96 



IV. BILE PIGMENTS 



cases is not quite correct; see p. 102). Bile pigments thus differ from 

 porphyrins in having an open chain tetrapyrroHc system instead of 

 the closed porphyrin ring. Related to this difference in structure 

 are fundamental differences in chemical behavior. The porphyrins 

 have a remarkably stable resonance structure. For this reason we 

 had to deal only with the "aromatic" porphyrins and the unstable 

 fully hydrogenated porphyrinogens. (A dihydroporphin ring plays 

 a role in chlorophyll chemistry, but it is also rather unstable and 

 easily transformed into the porphin ring.) In the open chain tetra- 

 pyrrolic pigments, however, a number of dehydrogenation stages 

 exist between the aromatic biliverdins (I), and the fully hydrogenated 

 leuco compounds (II), e.g. mesobilirubinogen (Fig. 2). Each differs 



HO 



A^^kAc^m^c^n^ 



H H 



C 

 H 



(I) 



H 



OH 



HONC]sjCnC 

 H H. h H. h H. 



(ID 



A. 

 N ^OH 

 H 



Fig. 2. Two hydrogenation stages of bile pigments. 



from the other in color, type of absorption spectrum, and properties 

 (cf. Table II, page 106). This explains the bewildering variety of 

 the bile pigments, their greater instability, and the fact that they 

 show all the colors of the rainbow. 



This greater complexity of bile pigment chemistry is compensated 

 for by a smaller variation in number and order of side chains of 

 naturally occurring bile pigments. So far only compounds with side 

 chains of proto (4 M, 2 V, 2 P) and meso type (4 M, 2 E, 2 P) (cf. 

 Chapter III) have been found in nature, and of these only type IX 

 (Fig. 3). 



This can be explained by the discovery of Lemberg (1681, 1682) 

 that bile pigments in animals are not formed from porphyrin but 

 from hematin compounds (Chapter X), and by the fact that proto- 

 porphyrin alone is found to occur in higher animals as iron complex 



