Practical Protein Chemistry - A. Darbre 1989

X-ray crystallography and electron microscopy
Electron microscopy
Image processing

The ultimate outcome of an Electron Cell/15.html">Microscopy study proper is obtaining an image on a photographic plate. In some cases, the molecules under investigation may be arranged into two-dimensional arrays, also referred to as two-dimensional crystals. For such objects, Methods exist that can significantly enhance image quality. One such image Processing tool is the optical diffractometer (Fig. 20.15). An electron micrograph of a crystal acts as a two-dimensional diffraction grating, producing a series of diffraction spots when illuminated by a beam of monochromatic light (Fig. 20.16). These spots can be viewed as the result of light beam diffraction by the averaged, repeating unit cell of the lattice Structure, whereas any deviations in The structure of individual Cells from the average are treated as noise, resulting in diffraction reflections lying outside the principal spots. Incorporating an additional lens into the diffractometer makes it possible to reproduce or reconstruct the original image. If the rays scattered by the non-periodic component (the noise component) are first removed by inserting a filter, the image of the averaged repeating structural unit can be successfully reconstructed. Similarly, an image formed by two overlapping lattices can be "resolved." To do this, one simply filters out (blocks) the diffraction maxima associated with one of the lattices and reconstructs the image using the remaining diffraction maxima. Clearly, in both of these Examples, the information content of the original micrographs is substantially improved.