Reversible hydration of pteridine. General acid-base catalysis, solvent deuterium isotope effects, and transition state characterization

The hydration of pteridine was investigated over the pH range 1 to 10 at 25.00, utilizing a spectrophotometric method. Above pH 7.5, the hydration of this heterocyclic Schiff base is catalyzed by hydroxide ions and by the basic component of a number of buffers (imidazole, 1,2-dimethylimidazole, borate, carbonate, triethylamine). The general base catalysis conforms to a Bronsted plot with an exponent β = 0.64 +- 0.03. Below pH 7.5, the reaction is catalyzed predominantly by hydronium ions. The value of 0.50 +- 0.02 for the ratio k/sub H3O+//k/sub D3O+/ indicates a preequilibrium protonation of pteridine. However, in a few instances the acidic component of buffers capable of bifunctional catalysis (H2PO4-, H2AsO4-, and HCO3- also exhibited an enhancement in the rate of hydration. The solvent isotope effect for water catalysis, k/sub H2O//k/sub D2O/, has a value of 3.4 +- 0.4 which is similar to those of a number of other reactions involving water in a cyclic mechanism. (auth)