The influence of the substrate temperature on the preparation of DNA films for studies under vacuum conditions

Experiments were carried out to determine the dependence of the physical form of supercoiled DNA films on the initial temperature of the substrate. Such films are often used in irradiation experiments involving low energy particles, like electrons or photons. In order to obtain absolute values for cross sections for such experiments, the spatial distribution of the sample in the film has to be well estimated. These investigations aim to correlate the size and form of a DNA film with the initial temperature of the substrate, on which the liquid sample is deposited prior to evacuation. From our previous studies we concluded that the presence of agents preventing DNA structural water evaporation is required in order to preserve the supercoiled plasmid DNA form under vacuum conditions. Therefore, we examined the temperature dependence on films prepared from plasmid suspended in solutions containing magnesium hydroxide, Tris-Cl buffer and various concentrations of sodium hydroxide. To visualise the films we used a conventional light microscope with a CCD camera and a scanning electron microscope. The results revealed a significant influence of the temperature of the substrate on both the area of the substrate covered by the film as well as on the spatial distribution of DNA molecules. An increase in the amount of sodium hydroxide that stabilises supercoiled DNA under vacuum increases DNA aggregation. After these investigations we conclude that the best temperature of the substrate to produce uniform and thin films should be between -5 °C and -10 °C for substrates which are not atomically flat and above 0 °C for atomically flat substrates.