The theoretical ab initio calculations performed in this work revealed two structural isomers of the anion of the canonical pair of nucleic acid bases, thymine and adenine, where the two bases are separated by more than 7 A but remain connected via an excess electron localized in the middle of the complex. In both systems the two monomers forming the complex point the positive poles of their dipoles at the electron - a configuration highly unstable for the neutral complex. (C) 2003 Elsevier B.V. All rights reserved.