MOLECULAR CONFIGURATION OF NUCLEIC ACIDS 21 



glycosidic links is the same. Attractive features of the base-pairing are 

 that the hydrogen bonds are reasonable lengths [ii] and almost linear, 

 and that the positions and directions of the glycosidic links in all four 



9 2A 



Fig. 7. The Hoogsteen base-pairing scheme. The glycosidic bonds have been 

 placed as far apart as seems reasonable. (I am indebted to Dr. M. Spencer for this 

 diagram.) 



nucleotides can be made exactly the same [lo]. This equivalence of 

 glycosidic links enables the arrangement of the deoxyribose and phosphate 

 atoms to be made identical in every nucleotide in the helical molecule. 



As alternatives to the Watson-Crick scheme, two schemes are worth 

 considering. The first is the Donohue pairing [12, 10, 13] shown in Fig. 6. 



