i6 



M. H. F. WILKINS 



Parts of the structure not included in the model appear roughly in 

 the picture, i.e. approximately half-height on the electron density 

 contour map. For example the positions of water molecules in 

 crystals of vitamin 6^2 have been determined ([7] and private 

 communication) when phases were calculated from the vitamin 

 molecule alone. 



Fig. 2. (Right). Fourier synthesis of section through DNA molecule. The 

 helix axis is vertical, approximately in the plane of the section, and on the right- 

 hand side of the diagram. (Left). Section through phosphate groups of two neigh- 

 bouring DNA molecules. A marks a peak that may correspond to a chloride ion. 

 Two smaller peaks lie to the right of A and probably correspond to water mole- 

 cules. The A' marks show positions of atoms in the molecular model. 



Fourier syntheses for DNA 



Fourier syntheses of sections through the structure of the lithium 

 salt of DNA are shown in Fig. 2. The model used for calculating the 



