Guanine is one of the purin bases. Purine contains a 6-membered ring, the pyrimidine, and a 5-membered ring, the imidazole. Guanine is derived from purine by amino substitution in the 2-position and hydroxy substitution in the 6-position. The N=C-OH tautomerizes to NH-C=O while the N=C-NH₂ does not tautomerize to NH-C=NH. Note that the amino group in guanine is pyramidal.