CHEMISTRY OF NUCLEOSIDES AND NUCLEOTIDES 143 



The pyrimidine ribonucleosides have been related by the conversion of 

 cytidine into uridine with nitrous acid.^^ 



d. Position of the Glycosidic Linkage 



The ease with which purine nucleosides may be liydrolyzed in acid 

 strongly suggests that the linkage between purine and sugar does not in- 

 volve a C — C bond, but is much more in keeping with the known properties 

 of A^'-glycosides. In view of the nucleoside conversions outlined above, the 

 primary amino groups in adenosine and guanosine cannot be involved. It 

 follows, then, that the N atoms at positions 1, 3, 7, or 9 may be involved. 

 Positions 1 and 3 were eliminated by methylation of xanthosine to give a 

 theophylline riboside (XI V).-^ 



The early work established that the sugar was attached to one of the imi- 

 nazole N atoms in the purine nucleosides. The assumption that these were 

 7-glycosides was corrected later. The ultraviolet absorption spectra of 

 adenosine and inosine closely resemble those of 9-methyladenine (XV) and 

 9-methylhypoxanthine but differ markedly from those of the corresponding 

 7-methyl derivatives.^^ Similarly, guanosine-- and xanthosine-^ show ultra- 

 violet absorption spectra very similar to those of the corresponding 9-methyl 

 and not the 7-methyl derivatives. It was concluded that the natural purine 

 ribosides are 9- and not 7-glycosides. 



This technique has also been applied to the purine deoxyribosides. 

 Adenine deoxyriboside-^ and guanine deo.xyriboside'-- show spectra very 

 similar to those of adenosine and guanosine, and it was concluded that these 

 must be 9-gl3^cosides. 



Chemical evidence supporting the 9-glycosidic structure of the purine 

 ribosides has been forthcoming from the periodate oxidation studies dis- 

 cussed in the next section. 



Chemical methods have been used for determining the position of the 

 glycosidic linkage in the pyrimidine ribosides. Since cytidine may be de- 



22 J. M. Gulland and L. F. Story, /. Chem. Soc. 1938, 692. 



23 J. M. Gulland, E. R. Holiday, and T. F. Macrae, J. Chem. Soc. 1934, 1639. 

 " J. M. Gulland and L. F. Story, J. Chem. Soc. 1938, 259. 



