BILANES, BILENES, AND RELATED SUBSTANCES 139 



by sodium amalgam reduction of mesobilirubin and by catalytic 

 hydrogenation of bilirubin or mesobilirubin (776,778,852). 



The condensation product with p-dimethylaminobenzaldehyde has 

 an absorption maximum found by most authors at 555-557 mn (Heil- 

 meyer, Turner, Niemann), while Watson observed it at 560-565 m/i. 

 457^ = 64.5 (1213, p. 215; 1608). 



M esohilene-(h) (urobilin IXa of Siedel, K-urobilin * of Watson) has 

 been obtained crystalline by Watson (2980-2982), Siedel (2550,2557) 

 and Fischer (822). Rather pure preparations had previously been 

 obtained by Terwen (1735). It crj'stallizes from chloroform or 

 acetone in orange-red needles or prisms. A melting point of 190°C. 

 is reported by Watson and by Fischer and Halbach for natural meso- 

 bilene-(b), while Siedel gives 177° C. for the synthetic substance. 

 The hydrochloride crystallizes from chloroform in small rectangular 

 or boat-shaped, orange-red crystals of melting point 199-200°. The 

 substance is optically inactive. 



Absorption spectra and complex salts. As would be expected from 

 the fact that resonance cannot occur beyond the two central pyrrole 

 rings, the absorption spectrum of mesobilene-(b) is similar in char- 

 acter to that of Q:,a:'-dimethylpyrromethenes, the bands lying, hrow 

 ever, about 10 rati toward the red. The band shift on hydrochloide- 

 formation, 40 ran toward the red, is the same in both classes. Table 

 IX summarizes the absorption spectra of mesobilene-(b), its hydro- 

 chloride, and its zinc complex salt. 



Heilmeyer observed an "alkaline" absorption curve with the maximum 

 at 510 van which gradually developed from an initial curve with an indis- 

 tinct maximum at 450 myu. Obviously the initial curve is the true "alkaline 

 curve" while the former is that of a complex salt. With the very dilute 

 solutions of urobilin used in spectrophotometry, very small amounts of metal 

 from solvent, glass, or metal rings of spectrophotometer cups suffice to form 

 complex salts, and this has confused many earlier observers. In fact uro- 

 bilinoids can be used as microchemical reagents for copper or zinc (252). 



By complex salt formation the absorption band is shifted about 

 60 niju toward the red. The zinc complex salts of urobilinoid sub- 

 stances were observed by the first investigators owing to their 

 extremely strong green fluorescence and are used for the recognition 

 of urobilins in the Schlesinger reaction. They are easily decomposed 

 by dilute acids with disappearance of the fluorescence. In this way 



* The term K-urobilin is, however, used by the Fischer school to denote a mixture 

 of side chain isomerides of mesobilene-(b) in contradistinction to mesobilene-(b) 

 IXa. 



