108 IV. BILE PIGMENTS 



change from one to the other is diminished. The number of con- 

 jugated bonds is diminished far more by the reduction of the central 

 methene group b of bilatrienes than by rechiction of the groups a or c. 

 Hence bihidienes-(a,c) are orange (rubins) with the main absorption 

 band in the bhie, while biladienes-(a,b) are violet (violins) with main 

 absorption in the yellow and green part of the spectrum. This holds 

 in spite of the fact that many of the pigments have indicator proper- 

 ties and a different absorption spectrum in acid and alkaline solution. 

 The complicated band spectra of the porphyrins are not seen in bile 

 pigments and the bands are usually less sharp, although the complex 

 salts of biliviolinoid pigments have a somewhat more complicated 

 and sharp absorption spectrum. 



The absorption bands of the various classes differ in height less 

 than in position, since hydrogenation has less effect on the total 

 number of double bonds than on their conjugation. Thus bilirubin 

 has two systems of five conjugated double bonds while biliverdin has 

 ten conjugated double bonds and one crossed double bond. Actually, 

 the double bond formulas can only give a rough picture of the reso- 

 nance of the molecules. 



Column 8 of Table II gives simpler pyrrolic compounds which 

 resemble the bile pigments of that particular class in their color, and 

 in the color, absorption, and fluorescence of their zinc complex salts 

 (column 9). Where the conjugation of double linkages is interrupted 

 by a methylene group, the group, isolated by this is no longer in 

 resonance with the remaining system; a resonating system without 

 such groups should therefore have similar spectroscopic properties. 

 Hence in mesobilene-(b) the two flanking pyrroles have little influ- 

 ence. It resembles Q!,Q!'-dimethylpyrromethenes and forms similar 

 green-fluorescing zinc complex salts. 



In bilirubin two a-hydroxypyrromethene groups are linked by 

 the methylene group and, like the ct-hydroxypyrromethenes, biliru- 

 bins do not form fluorescent zinc salts. This is probably due to the 

 fact that the hydroxylated pyrrolene ring in bilirubin and a-hydroxy- 

 pyrromethenes occurs in the tautomeric form: 



N C 

 H 

 rather than as: 



HO^N^C 



