PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



451 



NH 



a b 



Fig. 3. Structure of (a) adenine, (b) guanine (Broomhead''''^). 



elusion is reached for uracil, which should therefore be represented as 

 2 , 4(6)-pyrimidinedione. 



b. Purines 



X-ray studies of adenine hydrochloride were first made by Bernal and 

 Crowfoot," but no attempt was made to determine the positions of the 

 atoms in the unit cell. These have, however, been determined by Broom- 

 head'^"^^ for both adenine (VII) and guanine (VIII) hydrochlorides. More 

 precise data on adenine hydrochloride have been obtained by Cochran." 

 The bond distances and interbond angles are given in Fig. 3. The structures 

 of adenine and guanine are very similar, the main differences being that 



NH, 



O 



VII 



VIII 



\ 



X 



CH 



the amino nitrogen atom (Nio) of guanine appears to be displaced by 0.11 A. 

 from the plane containing the other atoms and that there is a difference of 



" J. D. Bernal and D. Crowfoot, Nature 131, 911 (1934). 

 12 J. M. Broomhead, Acta Cryst. 1, 324 (1948). 

 »3 J. M. Broomhead, Acta Cryst. 4, 92 (1951). 

 " W. Cochran, Acta Cryst. 4, 81 (1951). 



