CHEMISTRY OF NUCLEOSIDES AND NUCLEOTIDES 161 



tides known as yeast adenylic acid, guanylic, iiridylic, and cytidylic 

 acids.'--- '-^ Dilute alkali effects a nearly quantitative conversion to the nu- 

 cleotide mixture.'-^" Improved methods have been introduced from time to 

 time'^^ and cautious acid hydrolysis is convenient when only the more sta- 

 ble pyrimidine nucleotides are required.'-^-'" However, recent investigations 

 on these nucleotides using refined methods of ion-exchange and paper chro- 

 matography as well as selective fractional crystallization procedures, show 

 that they are not homogeneous and consequently the older isolation tech- 

 niques are no longer used. [Cf. Chapters 5-7, 11, 12.] By suitable combina- 

 tion of ion-exchange and paper chromatography it can be shown that the 

 nucleotides produced by hydrolysis of ribonucleic acid are mixtures of iso- 

 mers. Thus, yeast adenylic acid is a mixture of the two isomers adenylic 

 acid a and adenylic acid h. Similarly, guanylic, cytidylic, and uridylic acids 

 are all mixtures of a and b isomers.'-*"'^- Certain of the isomers may be 

 separated from each other by fractional crystallization of cyclohexylamine 

 and brucine salts. '^^- '^^ It should be explained that the terms a and 6 were 

 originally assigned arbitrarily and did not necessarily signify any structural 

 correlation between the different members. As will be seen later, it is now 

 known that cytidylic acid a is structurally related to uridylic acid a, sim- 

 ilarly cytidylic acid h is related to uridylic acid h. The isomerism in all 

 these nucleotides involves the position of the phosphate residue on C-2', 

 or C-3', in the respective nucleosides. 



Nucleoside-5'-phosphates may also be obtained from ribonucleic acid. 

 For reasons discussed in Chapter 12, chemical hydrolysis cannot give these 

 nucleotides. However, ribonuclease hydrolysis under conditions which do 

 not permit 5'-phosphatases to operate, followed by ion-exchange separation 

 of the products, yields 5'-phosphates.'^^ [Cf. Cohn, Chapter 6; Schmidt, 

 Chapter 15.] 



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