amino acid peptide
Amino acids are biologically important organic compounds which perform critical roles in processes such as neurotransmitter transport and biosynthesis. They contain amine (-NH2) and carboxylic acid (-COOH) functional groups as well as a side-chain (R group) specific to each amino acid usually. The key elements of an amino acid are carbon, hydrogen, oxygen, nitrogen and other elements which are found in the side-chains of certain amino acids. There are only 20 amino acids in the genetic code, and the 20 animo acids can be classified in many ways. They can be classified according to polarity (polar amino acids and no polar amino acids), pH level, and the type of side chain group, such as aliphatic, aromatic, containing hydroxyl or sulfur, etc.