136 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



(a) Make the filtrate alkaline with ammonia: precipitate 

 of xanthine-silver. For further treatment see chapter on 

 Muscular Tissue, page 25. 



(b) Suspend the precipitate of guanine-, adenine- and hypo- 

 xanthine-silver nitrate in water and decompose by passing in 

 hydrogen sulphide, filter from the silver sulphide, evaporate 

 to a small volume, make alkaline with ammonia, and let stand. 

 Guanine crystallizes out, while hypoxanthine and adenine 

 (besides ammonium nitrate) remain in solution. Filter, 

 wash a few times, and use the guanine for the reactions. 



Guanine, C 5 H 5 N 5 (amino-hypoxanthine, G 5 H 3 N 4 0(NH 2 ), 

 is insoluble in water, soluble in sodium and potassium hydrox- 

 ide solutions and also in acids forming salts. It is distin- 

 guished from the other xanthine bases by being almost insolu- 

 ble in ammonia. Of all the xanthine bases it gives the strong- 

 est so-called xanthine reaction and, like xanthine, gives a 

 reaction with hypochlorites. Like hypoxanthine and adenine 

 the compound with silver nitrate is insoluble in dilute nitric 

 acid. 



Reactions of Guanine. 



1. Try the xanthine reaction (see chapter on Muscular 

 Tissue, page 29). The residue left after evaporating the 

 nitric acid solution turns intensely dark red, even bluish red 

 on moistening with sodium hydroxide solution. 



2. Dissolve a small portion in hydrochloric acid and add 

 a little saturated aqueous solution of picric acid: a crys- 

 talline precipitate gradually forms. 



3. Mix in a watch-glass some sodium hydroxide solution 

 with a little chloride of lime and put into the mixture a grain 

 of guanine; around this there will be formed a dark-green 

 ring. This color soon turns to brown and then gradually 

 disappears. Xanthine gives the same reaction (originally 

 given by Hoppe-Seyler for xanthine). 



