98 IV. BILE PIGMENTS 



This experiment gave Fischer the key to the correct interpretation 

 of the structure of bile pigments (826,255 Jf.). So far bihrubinic acid 

 (Fig. 4) had been the only dipyrrolic product obtained from bilirubin 



M E M P 



HO 



H H2 H 



CH, 



Fig. 4. Bilirubinic acid. 



by mild reduction (872,873,2153). Now a pyrromethene was obtained 

 which had an unsubstituted a-position instead of the methyl group. 



M E 



P M M E 



neoxanthobilirubinic acid 



+ 



E M M P 



HO 



A'^c\ 



XH3 



H H 



isoxanthobilirubinic acid 



H H 



OH 



HO 



xanthobilirubinic acid 



+ 



E M M P 



HO 



WW 



N 



„ N ^H 

 H H 



isoneoxanthobilirubinic acid 

 Fig. 5. Decomposition of mesobilirubin in the resorcinol melt. 



This showed that the breakdown of mesobilirubin in the resorcinol 

 melt occurred on either side of a methane bridge combining two 

 pyrromethenes (Fig. 5) and thus led to the ultimate proof of the 



