X-RAY DIFFRACTION 



97 



(a) 



(b) 



(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- 



