430 CHEMISTRY OF THE FROTEIDS CHAP. 



tuted radical, e.g. in n-methylimido-azol ; or (2) if the amidin link 

 has disappeared by hydration ; or (3) if it has been converted into 

 urea. 



Quite an analogous behaviour is met with in the purin group. 

 Purin-bodies, in which the imide-hydrogen in No. 7 has not been 

 substituted, and in which the amidin-link has been preserved un- 

 changed (xanthin, hypoxanthin, guanin, adenin), yield with diazo- 

 bodies intensely coloured compounds, which closely resemble the 

 diazo-amino-compounds of the imido-azols. Substitution in the 

 pyrimidin-ring does not hinder the reaction, for it is given by 

 theophyllin (1*3- dimethylxanthin), but the reaction is prevented if 

 the No. 7 imid-hydrogen is replaced by methyl, as it is, for example, in 

 theobromine (3 '7- dimethylxanthin), or in caffeine (1 3'7- trimethyl- 

 xanthin) ; or if, as in uric acid, the imido-azol ring does not possess 

 the structure of a cyclic amidin, but that of a cyclic urea. 



It follows that the diazo-compound remainder must link on to the 

 purin-nucleus in No. 7, and therefore purin-bases + diazo-compounds 

 = diazo-amino-compounds, which are quite analogous to the imido- 

 thus : 



CH 3 . N CO 



. C C N N : N . C 6 H 4 S0 3 H 



| \ rH 

 S 



CH 3 . N C N 



The diazo-reaction may therefore be used for determining whether 

 No. 7 in the purin radical is substituted or not ; if it is not substituted 

 a red colour is obtained at once on dissolving the purin-derivative in 

 alkali, cooling it, and then adding a diazo-substance or diazotising 

 mixture. 



Burian l also points out that purin-bases must be preformed in 

 the nucleic acid molecule, as they are very readily separated from 

 the nucleic acid remainder. They are liberated partially by heating 

 nucleic acid to 60, and completely by boiling the same for ten 

 minutes in water or by dilute acids ; this fact, along with the observa- 

 tion that nucleic acids do not give the diazo-reaction described above, 

 led Burian to assume that the purin-bases are linked to the remainder 

 of the nucleic acid molecule by the No. 7 nitrogen. 



As nucleic acids are further very resistant to caustic potash, and 

 in this they resemble other organic phosphoric-acid amides, there exists 

 probably in nucleic acids a direct union between the phosphorus of the 

 1 R. Burian, Ber. d. deutsch. chem. Ges. 37. 708 (1904). 



