Tautomerism in the most-stable isomers of Al-guanine complexes and their cations is studied with density functional theory and second-order perturbation theory calculations. Electron propagator calculations on vertical ionization energies and Dyson orbitals provide information on the electronic structure in the most-stable neutral doublets, as well as in the corresponding singlets and triplets. The Al-guanine complex consists of a positively charged At ion with two localized valence electrons coordinated to a negatively charged guanine with an unpaired, delocalized pi electron. Three isomers have very similar energies; however, the most-stable form has a markedly different ionization energy. Ionization energies for the second and third forms almost coincide. Predicted ionization energies are in close agreement with recent spectra. In all three cases, the first ionization energy corresponds to a cationic, singlet final state where the unpaired, delocalized 7 electron on guanine has been removed, whereas the second ionization energy corresponds to the removal of an electron from a 3s-like orbital on the Al ion. Changes in Mulliken charges and optimized structures between neutrals and cations confirm these qualitative conclusions.