138 



IV. niLE PIGMENTS 



destroys the flanking hydroxypyrrole rings so that the GmeUn reac- 

 tion is negative. Synthetic bilenes-(b) with methyl instead of 

 hydroxy! groups in the a-position give the Gmelin reaction i8J^3). 



The bilenes-(b) form stable hydrochlorides and zinc complex salts 

 with green fluorescence similar to those of a,a'-dimethylpyTromethenes 

 (878;861, p. 7). 



M E M P 



P M M E 



HO' N C^N CHO 

 H H2 H 



+ 



H 



A A„A A 



HO 



H2 H 



OH 



M E MP P M M E 



N C 

 H H 



A A^A A^ 



N C VT C 

 H ^^ H 



H 



A A 



N 

 H 



OH 



Fig. 23. Synthesis of mesobilene-(b). 



Mesobilene-(b) and tetrahydromesobilene-(b) are more hydrophilic 

 than the other bile pigments. This is probably due to their relatively 

 strong basic character, as a result of which the pH values as base and 

 as carboxylic acid are less than two pH units apart. Thus the 

 substances are partly ionized even at their isoelectric point, while 

 most other bile pigments have an isoelectric zone at which they are 

 unionized. 



Mesohilene-{a) (cf. Table II) has been obtained as the product of incomplete 

 catalytic hydrogenation of mesobilirubin (addition of two hydrogen atoms 

 at the methene group c of mesobiladiene-(a,c). It has therefore been called 

 dihydromesobilirubin, a name which must not be confused with dehydro- 

 mesobilirubin (mesobiliverdin). The autoxidation or ferric chloride oxidation 

 of mesobilane begins with the oxidation of the methylene group b and hence 

 leads to mesobilene-(b), not to mesobilene-(a). Mesobilene-(a) gives no 

 Gmelin reaction, but the Ehrlich aldehyde and diazo reactions are positive. 

 Ferric chloride oxidizes it to a red compound (mesobilierythrin.^). It does 

 not form a fluorescent zinc salt. 



6.2.2. Properties of Mesobilane and Mesobilene-(b). Meso- 

 bilane (mesobilirubinogen, urobilinogen A of Watson) was prepared by 

 Fischer as a colorless crystalline substance of melting point 197-202°C. 



