Photoionization efficiency spectra of gas-phase Al-guanine-(NH3)(n), 0 less than or equal to n less than or equal to 2, have been measured in the 210-270 nm range. Within error the ionization energies, 4.65 +/- 0.08, 4.6 +/- 0.1, and 4.5 +/- 0.2 eV for n = 0, 1, and 2, respectively, are equivalent. This result is explained in terms of molecular geometries in which NH3 associates with the guanine constituent of Al-guanine specifically. The onset of signal in the Al-guanine photoionization efficiency spectrum is found to shift from 5.6 +/- 0.1 to 4.65 +/- 0.08 eV upon introduction of NH3 to the He carrier gas. The shift is indicative of formation of vastly different amounts of distinct isomers of Al-guanine in the presence or absence of NH3. The geometries, ionization energies, and heats of formation of the isomers have been determined using DFT calculations benchmarked against the experimental ionization energies. The most stable isomer is one of an Al atom complexed to an unusual tautomer of guanine in which, effectively, a proton has been transferred from the six-membered ring to the five-membered ring. To our knowledge, this is the first demonstration of the existence of this tautomer of guanine.