BILADIENES-(a,b) AND RELATED SUBSTANCES 127 



solvents and of its zinc complex salt are summarized. The absorption 

 curve of the zinc complex salt is very similar to that of a mesobili- 

 purpurin given in Figure 21. 



The zinc complex salt possesses an exceptionally strong red fluores- 

 cence. The copper complex has a similar absorption spectrum and 

 color, but does not fluoresce. A fluorescent mercury complex has 

 been described by Dhere and Roche {58^,585), but the preparation 

 which they studied was the "mesobiliviolin" mixture of Fischer. 



It is easy to differentiate the red fluorescence of mesobiliviolin-zinc 

 from that of porphyrins. Mineral acids decompose the zinc complex 

 and destroy its fluorescence, while porphyrins fluoresce in acid 

 solution. 



3.2.3. Occurrence of Mesobiliviolin in Nature. Certain red 

 and blue algae (c/. Section 7) contain a blue chromoprotein, phyco- 

 cyanin, which may be obtained crystalline. Lemberg (1673) first 

 isolated a bile pigment from phycocyanin by heating with concen- 

 trated hydrochloric acid at 80°C. in the absence of air. He named 

 the pigment "phycocyanobilin." By splitting the protein in boiling 

 10% methyl alcoholic alkali, mesobiliverdin was obtained. This 

 fact, together with the other properties of the bile pigment, led Lem- 

 berg to identify it with mesobiliviolin (1690). Analyses gave the 

 empirical formula C34H44O8N4 for phycocyanobilin, while for meso- 

 biliviolin a formula C33H40O6N4 would be expected, but Lemberg 

 emphasized at that time that noncrystalline bile pigments usually 

 give too high oxygen values. 



Siedel (2550) and Fischer (S61, p. 733) suggested that phycocyanobilin 

 may be a proto, not a meso, compound. Under the conditions of the forma- 

 tion of mesobiliverdin from phycocyanin, however, biliverdin is not trans- 

 formed into mesobiliverdin. Later (2553) Siedel confirmed the identity of 

 phycocyanobilin with mesobiliviolin. A purpurin structure is ruled out by 

 the fact that ferric chloride oxidizes phj^cocyanobilin into mesobiliverdin 

 (diene — > triene) (1G8S). 



Occurrence in feces. The "copromesobiliviolin," later more suitably called 

 mesobiliviolin, isolated by Watson from normal human feces (2971-2973), 

 is probably the same substance. It can be occasionally observed as by- 

 product of fecal porphyrin (1088,2837). Fischer and Halbach (822) did not 

 find it in feces, whereas Baar and Hickmans (107) occasionally found it in 

 large amounts. In view of the ease with which mesobilane is oxidized to 

 mesobiliviolin (cf. below), the latter is probably not preformed in feces but 

 arises by oxidation of mesobilane. Thus Watson (2905) observed meso- 

 biliviolin formation if mesobilane in hydrochloric acid solution was exposed 

 to air and found that the vield from feces became small when the extraction 



