Pyridine is aromatic with six pi electrons in a cyclic, conjugated pi-orbital system. The SP 2 nitrogen atom and the five carbon atoms contribute one pi electron each to the orbital system. Unlike pyrrole, however, the lone pair of electrons on the pyridine nitrogen atom is not part of the pi orbital system, but occupies an SP 2 orbital in the plane of the ring. Substituted pyridines occur in the B 6 -vitamins pyridoxal and pyridoxine, and these vitamins play important biological roles. Another heterocyclic form is pyrimidine, containing two nitrogen atoms and four carbon atoms in a six member ring.
Several examples are cytosine, uracil, and thymine which occur in nucleic acids.
The fused-ring aromatic heterocycles that contain both a benzene ring and a heterocyclic aromatic ring occur widely in biological molecules. Thus quinine, a quinoline derivative found in the bark of the tropical tree cinchona, is an important antimalarial drug. Lysergic acid, an indole derivative found in the ergot fungus that grows on grain, is the parent from which LSD is derived. Another series of fused-ring forms contains two nitrogen atoms in each of the two fused rings, and include the purines adenine and guanine also found in nucleic acids. Amides - Amides are amino derivatives of carboxylic acids, where the amino functional group replaces the hydroxyl group of the carboxyl functional group.