Vertical ionization energies of the nucleosides cytidine and deoxythymidine in water, the
lowest ones amounting in both cases to 8.3 eV, are obtained from photoelectron spectroscopy
measurements in aqueous microjets. Ab initio calculations employing a non-equilibrium
polarizable continuum model quantitatively reproduce the experimental spectra and provide
molecular interpretation of the individual peaks of the photoelectron spectrum showing also that
lowest ionization originates from the base. Comparison of calculated vertical ionization
potentials of pyrimidine bases, nucleosides, and nucleotides in water and in the gas phase
underlines the dramatic effect of bulk hydration on the electronic structure. In the gas phase, the
presence of sugar and, in particular, of phosphate has a strong effect on the energetics of
ionization of the base. Upon bulk hydration, the ionization potential of the base in contrast
becomes rather insensitive to the presence of the sugar and phosphate, which indicates a
remarkable screening ability of the aqueous solvent. Accurate aqueous phase vertical ionization
potentials provide a significant improvement to the corrected gas phase values used in the
literature and represent important information in assessing the threshold energies for
photooxidation and oxidation free energies of solvent-exposed DNA components. Likewise,
such energetic data should allow improved assessment of delocalization and charge hopping
mechanisms in DNA ionized by radiation.