Protonation of methyluracils in the gas phase: The particular case of 3-methyluracil

Abstract

The gas-phase structure of the protonated isomeric methyl uracils (1-Me-, 3-Me-and 6-Me-) was examined using mid-infrared multiple photon dissociation (IRMPD) spectroscopy performed at CLIO, the Orsay (France) Free Electron Laser facility. Experimental infrared spectra were recorded for protonated species generated by electrospray ionization, isolated and irradiated in a quadrupole ion trap, and compared to calculated infrared absorption spectra of the different low-lying isomers computed at the B3LYP/6-31++G(d,p) level of theory. For both protonated 1-Me-uracil and 6-Me-uracil, the global energy minima correspond to enolic tautomers, whose infrared absorption spectra were found to match very well with the experimental IRMPD spectra. A small fraction of another low energy lying keto tautomer is also present under electrospray conditions. Protonation of 3-Me-uracil by electrospray results exclusively in the formation of a keto form. 3-Me-uracil, which therefore constitutes a particular case in the series of pyrimidine nucleobases studied so far. Methylation of the N3 position of uracil prevents the interconversion between the keto and enol forms, as encountered for uracil, and uracil methylated on N1, C5, or C6. These data also give new insights about the unimolecular reactivity of protonated uracils.

Alkylation of uracil and thymine in the gas phase through interaction with alkylmercury compounds

“…The resulting IRMPD spectrum is reported in Figure 5a. First, one can see that this spectrum strongly differs from those recorded for the protonated forms of the four methyl-uracil isomers, and reported recently [81]. Therefore, this confirms the information deduced from CID experiments.…”

Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid

“…Such a rearrangement could be promoted by attractive interactions between the corresponding O(1) hydrogen atoms and the lone pairs of adjacent N(9) and N(2). Similar interactions have previously been recognized to occur in several molecular systems, including DNA/RNA bases and nucleosides and their methylated derivatives, both isolated and hydrated. − …”

“…Similar interactions have previously been recognized to occur in several molecular systems including DNA/RNA bases and nucleosides and their methylated derivatives, both isolated and hydrated. 25,26,27,28 Finally, atom-charge distributions of the six most stable [CA+H] + isomers were computed through natural population analysis. The results suggest that, when a proton binds to a cyameluric acid molecule for most favored protonation sites, the proton charge delocalizes over the entire species leading to similar charges (of  0.5) on the corresponding four H atoms, irrespective they are bound to N or O (Figure S13).…”

Gas-Phase Oxidation of the Protonated Uracil-5-yl Radical Cation

“…Thus, we set out to investigate the gas-phase reaction of the protonated uracil-5-yl radical cation, which should be in the di-enolic form. It has also been shown that studies on protonated uracils and uridines provide important insight into the stability and prevalence of various tautomeric forms. − Combining ion-trap mass spectrometry and the investigation of charge-separation distonic radical ions is a insightful technique for the study of radical reactivity without the perturbative presence of solvent. − …”