Water plays a central role in chemistry and biology by mediating the interactions
between molecules, altering energy levels of solvated species, modifying potential
energy profiles along reaction coordinates, and facilitating efficient proton transport
through ion channels and interfaces. This study investigates proton transfer in a
model system comprising dry and microhydrated clusters of nucleobases. With mass
spectrometry and tunable vacuum ultraviolet (VUV) synchrotron radiation, we show
that water shuts down ionization-induced proton transfer between nucleobases, which
is very efficient in dry clusters. Instead, a new pathway opens up in which protonated
nucleobases are generated by proton transfer from the ionized water molecule and
elimination of a hydroxyl radical. Electronic structure calculations reveal that the
shape of the potential energy profile along the proton transfer coordinate depends
strongly on the character of the molecular orbital from which the electron is removed,
i.e., the proton transfer from water to nucleobases is barrierless when an ionized state
localized on water is accessed. The computed energetics of proton transfer is in
excellent agreement with the experimental appearance energies. Possible adiabatic
passage on the ground electronic state of the ionized system, while energetically
accessible at lower energies, is not efficient. Thus, proton transfer is controlled
electronically, by the character of the ionized state, rather than statistically, by
simple energy considerations.