Ionization and fragmentation of uracil upon microhydration.

We study at the DFT level the ionization and the fragmentation of uracil in the presence of zero, one and two water molecules, to unravel the effect of microhydration on the reactivity of this nucleobase. We show that the microhydration lowers the adiabatic and vertical ionization potentials by 0.41 eV and 0.22 eV, respectively. Furthermore, microhydration increases the activation energies of the different dissociation channels up to 0.5 eV and restricts the formation of some fragments, in particular those corresponding to the C5-C6 fragmentation pathway. For the first time, our theoretical study shows new transition states and minima not found for the gas phase, hence indicating a change in the fragmentation mechanisms, as well as a stabilizing effect of microhydration, confirming previous experimental studies.