Scanning Tunneling Microscope Observation Of Plasmid Dna Under Electron Irradiation At 8-40 Ev

The structural changes in plasmid DNA adsorbed onto graphite following low-energy electron irradiation were investigated. Using a scanning tunneling microscope (STM), we observed networks or islands of DNA consisting of entangled molecules and compared the shapes of the DNA before and after electron irradiation at 8-40 eV field emitted from the tip of the STM. The shape of the DNA changed depending on the electron energy. Electrons with very low energy, such as 8 or 13 eV, extended the area of a DNA island, while the electrons at 18 or 38 eV degraded it. Both types of changes tend to saturate as the electron dose increases. We also discuss the above results in terms of the chemical reactions, such as strand breaks or molecular dissociation, induced by low-energy electrons.