The HF, B3LYP and MP2 methods along with the 6-311G(d,p) and 6-311++G(d,p) basis sets have been used to compute the relative stability, ionization potentials and spectral origins corresponding to the electronic lowest singlet pi pi* excited states of 28 possible guanine tautomers in the gas phase to supplement the experimental data. The CIS/6-311G(d,p) level of theory was used to optimize electronic singlet pi pi* excited state geometries. The TDDFT method and the 6-311G(d,p) basis set was used to compute spectral origins (0-0 transitions) using the ground and excited state optimized geometries. Significant variations in the ionization potentials and spectral origins among guanine tautomers were revealed. We believe that the computed results would be helpful to interpret the complex R2PI spectra of guanine obtained in the supersonic jet-beam experiments. (c) 2006 Elsevier B.V. All rights reserved.