DECOMPOSITION-PRODUCTS OF THE NUCLEIC ACIDS 175 



It does not occur among the decomposition-products of the nucleic 

 acids. Its derivatives Uracil, Cytosine and Thymine have the following 

 formulae : 



HN CO N=C.NH 2 HN CO 



| I 



OC CH OC CH OC C.CH 3 



I II 



HN CH HN CH HN CH 



Uracil. Cystosine. Thymine. 



Uracil is therefore dioxypyrimidine, cytosine is amino-oxypyrimidine 

 and thymine is methyluracil. Cytosine is transformed into uracil by 

 the action of nitrous acid. 



Each of these bases has been prepared synthetically; they are 

 known to occur in Nature, however, only as decomposition-products 

 derived from nucleic acids by hydrolysis. They are sparingly soluble 

 in cold water, more soluble in hot water. Cytosine dissolves in alcohol, 

 uracil with difficulty, and thymine not at all. Cytosine and thymine 

 are precipitated by phosphotungstic acid. Uracil is not. On heating, 

 thymine sublimes without decomposition, uracil partly decomposes and 

 partly sublimes, while cytosine undergoes decomposition. 



Cytosine and uracil give the Weidel Reaction as follows: To a 

 small quantity of solution chlorine water is added, and the mixture 

 boiled. The solution is evaporated to dryness and then exposed while 

 warm to the vapors of ammonia. A purple-red color develops. This 

 reaction is frequently referred to as the Murexide Reaction because it 

 is due to the formation of Ammonium Purpurate which is believed to 

 be identical with the scarlet dye found in the mollusc murex which 

 furnished the "purple" of the ancient Romans. An intermediate stage 

 in the reaction is the formation of Alloxan. : 



HN CO 



oc co 



HN CO 



Alloxan. 



and the test is only given by such substances as can be made to yield 

 alloxan by oxidation. The reaction is therefore not infrequently 

 alluded to as the Alloxan Reaction. Nitric acid may be used in the 

 place of chlorine as the oxidizing agent. 



Cytosine and uracil also give Wheeler and Johnson's Reaction: To 

 the solution of the substance bromine water is added drop by drop 

 until a permanent cloudiness appears. Baryta water is then added, 



