Abstract: The geometric structures, the interaction energies, the vibrational characteristics, and the electronic
structures of the complexes of the isoguanine (isoG) quintet coordinated with mono valent cations (Naþ, Kþ, Rbþ,
and Csþ) have been studied based on the nonplanar models. The geometry of the local minimum structure of the
Naþ-isoG quintet complex deviates significantly from the planar structure. The geometric characteristics of the NaþisoG
quintet complex support the experimental findings that Naþ is unlikely to induce the formation of the isoG
quintet-based pentaplexes. Similar to the guanine tetraplexes, the ionic selectivity of the isoG quintet-based pentaplexes
is largely dominated by the hydration energy of the cations. After hydration correction, the positive value of
the free energy difference for the formation of the Naþ-isoG quintet complex (DGf) suggests that the isoG quintet is
unable to capture the hydrated Naþ. The negative values of DGf for the Kþ and Rbþ complexes implies that both
ions have the tendency to be inserted into the isoG pentaplexes. This study suggests that, to elucidate the high Csþ
selectivity of isoG pentaplexes, it is necessary to extend the model from the isoG quintet to the isoG decamer.