title: Structure of protonated thymidine characterized by infrared multiple photon dissociation and quantum calculations.
abstract: BACKGROUND
  Many fundamental studies are motivated by the probable relationship between the presence of rare enol tautomers of nucleobases and point mutation developing during nucleic acid replication. The evaluation of the tautomeric behaviour of nucleobases is therefore of fundamental importance. This can be probed in the gas phase by combining action spectroscopy and mass spectrometry.
  
  
  METHODS
  Experimental Infrared Multiple Photon Dissociation spectra in the fingerprint region of electrospray-generated and subsequently selected ions were recorded at the CLIO free electron laser (FEL) facility, by coupling the FEL to a quadrupole ion trap, and compared to calculated harmonic vibrational infrared spectra of the different low-lying isomers computed at the B3LYP/6-31++G(d,p) level. Relative energies were refined using the extended basis set 6-311++G(3df,2p).
  
  
  RESULTS
  The Density Functional Theory (DFT) study shows that, as for protonated thymine, the global energy minimum of protonated thymidine corresponds to an enol tautomer, whose infrared absorption spectrum is found to be in very good agreement with the experimental IRMPD spectrum. A very weak IRMPD signal observed at ~1780 cm(-1) is very likely the signature of an oxo tautomer. Consequently, as for thymine, protonated thymidine generated by electrospray corresponds to a mixture of at least two tautomeric forms.
  
  
  CONCLUSIONS
  Tautomerization can be characterized by IRMPD spectroscopy. Interestingly, the dominant enolic tautomeric form(s) presently observed cannot be directly generated from the most stable neutral tautomer of the thymine residue.
authors.affiliation: Université d'Evry Val d'Essonne, Laboratoire Analyse et Modélisation pour la Biologie et l'Environnement, Boulevard François Mitterrand, 91025, Evry, France.|CNRS-UMR 8587.
authors.forename: Jean-Yves
authors.initials: JY
authors.lastname: Salpin
authors.affiliation: Université Paris Sud Orsay, Laboratoire de Chimie Physique, Bâtiment 350, 91405, Orsay, France.|CNRS - UMR 8000.
authors.forename: Debora
authors.initials: D
authors.lastname: Scuderi
mesh_terms: D015394:Molecular Structure; D017785:Photons; D013055:Spectrophotometry, Infrared / Q000295:instrumentation / Q000379:methods; D013936:Thymidine / Q000737:chemistry*
chemical_list: D013936:Thymidine