It is an unstable tautomer of formic acid. It exists in equilibrium with a tautomer CHC(O)CH=C(OH)CH. The compound exists in equilibrium with a minor tautomer, pyridin-4-ol. The triol tautomer, which may have aromatic character, predominates in solution. It has a tautomer known as 6-hydroxypurine. In solution, the 2"H"-isoindole tautomer predominates. The compound exists mainly as the enol tautomer. This species exists in equilibrium with an extremely minor tautomer P(OH). This ketone will prefer to exist in its enol tautomer in solution. Anthrone is the more stable tautomer relative to the anthrol. Upon deprotonation, the keto tautomer predominates. It is a tautomer of hyponitrous acid. It is a minor tautomer of hydrogen cyanide (HCN). The main active metabolite is 4-hydroxycyclophosphamide, which exists in equilibrium with its tautomer, aldophosphamide. Enolisation of RuBP is the conversion of the keto tautomer of RuBP to an enediol(ate).