Mechanisms of the deamination reactions of isoguanine with H2O, OH-, and OH-/H2O and of protonated isoguanine (isoGH(+)) with H2O have been investigated by theoretical calculations. Eight pathways, paths A-H, have been explored and the thermodynamic properties (Delta E, Delta H, and Delta G), activation energies, enthalpies, and Gibbs energies of activation were calculated for each reaction investigated. Compared with the deamination reaction of isoguanine or protonated isoguanine (isoGH(+)) with water, the deamination reaction of isoguanine with OH- shows a lower Gibbs energy of activation at the rate-determining step, indicating that the deamination reaction of isoguanine is favorably to take place for the deprotonated form isoG(-) with water. With the assistance of an extra water, the reaction of isoguanine with OH-/H2O, pathways F and H, are found to be the most feasible pathways in aqueous solution due to their lowest Gibbs energy of activation of 174.7 and 172.6 kJ mol(-1), respectively, at the B3LYP/6-311++G(d,p) level of theory.