110 



IV. BILE PIGMENTS 



shade, disregarding the possibility of mixtures of compounds. Later, appar- 

 ently pure colors turned out to be due to mixtures, e.g., the "green" stage 

 to mixtures of blue-green bilatriene with unaltered bilirubin and the blue 

 "cyanin" stage to mixtures of bilatriene with violet pigments {1680,2552). 

 The hydroxyl groups of bilirubin are not necessary for the reaction as was 

 shown by the fact that synthetic tetrapyrrodienes without these groups also 

 give the Gmelin reaction {SJfS). 



The explanation of the GmeUn reaction, at least in principle, 

 became clear only after the first change to blue-green had been proved 

 to be a dehydrogenation of bilirubin to a bilatriene (verdin) (Lemberg, 

 1680) and after Siedel {2552,2553,2555,2556) had found the explana- 

 tion for the fact that oxidation beyond this stage leads to a series of 

 compounds which resemble substances containing more hydrogen 

 than bilatrienes. If a bilatriene is oxidized, some of the pyrrole- 

 linking methene groups through which the conjugation of double 

 bonds runs are oxidized to carbonyl groups or disappear by addition 

 of nitrous acid or hydrogen peroxide (or in the presence of methyl 

 alcohol by two methoxyl groups), as shown in Figure 10. The 



/ N C N 



M = methyl 



^ N 



N^OH 

 H 



A.Ac'^\A 



N 



N 



/\no 



H OH 



OH 



^ N *-' N 

 /\0H 

 H OH 



OH 



A.Ac/\A 



N 



N 



/\0M 

 H OM 



OH 



Fig. 10. Gmelin reaction. 



