rULADrENES-(H,c). RUBINS 



121 



nation of the position of the absorption band in the orange, using a 

 Hartridge Reversion Spectroscope. BiUrubin yields an absorption 

 band at 635 m/i, nicsobilirubin at 6"25 m^u. 



Mesobilirnhin diinethijl ester hi/drochloride forms red hexagonal leaflets 

 from methyl alcoholic hydrochloric acid, melting point. 190°. 



4.3. Color Reactions 



The Gnielin reaction and related reactions have been discussed 

 above (Section 2.4.). 



Diazo reaction. The coupling of bilirubin with diazonium salts 

 (usually diazotized sulfanilic acid) to give azodyes was discovered 

 by P. Ehrlich in 1883 (651) and has been developed by van den 

 Bergh as the standard method of estimation of bilirubin in serum 

 {221,233,239, cf. Section 9 and Chapter XI). The reaction is more 

 complicated than was previously assumed and involves a splitting 

 of the bilirubin molecule at the central methylene group (861, p. 722; 

 820). The compound which actually couples with the diazobenzene 

 sulfonic acid is neoxanthobilirubinic acid (dehydronorbilinic acid) : 



HO ^N-^c\*c A^A^A^A o„ 



H 



H 



H. 



H H 



+ (ArNo)Cl 



H H 



+ 



cich/ n c n oh 



where Ar = aryl. Hence, while mesobilirubin reacts in the same 

 way, the bilatrienes do not give the reaction. 



4.4. Absorplioii Spectra and Metal Complexes 



Figure 13 shows the absorption curve of bilirubin in chloroform. 

 This solution obeys Beer's law. Bilirubin in blood serum (cf. Chapter 

 XI) has a somewhat weaker band at 460 m/x (c^m = 42). The 

 solutions in aqueous alkali resemble the colloidal solutions in cholic 



