Energetic and structural changes in the hydrogen bonds of guanine (G) and cytosine (C) base pair due to Pt complex binding are investigated by post-Hartree-Fock methods. When the ligands are NH3 or H2O the structure undergoes intermolecular proton transfer from G to C (denoted as G*C pair) resulting in an increase in the bonding energy of 3-10 kcal/mol. This renders the structure to be metastable due to: (a) successive processes of charge transfer from G to cisplatin thereby stabilizing the GC and G*C pairs and (b) an additional hydrogen bond between G and the ligand of Pt atom. (c) 2006 Elsevier B.V. All rights reserved.