Ionization of dimethyluracil dimers leads to facile proton transfer in the absence of H-bonds
Efficient proton transfer (PT) across π-stacked pairs of methylated nucleic acid bases (NABs, 1,3-dimethyluracil) is observed upon ionization, even though methylation blocks the H-bonding sites and eliminates the H-bonded conformers in the neutral. Ionization driven PT in dimers of uracil and 1,3-dimethyluracil are investigated by tunable vacuum-ultraviolet (VUV) synchrotron mass spectrometry and electronic structure calculations. Ionization initiates efficient PT in H-bonded NAB pairs and the methylated species exhibit similar behavior with 85% of the ionized dimers dissociating and is concurrent with a proton transfer. Deuterated dimethyluracil experiments reveal that PT occurs from the methyl groups and not from the aromatic CH sites. Calculations illuminate qualitative differences in the PT reaction coordinate in the π-stacked and H-bonded base pairs. PT in methylated dimers involves significant rearrangements of the two fragments facilitating a relatively low potential energy barrier of only 0.6 eV in the ionized dimer.