84 AARON BENDICH 



The Ring Index^'' for purines is also used in this text (VII), and, for con- 

 sistency and to indicate more clearly the interrelationships between purines 

 and pyrimidines, the style (VIII) for pyrimidines is used here. The hex- 



IN r 



3 



VII VIII 



Purine Pyrimidine 



agonal representations (VII and VIII) are preferred since they conform 

 closest to those revealed by X-ray studies.^* 



The actual structures of pyrimidines and purines depend upon many 

 conditions (see below) and, even when the effect of a condition (such as 

 pH) is understood, it is not always possible to write a single structure for 

 a particular compound. For the sake of brevity it is necessary, therefore, 

 to be arbitrary in representing the structures. Except where otherwise 

 noted, amino, hydroxy, mercapto, or other derivatives of these hetero- 

 cycles will be written as if they had been formed by a replacement of a 

 hydrogen atom, leaving the K^kul^-type ring intact. Thus uric acid is 

 referred to as 2 , 6 , 8-trihydroxypurine and 5-methylcytosine as 2-hydroxy- 

 5-methyl-6-aminopyrimidine, without regard to the obvious operation of 

 lactim-lactam or amino-imino tautomerism. It is to be understood that 

 such representations do not necessarily reflect an actual state of a molecule. 



3. Occurrence and Distribution 



a. Purines and Pyrimidines of Nucleic Acid Origin 



Thus far, only two purines are recognized as universal and normal con- 

 stituents of nucleic acids (PNA and DNA). Adenine^^ was discovered and 

 isolated from an acid hydrolysate of "nuclein" of beef pancreas in 1885 by 

 Kossel.2® '^^ He succeeded^^ in converting adenine into hypoxanthine (6-hy- 

 droxypurine) upon reaction with nitrous acid. Although Medicus^ proposed 



24 D. O. Jordan, Progress in Biophysics 2, 51 (1951) ; Ann. Rev. Biochem. 21, 209 (1952). 

 " Beilstein's Handbuch der organischen Chemie.," 4th ed., 26, 420 (1937). (Herein- 

 after referred to as "Beilstein.") 

 2« A. Kossel, Ber. 18, 79 (1885). 

 " A. Kossel, Z. physiol. Chem. 10, 248 (1886). 

 28 A. Kossel, Ber. 18, 1928 (1885). 



