Diverse mixtures of 2,4-dihydroxy tautomers and O4 protonated conformers of uridine and 2′-deoxyuridine coexist in the gas phase

The gas-phase conformations of protonated uridine, [Urd+H]⁺, and its modified form, protonated 2′-deoxyuridine, [dUrd+H]⁺, generated by electrospray ionization are investigated using infrared multiple photon dissociation (IRMPD) action spectroscopy techniques. IRMPD action spectra of [Urd+H]⁺ and [dUrd+H]⁺ are measured over the IR fingerprint and hydrogen-stretching regions. [Urd+H]⁺ and [dUrd+H]⁺ exhibit very similar IRMPD spectral profiles. However, the IRMPD yields of [Urd+H]⁺ exceed those of [dUrd+H]⁺ in both the IR fingerprint and hydrogen-stretching regions. The measured spectra are compared to the linear IR spectra predicted for the stable low-energy structures of these species computed at the B3LYP/6-311+G(d,p) level of theory to determine the tautomeric conformations populated by electrospray ionization. Both B3LYP and MP2 methods find O4 and O2 protonated canonical as well as 2,4-dihydroxy tautomers among the stable low-energy structures of [Urd+H]⁺ and [dUrd+H]⁺. Comparison between the measured IRMPD and calculated linear IR spectra suggests that these species exist in their ring-closed forms and that both 2,4-dihydroxy tautomers as well as O4 protonated canonical conformers coexist in the population generated by electrospray ionization for both [Urd+H]⁺ and [dUrd+H]⁺. The 2′-deoxy modification of [dUrd+H]⁺ reduces the variety of 2,4-dihydroxy tautomers populated in the experiments vs. those of [Urd+H]⁺.