Electronic Properties of DNA Base Molecules Adsorbed on a Metallic Surface

Abstract

The internal electronic structure of single deoxyribonucleic acid (DNA) base molecules (i.e., guanine, adenine, cytosine, and thymine) adsorbed on a metallic surface of Cu(111) is determined in detail using density functional theory (DFT) computations. In contrast to the intuitive beliefs that a molecule weakly interacts with a substrate and its electronic structure is only slightly perturbed, our simulations reveal strong hybridizations and interactions between molecular and metallic states. Stipulated by the symmetries of a base molecule and the Cu(111) surface, oxygen atoms of a base approach close to the substrate, breaking the parallel orientation of the π-system with respect to the surface. Such a behavior is the most pronounced for one oxygen containing bases, leading to the chemisorption of cytosine and guanine and to stronger hybridization of their electronic states with metallic ones. Oxygen free adenine, on the other hand, lies nearly flat on a Cu substrate and interacts weakly with the surface through physisorption. The calculated local electron density of states spectra demonstrate the absence of pure localized molecular states for all four DNA bases, yet they show the smallest delocalization for adenine and thymine and the largest for guanine and cytosine. The observed diversity of the geometrical and electronic structures of the nucleobases on the Cu substrate provides guidelines for interpreting DNA tunneling spectra in the scanning tunneling microscopy (STM) measurements. Our results open a new prospective for understanding biomolecule adsorbates and have an important implication for a possible differentiation of nucleotide sequences in DNA through STM.