218 



WALDO E. COHN 



ribose residues do not interfere (except in the case of adenosine, where ribose 

 replaces the only easily dissociable hydrogen). 



In acid solution, the amino groups of guanine, adenine, and cytosine are 

 cationic, with pK values increasing in that order (see Table I). Uracil and 

 thymine and their ribosides have no amino groups and are not sorbed to an 

 appreciable degree by cation exchangers in the hydrogen form. Hypoxan- 

 thine and xanthine and their ribosides, also not cationic, exhibit a small 

 degree of retention on cation exchangers." This retention may be ascribed 

 to a nonpolar attraction, seemingly general among the purines, giving them 



fi 



I U.^.J 



300 400 500 600 



VOLUME OF 2I1-HCI IN ml 



Fig. 2. Separation of purine and pyrimidine bases by cation exchange in an acid 

 system.^' 



Exchanger: Dowex-50-H+, ca. 300 mesh, 8.1 cm. X 0.74 cm. 2. 



Solution: 2 N HCl, 0.6 ml./min. 



Sorbed material: 0.5-1.0 mg. of each base in 7.5 ml. 2 A^ HCl. (Larger volumes of 



more dilute acid may be used.) 



a higher distribution coefficient than pyrimidines of equal pK values. Such 

 a family difference, noted in the nucleotides, ^^ may explain the greater re- 

 tention of adenosine over cytidine found on the carboxylic resin IRC-SO,^" 

 where ionic considerations alone would predict the reverse. 



2. C.\TioN Exchange 



In Fig. 2 is indicated the separation' of the four bases of ribonucleic acid 

 (such as could be derived by formic acid^ or perchloric acid hydrolysis^^) 



19 W. E. Cohn, /. Am. Chem. Soc. 72, 1471 (1950). 



2" W. Andersen, C. A. Dekker, and A. R. Todd, /. Chem. Soc. 1952, 2721. 



=*! W. E. Cohn, Science 109, 377 (1949). 



" E. Vischer and E. Chargaff, J. Biol. Chem. 176, 715 (1948). 



23 A. Marshak and H. J. Vogel, J. Biol. Chem. 189, 597 (1951). 



