Capture of an electron by either DNA base pair results in a major structural reorganization. One of the peripheral hydrogen bonds is significantly lengthened (0.22 angstrom in GC(-)) and the other significantly shortened (0.30 angstrom in AT(-)). These striking structural shifts are predicted by the MP2 method. We argue that the correct electronic description of the two anions is G[C-] and A[T-], displaying hydrogen bonding between the neutral purine base and the pyrimidine radical anion. This hypothesis is tested via higher level studies of thymine and its radical anion. The experimental conformation or denial of these predictions will provide a major step in our understanding of the structures of biomolecules. (C) 2011 Elsevier B.V. All rights reserved.