ABSTRACT
Attempts have been made to produce a useful, systematic naming system for tetrapyrroles. Bilirubins react with aromatic diazonium salts to give dipyrrole azo derivatives which are useful in characterization, especially the ordering of the pyrrole β-substituents. Oxidative degradation of bile pigments can lead to monopyrrole derivatives. Identification of the derivatives is useful in elucidation of the nature of the pyrrole β-substituents. A particularly useful method is that developed by W. Rüdiger using dichromatechromic acid to yield cyclic imides and formyl pyrroles. Keto-enol tautomerism between lactam and lactim forms of the 3-pyrrolin-2-one end rings of bilirubin and other bile pigments was mentioned by W. Kuster over 60 years ago, but the issue was not resolved until nearly 40 years later. Complete or partial infrared spectra of bilirubin have been published over the years. However, most spectra were determined on solid bilirubin dispersed in the solid phase or nujol mulls.
