PHOTOELECTRON IMAGING OF DNA - A STUDY OF SUBSTRATES AND CONTRAST

Photoelectron imaging is a surface technique in which low-energy electrons are ejected from the specimen, usually by UV light. In biological applications, the choice of substrates is an important factor. Substrates must be conductive, have low photoelectron emission, be optically flat and stable, and promote spreading of biological specimens. The results of a survey of substrate materials for photoelectron imaging of DNA and other biological macromolecules are reported. On the basis of material contrast observed in the photoelectric measurements, it is predicted that DNA complexes with small proteins should be observable by photoelectron microscopy. Calculations show that topographical contrast, in addition to material contrast, is important in imaging nucleic acids and proteins. Improved photoelectron images of duplex DNA were recorded using the best of the substrates examined: a thin layer of magnesium fluoride on chromium-coated glass. These photoelectron images were obtained without staining, coating, or metal-shadowing the DNA.