Purines contain four nitrogen atoms and are characterized by the fusion of pyrimidine with imidazole ring. Purine is a weak base and relatively unreactive towards electrophilic aromatic substitutions, similar to pyridines. There are many natural occurring purines available. Two of the five nucleic acid bases are adenine and guanine, which are derivatives of purine. Purines also play a vital role in biomolecules such as ATP, GTP, cyclic AMP, NADH, and coenzyme A. Purines can also function as neurotransmitters.