Protonation-induced tautomerization lowers the activation barriers for N-glycosidic bond cleavage of thymidine and 5-methyluridine

Abstract

Synergistic infrared multiple photon dissociation (IRMPD) action spectroscopy experiments and electronic structure calculations revealed that both 2,4-dihydroxy tautomers and O2 protonated conformers of protonated thymidine, [dThd+H]⁺, and protonated 5-methyluridine, [Thd+H]⁺ coexist in the gas phase with the 2,4-dihydroxy tautomers dominating the population. In the current study, the kinetic energy dependence of the collision-induced dissociation behavior of [dThd+H]⁺ and [Thd+H]⁺ are examined in a guided ion beam tandem mass spectrometer. The primary dissociation pathways observed involve N-glycosidic bond cleavage leading to competitive elimination of either protonated or neutral thymine. The potential energy surfaces (PESs) for these N-glycosidic bond cleavage pathways are mapped via electronic structure calculations for the mixture of 2,4-dihydroxy tautomers and O2 protonated conformers of [dThd+H]⁺ and [Thd+H]⁺ populated in the IRMPD experiments. The activation energies and heats of reaction predicted for N-glycosidic bond cleavage at the B3LYP and MP2(full) levels of theory are compared to the measured values. The agreement between experiment and theory indicates that B3LYP provides better estimates of the energetics of the species along the PESs for N-glycosidic bond cleavage of [dThd+H]⁺ and [Thd+H]⁺ than MP2, and that the 2,4-dihydroxy tautomers, which are stabilized by strong hydrogen-bonding interactions, control the threshold dissociation behavior of [dThd+H]⁺ and [Thd+H]⁺.