Owing to its unique properties, water pays 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 model systems such as dry and microhydrated clusters of nucleobases. By using mass spectrometry and tunable VUV radiation we found 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 produced by proton transfer from the ionized water molecule and elimination of the 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 molecular orbital from which 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 appearances energies. We also note that possible adiabatic process on the ground electronic state of the ionized system, which becomes energetically accessible at lower energies, is not efficient. Thus, in this system proton transfer is controlled electronically, by the character of the ionized state, rather than statistically, by simple energy considerations.

Proton transfer between nucleobases is mediated by water

246th National Meeting (2013)

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