We have simulated ionization of purine nucleic acid components in the gas phase and in a water environment. The vertical and adiabatic ionization processes were calculated at the PMP2/aug-cc-pVDZ level with the TDDFT method applied to obtain ionization from the deeper lying orbitals. The water environment was modelled via microsolvation approach and using a non-equilibrium polarizable continuum model. We have characterized a set of guanine tautomers and investigated nucleosides and nucleotides in different conformations. The results for guanine, i.e., the nucleic acid base with the lowest vertical ionization potential, were also compared to those for the other purine base, adenine. The main findings of our study are following: (a) Guanine remains clearly the base with the lowest ionization energy even upon water solvation. (b) The water solvent has a strong effect on the ionization energetics on guanine and adenine and their derivatives; the vertical ionization potential (VIP) is lower by about 1 eV for guanine while it is circa 1.5 eV higher in the nucleotides, overall resulting in similar VIPs for GMP, guanosine and guanine in water. (c) Water efficiently screens the electrostatic interactions between nucleic acid components. Consequently, ionization in water always originates from the base unit of the nucleic acid and all the information about conformational state is lost in the ionization energetics. (d) The energy splitting between ionization of two least bound electrons increases upon solvation. (e) Tautomerism does not contribute to the width of the photoelectron spectra in water. (f) The effect of specific short-range interactions with individual solvent molecules is negligible for purine bases, compared to the long-range dielectric effects of the aqueous medium.