Photoelectron spectroscopy and ab initio calculations employing a non-equilibrium
polarizable continuum model were employed for determining the vertical ionization potential of
aqueous protonated imidazole. The experimental value of 8.96 eV is in an excellent agreement
with calculations, which also perform quantitatively for ionization of aqueous alkali cations as
benchmark species. The present results show that protonation of imidazole shifts up its vertical
ionization potential in water by 0.7 eV, which is significantly larger than the resolution of the
experiment or the accuracy of the calculation. Therefore, this combined experimental and
computational approach can be applied for quantitatively analyzing the protonation state of
histidine, of which imidazole is the titratable side chain group, in aqueous peptides and proteins.