THE NUCLEOPROTEIDS. 283 



acid molecule, are very easily split off. Purine bases can be detected even 

 when the material is merely dissolved in water at 60 degrees. Boiling 

 with water for ten minutes is sufficient to separate practically all the 

 purine bases. We are justified in looking upon the purines as being 

 primary cleavage-products of the nucleic acid molecule, rather than 

 resulting from secondary causes. We may consider them as being com- 

 posed of a condensed nucleus, consisting of a pyrimidine and an imidazole 

 ring, as may be seen by the following structural formulae: 



(1) N=(6)CH (1) N-(6)CH 



(2) HC (5) C (7) NH (2) HC (5) CH (a)HC NH(rc) 



(3) N (4) C (9) N (3) N (4) CH (/?) HC N 



Purine Pyrimidine Imidazole 



Corresponding to this assumption, the purines show reactions, which are 

 characteristic of one or the other component. Imidazole possesses the 

 property of reacting with diazo-benzene chloride, forming a red, crystalline 

 product, which is (n) diazo-benzene-imidazole. This reaction also applies 

 to those imidazoles whose a, /?, or p. positions have been substituted; but 

 not to those in which the n-position is substituted. The purines act in 

 a very analogous manner. Substitution in the pyrimidine ring is without 

 influence on the appearance of this reaction. On the other hand, the re- 

 action is negative when the imide hydrogen of the imidazole ring, in position 

 7, is replaced. The reaction is positive in xanthine, hypoxanthine, guanine, 

 adenine, theophylline ; it fails with theobromine (3, 7-di-methyl-xanthine) 

 and with caffeine (1, 3, 7-tri-methyl-xanthine) . The nucleic acids, however, 

 even if liberally endowed with bases, do not react with diazo-benzene- 

 sulphonic acid. This last reaction only takes place when purines are being 

 split off at the same time. Taking the above facts into consideration, 

 we arrive at the conclusion that the purine bases are united with the nucleic 

 acid residue through its nitrogen atom in the seventh position. Another 

 series of observations indicate that the purine bases are linked in the first 

 place to the phosphoric acid portion. It is a very striking fact that those 

 purine bases, which are so easily split off by boiling water, are only released 

 with great difficulty by boiling with sodium hydroxide. Certain organic 

 phosphoric acid amides behave similarly. Burian believes that guanine is 

 linked in the nucleic acid molecule in the following way: 



NH 



HN CO P = 

 . C C N 



