purine
Purine is a heterocyclic nitrogen molecule consisting of a pyrimidine ring fused to an imidazole ring. It is the most widely distributed nitrogenous heterocycle in nature. Among the nine possible tautomers of the purine nucleus, the CH forms, as well as the N1H and N3H forms, can be neglected because very unlikely, only the N7H and especially N9H forms exist in neutral solution. The purine nucleus does not exist in nature. On the other hand, many amino, hydroxylated or methylated derivatives are found, often in the form of nucleosides, which are included in the general term purines. Two of the nucleic acid bases are purines: adenine and guanine. In DNA, these bases form hydrogen bonds with complementary pyrimidines, thymine and cytosine. Other purines include xanthine, hypoxanthine, theobromine, caffeine and uric acid. Purines, in addition to being components of DNA and RNA, are found in important biomolecules, such as ATP, GTP, cyclic AMP, NADP, SAM or coenzyme A.