Molecular structure and character of bonding of mono and divalent metal cations (Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Cu+) with guanosine: AIM and NBO analysis

The B3LYP/6-311++G (d,p) density functional approach was used to study the gas-phase metal affinities of Guanosine (ribonucleoside) for the Li+, Na+, K+, Mg2+, Ca2+, Zn2+, and Cu+ cations. In this study we determine coordination geometries, binding strength, absolute metal ion affinities, and free energies for the most stable products. We have also compared the results for Guanosine, with our previously reported results for 2'-Deoxyguanosine. Based on the results, it is obvious that MIA is strongly dependent on the charge-to-size ratio of the cation. Guanosine interacts more strongly with Zn2+ than do with Mg2+, Ca2+, and Cu+ and therefore stronger interactions lead to higher MIA. In both free molecules and their complexes, the Syn orientation of the base is stabilized by an intramolecular O5'aEuro"H center dot center dot center dot N3 hydrogen bond and the anti orientation of the base is stabilized by an intramolecular C-H center dot center dot center dot O hydrogen bond formed between the (C8-H8) and the O5' atom of the sugar moiety. It is also interesting to mention that linear correlation between calculated MIA values and the atomic numbers (Z) of the metal ions of Li+, Na+, and K+ were found. Furthermore, the influences of metal cationization on the strength of the N-glycosidic bond, torsion angles, angle of pseudorotation (P), and intramolecular C-H center dot center dot center dot O and O-H center dot center dot center dot O hydrogen bonds have been studied. Natural bond orbital (NBO) analysis was performed to calculate the charge transfer and natural population analysis of the complexes. Quantum theory of atoms in molecules (QTAIM) was also applied to determine the nature of interactions.