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 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.