128 IV. BILE PIGMENTS 



procedure was accelerated. In contradistinction to Watson (2989, p. 2488) 

 however, we have not observed mesobiHvioHn formation from tetrahydro- 

 mesobilane (cf. below). Hoesch {1301) had already observed that some 

 urines form biliviolin when treated with ferric chloride, while others do not. 

 The biliviolinoid pigments observed in dog bile {199), in gallstones {833) 

 and in gastric juice (1467) are almost certainly secondary oxidation products 

 of bilirubin. 



5.3. Mesobilierythrin (Lemberg), Mesobilirhodin (Siedel), 

 and Phycoerythrobilin 



This type of red bile pigment was first obtained by Lemberg {1690) 

 as the prosthetic group of phycoerythrin {cf. Section 7) and called 

 'phycoerythrobilin. It could be obtained only in the form of its 

 chloroform-soluble methyl ester, which probably still contained a 

 small peptide chain. A spectroscopically identical compound was 

 shown to occur in the "mesobili violin" mixture obtained from meso- 

 bilane or mesobilene-(b) by ferric chloride oxidation, and named 

 mesobilierythrin {1690) . * 



Treatment with ferric chloride in hydrochloric acid transformed , it into 

 mesobiliviolin and from this it was concluded that mesobilierythrin repre- 

 sented an oxidation stage between mesobilene-(b) and mesobiliviolin. The 

 fact that on heating with alkali in methanol phycoerythrin yielded a mix- 

 ture of mesobiliverdin and mesobilirubin, while phycocyanin gave only the 

 former {1690), supported the assumption that phycocyanobilin (mesobili- 

 violin) was an oxidation product of phycoerythrobilin. If mesobiliviolin is a 

 biladiene, this cannot be correct, since there is no other stage of dehydro- 

 genation available between bilene and biladiene. Mesobilierythrin must then 

 be an unstable isomeride of mesobiladiene, which on heating with acid alone 

 is isomerized. The faster change of color to blue on heating in acid in the 

 presence of air may be due to further oxidation of the mesobiliviolin to 

 mesobiliverdin. 



Siedel separated the compound from the "mesobiliviolin" mixture by 

 chromatography and called it "mesobilirhodin" (2550). He also obtained it 

 by synthesis from formylneobilirubinic acid and isoneoxanthobilirubinic acid 

 (Fig. 16). According to this, mesobilierythrin is an isomeride of the meso- 

 biliviolin (Fig. 17), differing from it only in the position of one double bond 

 (5'b in the violin, -I'b in the erythrin). One may ask whether the erythrin 

 formula of Siedel might not just represent a resonating form of this violin, 

 if one assumes hydrogen linkage between the nitrogen atoms {cf. Fig. 18). 

 We have seen in Chapter III, Section G.3., that no reliable evidence for 



* Siedel has given as the reason for his altering the name erythrin to rhodin that 

 the former was used for tlie' prosthetic group of phycoerythrin^ This is not so; it 

 had, in fact, heen given to the product ohtained by oxidation of mesobilane which 

 forms a component of Fischer's "mesobiliviolin" mixture. 



