Sulphite-ion-catalysed reversion of 5,6-dihydrothymine-6-sulphonate to thymine. A general-base-catalysed reaction

Abstract

5,6-Dihydrothymine-6-sulphonate (Thy-SO₃^–) was isolated as its sodium salt from a reaction mixture of thymine and sodium hydrogensulphite. Thy-SO₃-was stable in unbuffered aqueous solution, but reverted to thymine in the presence of general bases; e.g., sulphite (SO₃^2–) and phosphate (HPO₄^2–) ions were efficient catalysts for this reversion in neutral solutions. The rate of the reversion was first-order in base, and the rate of addition of hydrogensulphite to thymine was proportional to the square of the hydrogensulphite concentration. The pH-independent equilibrium constant of the reaction, thymine + HSO₃^–⇌ Thy-SO₃^–, was determined, and it was in good agreement with the expected equilibrium constant that was calculated from the observed rates of the forward and the reverse reactions. The rate of reversion of 5-[²H]Thy-SO₃^– to thymine in phosphate buffers was 4.11-fold lower than that of Thy-SO₃^–, indicating that the proton transfer at position 5 is the rate-determining step in the reversible reaction. Mechanisms that are consistent with the observed kinetics are discussed.