Infrared multiple photon dissociation action spectroscopy of protonated uracil and thiouracils: Effects of thioketo-substitution on gas-phase conformation

The gas-phase structures of protonated complexes of uracil and five thiouracils including 2-thiouracil (2SU), 5-methyl-2-thiouracil (5Me2SU), 6-methyl-2-thiouracil (6Me2SU), 4-thiouracil (4SU), and 2,4-dithiouracil (24dSU) are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical electronic structure calculations. IRMPD action spectra of all six protonated complexes exhibit both similar and distinct spectral features over the range of similar to 1000-1900 cm(-1), such that the complexes are easily differentiated by their IRMPD action spectra. Absence of the carbonyl stretch at similar to 1825 cm(-1) in the IRMPD spectra for the H+(U), H+(2SU), H+(5Me2SU), and H+(6Me2SU) complexes suggests that the binding of a proton preferentially stabilizes alternative tautomers of the nucleobases in these complexes such that no free carbonyl stretch is observed. In contrast, the intense band at similar to 1825 cm(-1) in the IRMPD action spectrum of H+ (4SU) indicates that a free carbonyl group is still present in this complex. Measured IRMPD action spectra are compared to linear IR spectra calculated at the B3LYP/6-31G(d) level of theory to identify the structures accessed in the experimental studies. On the basis of these comparisons and energetics derived from the calculations, protonation results in preferential stabilization of a minor tautomer of the nucleobase in the H+(U), H+(2SU), H+(5Me2SU),H+(6Me2SU), and H+(24dSU) complexes, where both keto (thioketo) groups are converted to hydroxy (sulfhydryl) groups by proton binding and proton transfer from the neighboring N3H group. In contrast, the proton preferentially binds at the 4-thioketo position to the canonical keto-thioketo tautomer in the H+(4SU) complex. Additional band(s) are present in the IRMPD action spectra of the H+(U) and H(4SU) complexes that suggest that a small population of excited low-energy conformers are also accessed in those systems. (C) 2010 Elsevier B.V. All rights reserved.