Electron Microscopy as a Structure Analyzing Tool
EM may be used as a tool to obtain high-resolution images of membrane proteins, much the same as
X-ray crystallography is used for water soluble proteins. Low-dose radiation was used in Unwin & Henderson's
work (1975) on unstained membranal specimens, so that structural damage was minimized. Membranes are
tilted at various angles so that different views of the structure projected onto the interference plane are
obtained. The information is recombined using Fourier techniques. This technique provides information
with a resolution lower than x-ray, due to the longer wavelength of the electron beam. However, X-ray
cannot be used on membranal systems,since it is necessary that the sample will be wholly water-soluble, for crystallization purposes.
A refinement of this technique is electron-cryo microscopy. This involves cooling the staged specimen to extremely low temperatures (down to liquid helium!), and thus minimizing beam-induced movement of the specimen. An additional refinement is stepwise scanning of the image using an electron beam with a 500-1000A diameter. Again, this minimizes beam-induced movement.