecules (containing the radical group NH2) linked together in various combinations to form proteins in living things. Synthesized by living cells or obtained as essential components of the diet of human and animals, these twenty (20) amino acids are divided into four (4) groups on the basis of their side-chain properties:1.Neutral, hydrophobic side chains,2.Neutral, hydrophilic side chains,3.Acid, hydrophilic side chains,4.Basic, hydrophilic side chains.In addition to the twenty common amino acids there are less common derivatives (e.g. hydroxyproline, found in collagen) formed by the modification of a common amino acid.