BILADIENES-(a,b) AND REL,\TED SUBSTANCES 131 



<;ver, that two atoms of iodine were required for the oxidation of l)inverdin- 

 zinc to biHpurpurin-zinc. He assumed for (what we now call) bilipurpurins 

 a structure of dehydrobilatrienes. while Fischer and Reinecke (870) formu- 

 lated similar substances obtained by oxidation of biliverdins with quinone 

 in methanol as dimethoxybilatrienes with methoxyl on the carbon atoms 

 between pyrrole nuclei. The correct mechanism of the reaction has been 

 demonstrated by Siedel (cf. Section ^2. 4.) but the structure of the various types 

 of compounds in this class cannot yet be considered finally established. 

 Later work of Siedel and co-workers and of Lemberg and Lockwood then 

 revealed how complicated these reactions are. These investigations, carried 

 out during the war in Germany and Australia, have still to be correlated. 



Siedel and co-workers (2553, 2555, 2556) studied the oxidation of symmetri- 

 cally substituted raesobiliverdin XII under various conditions, and isolated 

 a variety of purpurins by chromatography, some in crystalline state. In some 

 instances the substances were pure enough for analysis, in others they were 

 characterized only by the position of the absorption maximum of their zinc 

 complex, which is not a satisfactory criterion {cf. footnote to Table VI). 



Lemberg and Lockwood {1711, cf. also 1681,1712,2666) showed that the 

 mesobilipurpurin and bilipurpurin produced by the oxidation in methanol 

 with two atoms of iodine per mole of the corresponding biliverdin zinc com- 

 plexes (bilipurpurins type I, then called biliviolins type II, cf. Section "2.4.) 

 were unstable. In the presence of air they underwent autoxidation to weakly 

 basic bilipurpurins type II (then called biliviolins type III), while under 

 nitrogen they isomerized to yellow compounds (bilichrysins).* The latter 

 could be obtained crystalline and the analyses showed that their tetrapyr- 

 rolic system contained three oxygen atoms in addition to the four oxygens 

 present in the two carboxylic acid groups. The scheme of Figure '■20 may 

 be tentatively suggested for these reactions. 



5.4.2. Properties. In view of the confusing variety of similar compounds 

 and the fact that they have so far not yet been found in nature, we restrict 

 ourselves to describing only some of their main characteristics. 



The absorption spectra of purpurin solutions in aqueous hydrochloric 

 acid and those of the zinc complex salts in methanol are practically indis- 

 tinguishable from those of violins {cf. Table VI). The absorption curve of 

 mesobilipurpurin I-zinc (formed from mesobiliverdin-zinc with iodine in 

 methanol) is given in Figure "21. The fluorescence spectrum of this compound 

 has been studied by Dhere {570); it shows only one emission band lying 

 toward the infrared of the first absorption maximum. Both as zinc complex 

 and as hydrochloride mesobilipurpurin I shows only weak absorption bands 

 in the ultraviolet {1324). Bilipurpurins are able to form hemochromes 

 {1681). The absorption maxima of the protobilipurpurin-zinc complexes 



* Siedel and Frowis (25-55) obtained apparently similar substances by reduction 

 of bilipurpurins with zinc dust in acetic acid. In our experiments reduction was 



excluded. The compounds of Siedel and Frowis probaljly contain the ^ 



c 



grouping instead of the n.v^^/'^ groupings of the chrysins. 



