van der Waals DFT ONIOM study of the adsorption of DNA bases on the Cu(111) nanosurface

In this work, the adsorption of DNA bases on the Cu(111) nanosurface was studied to investigate the electronic structure of the bases on the Cu surface and to find the effect of van der Waals interactions on the adsorption mode of the bases. The calculations were performed in the ONIOM scheme using two DFT functionals (PW91PW91 and wB97XD). It was found that using the long-range corrected DFT functional such as wB97XD (which considers the vdW interactions) decreases the tilt angle of the bases on the surface. The main molecular orbitals responsible for the interaction and charge transfer between each base and Cu surface were determined. The HOMO-4, (HOMO, HOMO-2, and HOMO-3), (LUMO, HOMO-3, HOMO-4, and HOMO-6), and (LUMO and HOMO-4) are responsible for the interaction and charge transfer with the surface for adenine, cytosine, guanine, and thymine, respectively. The quantum theory of atoms in molecule (QTAIM) was used for better characterization of the interaction of the bases with the surface by a topological analysis of the electron density. The results showed that the most interacting atom of adenine with the Cu surface is the N atom of the NH2 group, whereas the most interacting atom of cytosine and guanine were the O atoms of the carbonyl groups. The QTAIM analysis also showed that the interaction of the O atom of cytosine with the Cu surface is stronger than that of guanine. For TY, the O and N atoms have higher interaction with the Cu surface compared to other atoms. Based on the calculated values of the electron density (ρBCP) and the positive sign of the Laplacian of the electron density (∇2ρBCP) at the bond critical points of the atom-atom interactions, it was concluded that the interaction between the bases and the surface can be categorized as closed-shell interactions.