I962 M. H. F.WILKIN S 



crystals. Techniques such as 3 -dimensional Fourier synthesis (see Fig. 7) can 

 be used and the structure determination made reasonably reliable. 



The A configuration 



In this conformation, the molecule has 11 nucleotide pairs per helix turn; the 

 helix pitch is 28 A. The relative positions and orientations of the base, and 

 of the dcoxyribose and phosphate parts of the nucleotides differ considerably 

 from those in the B form; in particular the base-pairs are tilted 20° from 

 perpendicular to the helix axis (Fig. 8). 



The A form of DNA (Fig. 1) was the first crystalline form to be ob- 

 served. Although it has not been observed in vivo, it is of special interest 

 because helical RNA adopts a very similar configuration. A full account of 

 A DNA will shortly be available. A good photograph of the A pattern is 

 shown in Fig. 9. 



The C configuration 



This form may be regarded as an artefact formed by partial drying. The helix 

 is non-integral, with about 9] nucleotide pairs per turn. The helices pack 

 together to form a semi-crystalline structure; there is no special relation 

 between the position of one nucleotide in a molecule and that in another. 

 The conformation of an individual nucleotide is very similar to that in the B 

 form. The differences between the B and C diffraction patterns are accounted 

 for by the different position of the nucleotides in the helix. Comparison of 

 the forms provides further confirmation of the correctness of the structures. 

 In a way, the problem is like trying to deduce the structure of a folding chair 

 by observing its shadow: if the conformation of the chair is altered slightly, 

 its structure becomes more evident. 



The Helical Structure of RNA Molecules 



In contrast to DNA, RNA gave poor diffraction patterns, in spite of much 

 effort by various workers including ourselves. There were many indications 

 that RNA contained helical regions, e.g. optical properties of RNA solutions 

 strongly suggested (e.g. Doty 21 ) that parts of RNA molecules resembled 

 DNA in that the bases were stacked on each other and the structure was 

 helical; and X-ray studies of synthetic polyribonucleotides suggested that 



s-49 



