BILANES, BILENES, AND RELATED SUBSTANCES 143 



Spectroscopic differentiation betioeen mesobHene-{b) and tetrahydromeso- 

 bilene-{b). Measured in the Hartridge Reversion Spectroscope, the hydro- 

 chloride of tetrahydromesobilene-(b) has an absorption band at ^9^1 m/x, 

 ^20 A less toward the infrared than tliat of mesobilene-(b) (Leuiberg, lCy77, 

 1713; cf. 3001). The same difference has been observed spectrophotometri- 

 cally by Pruckner and Stern {:il90), cf. Tables IX and X, together with 

 similar diflFerences in the position of the weak bands in the ultraviolet. 

 Another spectroscopic difference between the two compounds was noted as 

 early as 1897 by Hopkins and Garrod {133If). When a solution of tetra- 

 hydromesobilene in sodium bicarbonate is acidified by a slight excess of very 

 dilute sulfuric acid, an absorption band at 530 mju ("E" band) appears. 

 This phenomenon is not given by mesobilene-(b). Siedel could not observe 

 this, but it was confirmed by Lemberg and collaborators {1713). 



Absorption spectra. Apart from this slight shift of the absorption 

 maxima toward the ultraviolet, the colors and absorption spectra of 

 tetrahydromesobilene-(b) and of its compounds closely resemble 

 those of mesobilene-(b) {cf. Table X). The extinction coefficients 

 are higher than those of mesobilene-(b), but this may be due to the 

 fact that the latter, being less stable, may not yet have been obtained 

 so pure as tetrahydromesobilene-(b). 



The fluorescence spectrum of the urobilin-zinc complexes shows 

 only one broad emission band extending from the position of the 

 absorption maximum through the green part of the spectrum (Dhere 

 and Roche, 581^). 



6.4. d-Urobilin 



In preliminary publications {2513,2515,2998) Watson and collab- 

 orators report the isolation of a dextrarotatory urobilin from infected 

 bile. 



The strong optical dextrorotation, [a]."° = +4000°, excludes the possi- 

 bility that d-urobilin is a mesobilene-(b) containing an optically active 

 impurity. The simplest explanation would be that bacteria different from 

 those reducing mesobilane in the feces to /-tetrahydromesobilane {cf. Chap- 

 ter XI) reduce it in the bile to an enantiomorph rf-tetrahydromesobilane. 

 Two observations do not favor this assumption. The position of the absorp- 

 tion band of c?-urobilin is said to coincide with that of mesobilene-(b), not 

 of (/-tetrahydromesobilene-(b); and by heating in dio.xane-hydrochloric acid 

 the formation of mesobiliviolin and mesobiliverdin was observed. The latter 

 also corresponds to the behavior of mesobilene rather than of tetrahydro- 

 mesobilene; one may think of a structure with a different position of the 

 hydrogen atoms in the pyrrole rings I and IV as in the formula suggested 

 by Siedel for tetrahydromesobilene-(b), cf. Figure '■2.J. Watson claimed that 

 bacteria play no role in the observed conversion of mesobilane to (/-urobilin. 



