MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 



The X-ray Diffraction Patterns of DNA and the Various Configurations of 



the Molecule 



X-ray diffraction analysis is the only technique that can give very detailed 

 information about the configuration of the DNA molecule. Optical tech- 

 niques, though valuable as being complementary to X-ray analysis, provide 

 much more limited information - mainly about orientation of bonds and 

 groups. X-ray data contributed to the deriving of the structure of DNA 

 at two stages. First, in providing information that helped in building the 

 Watson-Crick model ; and second, in showing that the Watson-Crick pro- 

 posal was correct in its essentials, which involved readjusting and refining 

 the model. 



The X-ray studies (e.g. Langridge ct al. 19 , Wilkins 20 ) show that DNA mol- 

 ecules are remarkable in that they adopt a large number of different con- 

 formations, most of which can exist in several crystal forms. The main 

 factors determining the molecular conformation and crystal form are the 

 water and salt contents of the fibre and the cation used to neutralize the 

 phosphate groups (see Table i). 



I shall describe briefly the three main configurations of DNA. In all cases 

 the diffraction data are satisfactorily accounted for in terms of the same basic 

 Watson-Crick structure. This is a much more convincing demonstration of 

 the correctness of the structure than if one configuration alone were studied. 

 The basic procedure is to adjust the molecular model until the calculated 

 intensities of diffraction from the model correspond to those observed (Lang- 

 ridge ct al. 19 ). 



As with most X-ray data, only the intensities, and not the phases, of the 

 diffracted beams from DNA arc available. Therefore the structure cannot 

 be derived directly. If the resolution of X-ray data is sufficient to separate 

 most of the atoms in a structure, the structure may be derived with no 

 stereochemical assumption except that the structure is assumed to consist of 

 atoms of known average size. With DNA, however, most of the atoms 

 cannot be separately located by the X-rays alone (see Fig. 7). Therefore, 

 more extensive stereochemical assumptions are made: these take the form 

 of molecular model-building. There arc no alternatives to most of these 

 assumptions but where there might be an alternative, e.g. in the arrangement 

 of hydrogen bonds in a base-pair, the X-ray data should be used to establish 

 the correctness of the assumption. In other words, it is necessary to establish 

 that the structure proposed is unique. Most of our work in recent years has 



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