98 METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE 



Fig. 49. Drawing of a model of the myoglobin molecule. The model shows 

 the shape of the chain seen at 6 A resolution. The dark disc-shaped region is 

 the haem group. 



putable in a straightforward way. Thus if the structure of the molecule 

 being studied is either known or suspected, the pattern can be computed 

 and compared with the experimental pattern. A skilled worker can use 

 the comparison to make reasonable guesses about what modifications 

 may be needed to make the theoretical pattern agree better with the 

 observed pattern. Thus, by a series of trial and error modifications, it is 

 frequently possible to obtain a structure which fits the data very well. 

 The phase problem still exists, but known bond lengths and angles and 

 auxiliary information from infrared spectroscopy and other chemical 

 means can frequently decide uniquely among alternatives. 



Through various clever stratagems and much hard work, the structures 

 of several complex molecules of biological interest have been obtained 

 in recent years. One of these is sketched in Fig. 49. The number of 

 molecules whose structures have been determined is too small for any 

 generalizations about three-dimensional structures to have emerged. But 

 despite the complexities and the time needed for such x-ray diffraction 

 studies of molecular architecture, there is rapidly increasing interest in 

 these methods because at present the information can be obtained in no 

 other way. 



