Origin of Na+/K+ Selectivity of the Guanine Tetraplexes in Water: The Theoretical Rationale

Theoretical study of the “sandwich” models of the metal ion (Na + and K + ) G-tetrad complexes has been performed at the HF/6-311G(d,p)//HF6-31G(d,p) level. The distances between cations and the O6 atoms of guanines have been predicted to be 2.826 Å for sodium and 2.951 Å for potassium in the G4−M−G4(inter) model and 2.791 Å for sodium and 2.927 Å for potassium in the G4-M−G4(intra) model. These results are consistent with the Na + −O6 distance of 2.75 ± 0.02 Å in the corresponding crystal structures. The binding energy between the cations and the G tetrad in the tetraplexes is greater for Na + than for K + . This suggests the domination of the electrostatic interaction in the cation-tetraplexes systems. After the hydration energy correction, the stability sequence of monovalence cations in the guanine tetraplexes is found to be K + > Na + . This finding gives the first direct theoretical evidence to support the hypothesis of Hud et al. that the preferred coordination of K + over Na + in G tetraplexes is actually dominated by relative energies of hydration.