RELATION OF COLOR AND LIGHT ABSORPTION TO STRUCTURE 107 



7n5-dihydrobilin-4,5-dipropionic acid (or-bilidiene-(2'a, ll'y)- instead 

 of (-77W-dihydrobilin-). 



Simplified nomenclature. For our purpose, a greatly simplified 

 nomenclature is sufficient, which in principle follows Siedel's second 

 suggestion and uses, where necessary, the numbering indicated above 

 in B, but with a, b, and c instead of a, 0, and 7, and without ms. 

 This obviates any confusion with the numbering of methene groups 

 (a, /3, 7, 5) in protoporphyrin. The term bilane is used for the fully 

 hydrogenated tetrapyrrane including the two hydroxyl groups in 1' 

 and 8'. 



Bilirubin thus becomes simply biladiene-(a,c) and biliverdin, bila- 

 triene. This sj'stematic nomenclature should not exclude the use of 

 the older terms, such as bilirubin or biliverdin, where their meaning 

 is sufficiently precise. 



Table II shows that the better known naturally occurring bile 

 pigments can be readily brought into this systematic nomenclature 

 (column 1). 



In some cases the chemical structure has not been elucidated 

 completely. Reference to Table III will show that other bile pigments 

 of different, though related, structure may have spectroscopic proper- 

 ties so similar to those of some described in Table II that a differen- 

 tiation with small amounts of material obtained from biological 

 sources may be impossible. In such cases, terms such as "urobili- 

 noid," "biliviolinoid," etc. must still be used, but it should be clear 

 that they imply a certain type of bile pigment of similar color and 

 spectroscopic properties rather than a distinct chemical entity.- The 

 position of the double bonds, which is usually not required for the 

 nomenclature, is given in column 2 making use of the numbering of 

 Figure 9B; the customary type names are given in column 3, and 

 examples of bile pigments belonging to this class in column 4. 



2.3. Relation of Color and Light Absorption to Structure 



Table II gives the classes of bile })igments in order of increasing 

 hydrogenation. It can be seen that, with decreasing number of 

 conjugated double bonds (column 7, fornndas in column 5), the color 

 of the pigipents changes from green through violet, red, orange, and 

 yellow to colorless, the absorption bands being shifted toward shorter 

 wavelengths. This follows the general rule found for polyenes by 

 Hausser and collaborators (1184), which is probably explained by 

 the creation of a large number of resonance states with increased 

 number of conjugated bonds, so that the energy required for the 



