Scanning tunneling spectroscopy (STS) of DNA and G4-DNA molecules*

Attempts to resolve the energy level structure of single DNA molecules by STS span over the last two decades, thanks to this technique ability to probe the local density of states of objects deposited on a surface. Success was hindered by extreme technical difficulties in stable deposition and reproducibility. By using STS at 78 \r{ }K, for the first time we disclose the energy spectrum of poly(G)-poly(C) DNA molecules deposited on gold. The tunneling current-voltage characteristics and their derivative curves exhibit a clear gap and a peak structure around the gap. By means of ab initio Density Functional Theory calculations the character of the observed peaks is assigned to orbitals originating from the different molecular components. Limited fluctuations in the I-V curves are observed and statistically characterized.