Tautomerism of Uracil Probed via Infrared Spectroscopy of Singly Hydrated Protonated Uracil

Abstract

Tautomerism of the nucleobase uracil is characterized in the gas phase through IR photodissociation spectroscopy of singly hydrated protonated uracil created with tandem mass spectrometric methods in a commercially available Fourier transform ion cyclotron resonance mass spectrometer. Protonated uracil ions generated by electrospray ionization are re-solvated in a low-pressure collision cell filled with a mixture of water vapor seeded in argon. Their structure is investigated by IR photodissociation spectroscopy in the NH and OH stretching region (2500-3800 cm(-1)) with a tabletop IR laser source and in the 1000-2000 cm(-1) range with a free-electron laser. In both regions the IR photodissociation spectrum exhibits well-resolved spectral signatures that point to the presence of two different types of structure for monohydrated protonated uracil, which result from the two lowest-energy tautomers of uracil. Ab initio calculations confirm that no water-catalyzed tautomerization occurs during the re-solvation process, indicating that the two protonated forms of uracil directly originate from the electrospray process.

Microhydration of protonated nucleic acid bases and protonated nucleosides in the gas phase

“…The gasphase 1a ¡ 1d interconversion via the 1,3 proton transfer requires an energy barrier in excess of 35-40 kcal/mol [29,31] to be overcome, making it energetically inaccessible at thermal energies. Similarly, the water-catalyzed isomerization of 1a into 1d by 1,3 proton transfer converting 1b ¡ 1e is relatively high in energy (16.6 kcal/mol), and is also not expected to occur in the collision cell [22]. Note that 1b is more stable than 1e and 1f by 1.7 and 3.1 kcal/mol [22], respectively.…”

“…To our best knowledge, the stabilities of protonated adenine tautomers and their complexes with water molecules [20] have only been investigated computationally, while there have been some experimental and theoretical studies on the hydration of protonated and deprotonated forms of the four mononucleotides (dAMP, dCMP, dGMP, and dTMP) [21]. More recently, the possible tautomeric forms of the monohydrated protonated uracil have been examined in the gas phase by using IR multiple-photon dissociation spectroscopy (IR-MPD) in combination with computational study [22].In this paper, we present the first experimental results on the energetics of hydration of protonated nucleic acid bases, NABs (uracil, Ura; thymine, Thy; adenine, Ade; and hypoxanthine, Hyp), and protonated nucleosides, NSDs (uridine, Urd; thymidine, Thd; adenosine, Ado; and inosine, Ino). These results were obtained from high-pressure mass spectrometry equilibria measurements in the gas phase.…”

“…To our best knowledge, the stabilities of protonated adenine tautomers and their complexes with water molecules [20] have only been investigated computationally, while there have been some experimental and theoretical studies on the hydration of protonated and deprotonated forms of the four mononucleotides (dAMP, dCMP, dGMP, and dTMP) [21]. More recently, the possible tautomeric forms of the monohydrated protonated uracil have been examined in the gas phase by using IR multiple-photon dissociation spectroscopy (IR-MPD) in combination with computational study [22].…”

Diagnosing the Protonation Site of b2 Peptide Fragment Ions using IRMPD in the X–H (X = O, N, and C) Stretching Region

“…IR spectra in the 2800-3800 cm -1 spectral range of the clusters ions were recorded using a 7 Tesla Fourier transform ion cyclotron resonance (FT-ICR) tandem mass spectrometer (Bruker Apex Qe) coupled with an optical parametric oscillator/amplifier (OPO/OPA from LaserVision) laser system [32,33]. This laser system is pumped by an Innolas Spitlight 600 non-seeded Nd:YAG (1064 nm, 550 mJ/pulse, bandwidth~1 cm -1 ) laser running at 25 Hz and delivering pulses of 4-6 ns duration.…”

Chirality Effects on the IRMPD Spectra of Basket Resorcinarene/Nucleoside Complexes

“…It is a well-known feature of the IRMPD spectra of noncovalent adducts that contain NH and OH hydrogen-bond donors that their stretching vibrations can be more or less red-shifted and broadened, depending upon their relevant dissociation threshold. [34,46,47,[50][51][52][53][54][55][56] The broad resonances at 3100-3300 cm À1 …”