MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 1 7 



phases was the final model (Model 3) described in Langridge et al. [3, 4]. 

 The section on the right in Fig. 2 is along the helix axis of a DNA 

 molecule and shows the bases stacked on one another and confined to the 

 central part of the molecule. Parts of the deoxyribose sugar ring are shown 

 where the section passes through them. The section on the left is parallel 

 to the helix axis but removed from it and passes through various phosphate 

 groups. Some of the phosphate groups belong to one molecule and others 



Fig. 3. Typical Fourier synthesis in plane of a base-pair. The positions of 

 atoms in the model are shown. + marks the position of the helix axis. The base- 

 pairs and deoxyribose rings show clearly in the synthesis. 



to an adjacent molecule in the crystal. The resolution is insufficient to 

 show separately the oxygen atoms of the phosphate groups. The main 

 interest of this section is that in the region occupied by water and between 

 the DNA molecules, several peaks appear on the contour map. Possibly 

 several of these peaks correspond to water molecules and they do in fact 

 occur in stereochemically likely positions. One peak at A is higher than 

 the others and might be due to a chloride ion, for it is necessary that 

 chloride be present in the DNA for the crystalline structure to form. 



