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

The gas phase structures of sodium cationized 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. The IRMPD spectra of all six sodium cationized complexes exhibit both characteristic and unique spectral features over the range from similar to 1000 to 1900 cm(-1) such that the complexes are easily differentiated. The intense band at similar to 1800 cm(-1) in the IRMPD action spectrum of Na(+)(U) indicates that, as expected, a free carbonyl group is present in this complex. Absence of an intense band at similar to 1800 cm(-1) in the IRMPD action spectra for Na(+)(2SU), Na(+)(5Me2SU), Na(+)(6Me2SU), and Na(+)(4SU) complexes suggests that either sodium cationization preferentially stabilizes a minor tautomer of the nucleobase, or that the sodium cation binds to the keto group in these complexes, such that no free carbonyl stretch is observed. 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. Based on these comparisons and the energetic predictions from theory, sodium cations preferentially bind at the 4-keto position of the canonical 2,4-diketo or 2-thioketo-4-keto tautomer in the Na(+)(U), Na(+)(2SU), Na(+)(5Me2SU), and Na(+)(6Me2SU) complexes. In contrast, sodium cationization results in preferential stabilization of a minor tautomer of the nucleobase in the Na(+)(4SU) and Na(+)(24dSU) complexes, where proton transfer from the N3H group to the 4-thioketo group, facilitates strong binding of the sodium cation via chelation with the O2(S2) and N3 atoms. (C) 2011 Elsevier B.V. All rights reserved.