PHYSICAL PROPERTIES OF NUCLEIC ACIDS 



449 



CI 



NH. 



CH3 NHj 



Fig. 1. Structure of (a) 2-amino-4-methyl-6-chloropyrimidine, (b) 2-amino-4,6- 

 dichloropyrimidine, (c) 4-amino-2,6-dichloropyrimidine, and (d) 5-bromo-4,6-di- 

 aminopjTimidine (Clews and Cochran''^). 



The position of the hydrogen atom in the — NH^ — CO — • groups, i.e., 

 whether the group exists in the keto or enol form, may be determined from 

 the C — bond length in 2-hydroxy-4,6-dimethylpyrimidine obtained by 

 Pitt,^ the structure of which is shown in Fig. 2. The length of the C^^^OH 

 bond is 1.25 ± 0.04 A. and is clearly different from the phenolic hydroxyl 

 group in resorcinol (C — ^OH distance 1.36-1.37 A.) and is to be compared 

 to that in oxalic acid dihydrate (1.24-1.30 A.). The bond between the 

 carbon and oxygen atoms thus possesses considerable double bond charac- 

 ter, but the hydrogen atom is nevertheless covalently bound to the oxygen 

 and takes part in a hydrogen bond between the oxygen atom and a water 

 molecule. In aqueous solution, the position of the hydrogen atom is not 

 necessarily the same as in the crystal, and evidence from the ultraviolet 

 absorption spectra of various pyrimidines shows that the keto form is 

 predominant in solution. At room temperatures, in neutral aqueous solution, 



^ G. J. Pitt, Ada Cryst. 1, 168 (1948). 



