title: N3 and O2 protonated tautomeric conformations of 2'-deoxycytidine and cytidine coexist in the gas phase.
abstract: 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.
authors: †Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.
authors: R R
authors: RR
authors: Wu
authors: †Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.
authors: Bo
authors: B
authors: Yang
authors: †Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.
authors: C E
authors: CE
authors: Frieler
authors: ‡Institute for Molecules and Materials, FELIX Facility, Radboud University Nijmegen, Toernooiveld 7, 6525 ED, Nijmegen, The Netherlands.
authors: G
authors: G
authors: Berden
authors: ‡Institute for Molecules and Materials, FELIX Facility, Radboud University Nijmegen, Toernooiveld 7, 6525 ED, Nijmegen, The Netherlands.|§van't Hoff Institute for Molecular Sciences, University of Amsterdam, Amsterdam, The Netherlands.
authors: J
authors: J
authors: Oomens
authors: †Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.
authors: M T
authors: MT
authors: Rodgers
mesh_terms: D003562:Cytidine / Q000737:chemistry*; D003841:Deoxycytidine / Q000737:chemistry*; D005740:Gases / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D016018:Least-Squares Analysis; D008956:Models, Chemical; D015394:Molecular Structure; D009584:Nitrogen / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization; D013055:Spectrophotometry, Infrared; D014732:Vibration
chemical_list: D005740:Gases; D011522:Protons; D003841:Deoxycytidine; D003562:Cytidine; D006859:Hydrogen; D009584:Nitrogen; D010100:Oxygen