title: 2,4-Dihydroxy and O2 Protonated Tautomers of dThd and Thd Coexist in the Gas Phase: Methylation Alters Protonation Preferences versus dUrd and Urd.
abstract: The gas-phase structures of protonated thymidine, [dThd + H](+), and its modified form, protonated 5-methyluridine, [Thd + H](+), are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy combined with electronic structure calculations. IRMPD action spectra are measured over the ranges extending from ~600 to 1900 cm(-1) and ~2800 to 3800 cm(-1) using the FELIX free electron laser and an optical parametric oscillator/amplifier (OPO/OPA) laser system, respectively. Comparisons between the B3LYP/6-311+G(d,p) linear IR spectra calculated for the stable low-energy conformers and the measured IRMPD spectra are used to determine the most favorable tautomeric conformations of [dThd + H](+) and [Thd + H](+) and to identify those populated in the experiments. Both B3LYP and MP2 levels of theory predict a minor 2,4-dihydroxy tautomer as the ground-state conformer of [dThd + H](+) and [Thd + H](+) indicating that the 2'-hydroxyl substituent of Thd does not exert a significant impact on the structural features. [dThd + H](+) and [Thd + H](+) share parallel IRMPD spectral profiles and yields in both the FELIX and OPO regions. Comparisons between the measured IRMPD and calculated IR spectra suggest that minor 2,4-dihydroxy tautomers and O2 protonated conformers of [dThd + H](+) and [Thd + H](+) are populated in the experiments. Comparison of this work to our previous IRMPD spectroscopy study of protonated 2'-deoxyuridine and uridine suggests that the 5-methyl substituent alters the preferences of O2 versus O4 protonation.
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, MI, 48202, USA.
authors.forename: R R
authors.initials: RR
authors.lastname: Wu
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, MI, 48202, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Yang
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, MI, 48202, USA.
authors.forename: C E
authors.initials: CE
authors.lastname: Frieler
authors.affiliation: FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7, 6525 ED, Nijmegen, The Netherlands.
authors.forename: G
authors.initials: G
authors.lastname: Berden
authors.affiliation: FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7, 6525 ED, Nijmegen, The Netherlands.|van't Hoff Institute for Molecular Sciences, University of Amsterdam, 1090 GD, Amsterdam, The Netherlands.
authors.forename: J
authors.initials: J
authors.lastname: Oomens
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, MI, 48202, USA. mrodgers@chem.wayne.edu.
authors.forename: M T
authors.initials: MT
authors.lastname: Rodgers
mesh_terms: D005740:Gases / Q000737:chemistry; D007536:Isomerism; D013058:Mass Spectrometry; D008745:Methylation; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D011522:Protons; D013936:Thymidine / Q000737:chemistry*; D014529:Uridine / Q000031:analogs & derivatives* / Q000737:chemistry
chemical_list: D005740:Gases; D011522:Protons; C009182:ribothymidine; D010100:Oxygen; D013936:Thymidine; D014529:Uridine
keywords: 5-Methyluridine; Electronic structure theory; IRMPD action spectroscopy; Protonation; Thymidine