124 AARON BENDICH 



Guanine and xanthine, but not adenine or hypoxanthine, react with the 

 Folin phenol reagent to give a blue color in an analytical procedure.^^^ 



e. Salt Formation 



The purines and pyrimidines form a variety of salts with many acids 

 and metal ions, a detailed hst of which has been compiled by Levene and 

 Bass.^^ Many investigators have, since 1858,^^ employed anmioniacal silver 

 nitrate to precipitate, isolate, and estimiate purines as sparingly soluble 

 silver salts.^^-*^^'*^^ Silver salts of the purines may also be formed quanti- 

 tatively in dilute sulfuric acid solution^^^ ■^'^^ and the purines are easily re- 

 generated upon treatment with hydrochloric acid. The pyrimidines of 

 nucleic acid origin do not precipitate as silver salts under acid conditions, 

 but do so in alkali.^' Poorly soluble cuprous-purine complexes are formed 

 when purines are boiled in the presence of cupric sulfate and sodium bi- 

 sulfite^^^ '^^^^ or cuprous oxide.^^^ The purines can be recovered following 

 reaction with hydrogen sulfide. The common purines and pyrimidines form 

 relatively insoluble complexes with mercury^^" when treated with mercuric 

 acetate solution at pH 6.2. Silver salts and chloromercuric derivatives of 

 the purines^*^ '^^^ condense with acetohalo-sugars to form purine nucleo- 

 sides.^^* -^^^ 



The picrates of many purines and pyrimidines are easily crystallizable, 

 well-defined salts and are valuable for purposes of isolation, characteriza- 

 tion, and quantitative estimation. Properties of certain picrates are given 

 in Table II. Picrate formation is the basis of a quantitative procedure for 

 adenine and guanine^^' ■^^^'^ The crystalline argentipicrate is used in a de- 

 termination of hypoxanthine.^"" The presence of a high -intensity band for 

 picric acid in the spectral region 350-400 mju and the absence of signficant 

 light absorption for many purines and pyrimidines in this region lend them- 

 selves to a simple micromethod for the determination of the molecular 



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