Three Pyrimidine Decarboxylations in the Absence of a Catalyst

The epigenetic modification of DNA by 5-methylation of cytosine residues can be reversed by the action of the TET family of dioxygenases that oxidize the methyl group to produce 5-carboxycytosine (5caC), which can be converted to cytosine in a final decarboxylation step. Likewise, 5-carboxyuracil (5caU) is decarboxylated to uracil in the last step in pyrimidine salvage. In view of the extreme difficulty of decarboxylating derivatives of orotic acid (6caU), it seemed desirable to establish the rates of decarboxylation of 5caC and 5caU in the absence of a catalyst. Arrhenius analysis of experiments performed at elevated temperatures indicates that 5caU decomposes with a rate constant of 1.1 × 10^-9 s^-1 (ΔH^⧧ = 25 kcal/mol) in a neutral solution at 25 °C. The decomposition of 5caC is somewhat slower (k₂₅ = 5.0 × 10^-11 s^-1; ΔH^⧧ = 27 kcal/mol) and leads to the initial accumulation of cytosine as an intermediate, followed by the relatively rapid deamination of cytosine (k₂₅ = 1.9 × 10^-10 s^-1; ΔH^⧧ = 23.4 kcal/mol). Both 5caC and 5caU are decarboxylated many orders of magnitude more rapidly than 6caU is (k₂₅ = 1.3 × 10^-17 s^-1). Ab initio simulations indicate that in all three cases, the favored route of spontaneous decarboxylation in water involves direct elimination of CO₂ with the assistance of an explicit water molecule.