purine, any of a class of organic compounds of the heterocyclic series characterized by a two-ringed structure composed of carbon and nitrogen atoms. The simplest of the purine family is purine itself, a compound with a molecular formula C₅H₄N₄. Purine is not common, but the purine structure occurs in many natural substances.

Uric acid, the first purine derivative to be discovered, was isolated in 1776 from urinary calculi; xanthine was obtained from the same source in 1817. Xanthine also occurs in tea, as does caffeine, another purine compound. Guanine, found in guano, the accumulated excrement and dead bodies of birds, bats, and seals, and adenine were identified in 1891 as products of the chemical decomposition of nucleic acids, the cell constituents that determine hereditary characteristics. Purine itself and several purine compounds were synthetically prepared from uric acid in the 1890s.