102 



IV. BILE PIGMENTS 



Since they are formed, however, by opening the porphyrin ring of 

 heraatin {cf. Chapter X) as pictured in Figure 7 (IX — > IXa), the 

 correct formulas are cycUc rather than linear. There is, of course. 



OH 



OH 



Fig. 7. Isomeric bile pigments that might be derived from protoporphyrin, IX. 



free rotation around the single bonds between the combining carbon 

 atoms and the pyrrole rings, allowing the molecule to assume a 

 variety of shapes in space. Thus bilirubin forms extremely unstable 

 films of weak solid or viscous liquid type, occupying up to 140 A^ in 

 contradistinction to protoporphyrin and hematin {38). Only the 

 linear formulas of the leuco compounds with three methylene groups 

 between the four pyrroles are, however, strictly identical with their 



