Electron exposure-dependent contrast transfer

Abstract

The use of electron microscope images for high resolution biological structure analysis has the important advantage that the structural phase information, which is not easily obtained in x-ray or electron diffraction studies, may be easily obtained from images by suitable numerical computations. It has been shown that the phase error determined from image data is much smaller than that which is generally achieved in the use of the isomorphous replacement method in protein x-ray crystallography (1). Due to the constraints imposed by electron radiation damage in the specimen, however, it is quite difficult to obtain high resolution image data for most biological specimens. The use of low-dose image recording techniques, and subsequent computer averaging, has made it possible to obtain 7 Å resolution images of a crystalline specimen embedded in glucose (1). The limitation at this resolution could be caused by instrumental factors and/or poor signal transfer in the recording medium when it is exposed to only a small number of electrons.