x THE PROTEIDS 431 



nucleic acid remainder and the No. 7 nitrogen of the purin-base. The 

 union of guanin in nucleic acid would therefore be represented by the 

 formula 



HN CO 



' I I /P= 



NH 9 - C C N; 



II II >CH. 



N C N' 



These results of Burian have been criticised by Steudel, 1 who 

 found that, if sodium hydrate was used, thymin gave colour-reactions 

 although no nitrogen is present in the No. 7 position ; but Burian 

 points out that the diazo-reaction is so very sensitive that Steudel 

 ought to have isolated and analysed the diazo-benzene sulphonic acid 

 derivative of thymin to make sure that the thymin did not contain 

 traces of guanin ; and, even if thymin does give a colour reaction, 

 then the diazo-remainder may perhaps link on to the No. 4 carbon, 

 giving rise to an azo-dye. 



The essential points in favour of Burian's 2 view are : 



(1) Uric acid does not give the colour reaction, although it only 



differs from xanthin, which does give the reaction, in the 

 structure of its imido-azol ring. 



(2) Theophyllin gives the * diazo-reaction,' while theobromine and 



caffeine do not. 



CH 3 . N CO HN CO OH. . N CO 



II II II 



OC C NH OC C N.CH 3 OC C N.CH 3 



\ 



CH 



\ 



CH 



\ 



CH 



CH 3 . N C N CH . 3 N C N CH 3 . N C N 



Theophyllin. Theobromine. Caffeine. 



Therefore substitution in the pyrimidin ring does not matter, while 

 substitution of the imide-hyrogen-atom in No. 7 prevents the reaction. 



The liberation of Purin-compounds by the Autodigestion oj 

 Nucleo-proteids. 



The autodigestion of nucleo-proteids has been investigated by 

 many observers, and the historical account of it given here is based on 

 the recent article by Walter Jones. 3 



1 H. Steudel, Zeit. f. physiol. Ghem. 42. 165 (1904). 

 2 R. Burian, ibid. 42. 297 (3904). 3 Walter Jones, ibid. 42. 35 (1904). 



