By a combination of ab initio calculations and photoelectron
spectroscopy, we demonstrate that the DNA surrounding has only a negligible
effect on ionization energies of nucleobases in the native aqueous environment. The
aqueous solution thus effectively screens the sugar.phosphate backbone and the
neighboring nucleobases. Consequently, vertical ionization potentials of nucleobases
in aqueous DNA can be reliably derived from the corresponding values for its
building blocks in water, that is, aqueous nucleobases, nucleosides, or nucleotides.