DFT investigation of the intermolecular interactions of a thieno-separated tricyclic guanine analog with gold nanoclusters

The interactions between a thieno-separated tricyclic guanine analog (ttG) and gold nanoclusters Aun (n = 2, 4, 6, and 8) were studied theoretically at the DFT level. Geometries of neutral complexes were optimized using the B3LYP functional with the 6-31+G(d,p) basis set for ttG and the LANL2DZ basis set for gold clusters. The binding modes, interacting strength, and the charge transfer properties of different Aun–ttG complexes were investigated. Natural population analysis was performed for NBO charges. It was found that gold nanoclusters form stable complexes with ttG and the complexation results in a substantial amount of electronic charge being transferred from ttG to the gold clusters. Furthermore, binding of Aun is the most strong at N3 site, followed by the N7 site, and the binding at O6 site is relatively weaker. In addition, binding of Aun at N3 site will increase the planarity of ttG moiety. The vertical first ionization potential, electron affinity, fermi level, and the HOMO–LUMO gap of ttG and Aun–ttG complexes were also analyzed.