We investigated structures and electronic properties of DNA base pairs containing the lactim forms of guanine (G*) or thymine (T*) as well as the imino forms of cytosine (C*) or adenine (A*) by ab initio MP2 and DFT methods. The found stable structures of G*-T and G-T* base pairs are the nonplanar ones. The obtained formation energies indicate that G*-T and A-C* are more favorable than G-T* and A*-C. The nature of their hydrogen bonding was discussed based on the energy decomposition analysis of Morokuma-Kitaura and the reduced-variational-space methods to elucidate the possibility of the tautomerization of DNA base pairs. (c) 2005 Elsevier B.V. All rights reserved.