COLOR REACTIONS OF NUCLEIC ACID COMPONENTS 305 



with ether to remove the laevuhc acid produced from the sugar. The re- 

 action can be obtained with solutions containing only 10 ng. per cc. of 

 thymine. The involved procedure for the purification of the base requires 

 much higher quantities when thymine is determined in DNA or PNA. The 

 values for thymine obtained on preparations of nucleic acids are in the range 

 found by other authors with these methods. The reaction of Hunter is by 

 no means characteristic for thymine, but is given by carbonyl compounds 

 which are capable of enolization in Na2C03 solutions (such as acetone). 



3. Determination of Cytosine and Uracil 



The method of Soodak et a/.** is based on the reduction of arsenotungstic 

 acid (uric acid reagent) by brominated cytosine and uracil. Solutions con- 

 taining no more than 5 jug. per cc. of cytosine can still give a clearly visible 

 color in mixtures of the two pyrimidines; both of them can be determined 

 after quantitative removal of cytosine by a zeolite, Decalgo. Thymine and 

 purines do not interfere. 



^3 M. Soodak, A. Pircio, and L. R. Cerecedo, /. Biol. Chem. 181, 713 (1949). 



