G-tetrad and other 6-thioguanine (SG) incorporated tetrads have been studied in this Letter. The geometries, energies, charge distributions have been discussed. The effects of different cations (K- and Na+) on the various tetrads have also been studied. The outcomes show that as the SG number increases the tetrad becomes more and more unstable. The Na+ binds more tightly with the tetrad than that of K- without hydration correction, while considering hydration effects the stability sequence changes to K+ > Na+. Electrostatic potential map of the tetrads have been plotted and the binding sites of cations have been also shown. (C) 2004 Elsevier B.V. All rights reserved.