N3 and O₂ protonated tautomeric conformations of 2′-deoxycytidine and cytidine coexist in the gas phase

Infrared multiple photon dissociation action spectra of the protonated forms of the cytidyl nucleosides, 2′-deoxycytidine, [dCyd+H]⁺, and cytidine, [Cyd+H]⁺, are acquired over the IR fingerprint and hydrogen-stretching regions. Electronic structure calculations are performed at the B3LYP/6-311+G(d,p) level to determine the stable low-energy tautomeric conformations of these species generated upon electrospray ionization (ESI) and to generate the linear IR absorption spectra of these protonated nucleosides. Comparison between the experimental and theoretical spectra allows the tautomeric conformations of [dCyd+H]⁺ and [Cyd+H]⁺ populated by ESI to be determined. B3LYP predicts N3 as the preferred protonation site for both [dCyd+H]⁺ and [Cyd+H]⁺, whereas MP2 suggests that protonation at O₂ is more favorable. The 2′-hydroxyl substituent does not significantly alter the structures of the B3LYP N3 and MP2 O₂ protonated ground tautomeric conformations of [dCyd+H]⁺ vs [Cyd+H]⁺. [dCyd+H]⁺ and [Cyd+H]⁺ exhibit very similar spectral signatures in both regions. Nonetheless, the 2′-hydroxyl does affect the relative intensities of the IRMPD bands of [dCyd+H]⁺ vs [Cyd+H]⁺. The spectral features observed in the hydrogen-stretching region complement those of the fingerprint region and allow the N3 and O₂ protonated tautomeric conformations to be readily distinguished. Comparison between the measured and computed spectra indicates that both N3 and O₂ protonated tautomeric conformations coexist in the experiments, and the populations are dominated by the most stable N3 and O₂ protonated tautomeric conformations. Least-squares fitting of the IRMPD spectra to the IR spectra for these most stable conformers suggests relative populations of ∼55% N3 vs 45% O₂ protonated conformers of [dCyd+H]⁺, whereas ∼47% N3 vs 53% O₂ protonated conformers of [Cyd+H]⁺. This change in the preferred site of protonation indicates that the 2′-hydroxyl substituent plays an important role in controlling the reactivity of the cytidyl nucleosides.