STRUCTl'RE OF VERDOHEME 



465 



compounds, is extremely firmly bound in azaheme compounds — 

 which is in agreement with the reclosure of the ring. 



Nevertheless, the structure suggested by Lemberg for verdoheme 

 requires a modification. We hav^e seen in Chapter IV, Section 3.3. 

 that a formula similar to that given in Figure 2 for verdohemochrome 

 has to be assumed for the zinc complex salt of biliverdin (which is 

 readily obtained from biliverdin), whereas all attempts to form verdo- 

 hemochrome from biliverdin have failed; the two compounds, bili- 

 verdin zinc and verdohemochrome, also have rather different absorp- 

 tion spectra. That they must nevertheless be closely related to each 

 other is not only supported by the conversion of verdohemochrome 

 to biliverdin by acids, but also by other observations. Oxidation of 

 the zinc or copper complexes obtainable from verdohemochrome by 

 exchange of iron with these metals leads to complexes of biliviolinoid 

 type, spectroscopically indistinguishable from the biliviolinoid com- 

 plexes obtained by oxidation of biliverdin zinc {1681). The structure 

 of these biliviolinoid pigments (bilipurpurins) has been discussed in 

 Chapter IV, Section 5.4. 



These facts and (compare page 463) a number of other observations 

 are best accounted for by the assumption that verdoheme contains a 

 cyclic anhydride of biliverdin with a ring system closed by a labile 

 oxygen bridge. Formulas such as A and B in Figure 5 are suggested 



-,-h 



Fig. 5. Structure of verdohemochrome. 



for verdohemochrome. Formula B was suggested by Lemberg (1687) 

 in 1943, but formula A appears to be more likely. 



By the action of acid on the yellow dithionite reduction product of 

 verdohemochrome a yellow compound is obtained which differs from 

 all known bile pigments. This may be explained in the way suggested 



