Modulation of structural, energetic and electronic properties of DNA and size-expanded DNA bases upon binding to gold clusters

Gold cluster-nucleobase complexes have potential applications in designing and fabrication of novel electronic nano- devices, and there has been a surge in research activities in this area recently. Binding of gold clusters (Au-3 and Au-4) with DNA bases and size-expanded DNA bases (x-bases) have been studied using density functional theory employing high quality basis set. A comprehensive attempt has been made to examine several gold-nucleobase complexes with respect to change in the orientation of Au clusters with respect to all the titratable sites of the bases. Geometric and electronic features of these complexes provided evidences for existence of non-conventional hydrogen bonds, which was further substantiated via vibrational frequency and natural bond orbital (NBO) analysis. The nucleobases, both canonical and size-expanded forms, form stable complexes with both the gold clusters considered here. The natural population analysis (NPA) and NBO analysis indicated that complexation greatly affects the charge distribution on the DNA bases due to charge transfer between base and gold cluster. Upon complexation, a marked decrease in the HOMO-LUMO gaps and ionization potentials was observed, which was more profound in case of x-bases due to the extended pi-conjugation of the fused benzene rings. Such electronic effects driven by structural perturbations in nucleobases are expected to provide a better test bed for designing charge transfer driven nano-devices. This study demonstrates that combining structural modifications to DNA bases and subsequent binding to gold nanoparticles can be effectively used to modulate and design materials with desired optico-electronic properties.