Are guanine tetrads stabilized by bifurcated hydrogen bonds?

We have carried out Hartree-Fock and B3LYP hybrid density functional studies with various basis sets to analyze the geometries and relative energies of different guanine tetrad conformations with Hoogsteen-type and bifurcated hydrogen bonds. B3LYP/DZVP calculations yield energy minima for nonplanar tetrad structures with both types of hydrogen bond patterns. At the highest B3LYP/6-311+G(2d,p)//B3LYP/6-311G(d,p) level of theory, a planar conformation with a bifurcated hydrogen bond pattern is the most stable structure we found, whereas at the B3LYP/6-311G(d,p) level, the nonplanar Hoogsteen tetrad is the more stable one. The energy differences between structures with Hoogsteen-type and bifurcated hydrogen bonds as well as between nonplanar S-4 symmetric and planar C-4h symmetric structures are very small and depend on the choice of the basis. Finally, HF/6-311G(d,p) calculations show a significantly different potential energy surface compared to the B3LYP method because the local energy minima for the Hoogsteen-type structures are missing.