CHEMISTRY OF NUCLEOSIDES AND NUCLEOTIDES 



169 



these are 2'- and 3'-phosphates, but not necessarily respectively (XXVIII, 



XXIX). 



P03H, 



+0- 



POjH. 



O OH 

 I I 

 ■-CH-CH-CH-CH-CHj-OH 



-CH-( 



-0- 



XXVIII 



Guanosine-2'-phosphate 



h. Cyiidylic Acids a and b 



OH 

 I I 

 ■-CH-CH-CH-CH-CH^-OH 



I 



H^Ni** 



XXIX 



Guanosine-3'-phosphate 



As the pyrimidine nucleotides are more stable than, purine nucleotides 

 towards acids, they may be prepared by hydrolysis of ribonucleic acid with 

 dilute sulfuric acid.'*^' ^^^ By fractional crystallization of the barium salts 

 of the resulting pyrimidine nucleotide mixture, materials known as cytidylic 

 acid and uridylic acid are obtained. ^^' '*^' ^** Cytidylic acid yields cytidine 

 on neutral hydrolysis.'** In view of the stability of the pyrimidine nucleo- 

 tides it is not possible to hydrolyze them to free pyrimidines and ribose 

 phosphates. However, cytidylic acid may be reduced catalytically to dihy- 

 drocytidylic acid which, like the corresponding nucleoside dihydrocytosine, 

 is readily hydrolyzed by dilute acids to the dihydropyrimidine."^ Whereas 

 the phosphate residue in cytidylic acid is rather resistant towards hydroly- 

 sis, that in dihydrocytidylic acid has the same order of stability as the 

 phosphate residue in yeast adenylic acid. 



Recently it has been shown that cytidylic acid from ribonucleic acid is a 

 mixture of a and h isomers. '^^''' '^'^ These may be separated by ion-exchange 

 chromatography or by careful fractionation of their salts, and, since they 

 are readily interconvertible in acid solution,'''" do not consume periodate, 

 and are both formed by alkaline hydrolysis of synthetic cytidine-2',3'- 

 hydrogen phosphate,'" they must be 2'- and 3'-phosphate (XXX, XXXI). 

 No decision has yet Vjeen made as to which is the 2'- and which is the 

 3'-phosphate on purely chemical grounds. However, careful measurements 

 of physical properties, e.g., spectra, dissociation constants, etc, suggest that 

 cytidytic acid a is the 2'-phosphate and cytidylic acid h the 3'-phos- 

 phate.'«9-"" 



185 p. A. Levene, Biochem. Z. 17, 120 (1909). 

 i8« P. A. Levene and W. A. Jacobs, Ber. 44, 1027 (1911). 

 »" P. A. Levene, Proc. Soc. Exptl. Biol. Med. 15, 21 (1917). 

 188 P. A. Levene, J. Biol. Chem. 41, 1 (1920). 



