. Elementary biophysics: selected topics. Biophysics. (c). Figure 48 depending on where they are, the contributions of the two atoms differ widely. Thus, by combining the results of several different reinforce- ment situations, it is possible to deduce the correct structure of molecules containing a few atoms. However, if the number of atoms in the molecule is very large, as it is for almost all biologically important molecules whose structure is unknown, it is not feasible to deduce the structure from the observed intensities alone. We now turn briefly to a method other than the powder method
. Elementary biophysics: selected topics. Biophysics. (c). Figure 48 depending on where they are, the contributions of the two atoms differ widely. Thus, by combining the results of several different reinforce- ment situations, it is possible to deduce the correct structure of molecules containing a few atoms. However, if the number of atoms in the molecule is very large, as it is for almost all biologically important molecules whose structure is unknown, it is not feasible to deduce the structure from the observed intensities alone. We now turn briefly to a method other than the powder method for obtaining properly oriented crystals. If a single crystal is placed in the path of the x-ray beam, it will not in general be at the proper angle to give reflections. But if the crystal is rotated slowly, it will at some in- stant be in the proper position to give reinforcement for a particular set of planes, and the x-rays will form a spot on the photographic film. When rotated to a somewhat different position, still another set of planes will be at the proper angle for reinforcement, and a spot will be formed at a different place on the film. Thus, by rotating the crystal, a set of spots of various intensities is obtained. The analysis is similar to that for the powder method, in that the distances between spots are related to the crystal plane spacings and the intensities of the spots are related to the structures of the (polyatomic) molecules at the crystal positions. The materials of biological interest usually do not lend themselves to the powder method, and other methods, especially rotating crystal methods, are usually employed. As pointed out in the first paragraph of this section, it must be realized that, given the structure, the x-ray diffraction pattern is com-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble t
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