CHEMICAL STRUCTURE OF PYRROLE PIGMENTS 3 



2. FUNDAMENTALS OF THE CHEMICAL STRUCTURE 

 OF PYRROLE PIGMENTS 



Practically all the pyrrole pigments occurring in nature contain 

 four pyrrole rings linked by four single carbon atoms to a system in 

 which a high degree of resonance is produced by conjugation of a 

 large number of double bonds. We can distinguish, first, two types. 

 In the porphyrins, and in the hematin and chlorophyll compounds, 

 we have a system in which the four pyrroles are kept together by 

 four single carbon atoms in the form of a closed planar ring system 



P'igure 2 



^N^C^N^C^N^C^N^ 



Figure 3 



(Fig. 1). In the bile pigments one of these carbon atoms is missing 

 and the system is therefore less rigid, and (at least in some sub- 

 stances) can be pictured equally well as an open ring system (Fig. 2) 

 or as a tetrapyrrole chain (Fig. 3). It will be shown that, while the 

 latter is customary, the former rather is correct. Of the closed ring 

 tetrapyrrole substances two main types can be distinguished (c/. 

 Table I). The first group comprises the porphyrin derivatives proper, 

 which possess the most stable type of ring system (porphin), com- 

 parable to the fully aromatic system in benzene (Fig. 4). 



