CHEMICAL STRUCTURE OF PYRROLE PIGMENTS 5 



rhodins, prepared in the test tube from chlorophylls a and b are such 

 dihydroporphyrin derivatives. We shall not discuss at this stage the 

 side chains with which the eight jS-positions of the pyrrole rings are 

 substituted, but it must be mentioned that in chlorophyll and bac- 

 teriochlorophyll one of these side chains is linked with one of the 

 four carbon atoms which connect two pyrrole rings, forming a fifth 

 (isocyclic) ring (Fig. 6). In the chlorophylls, the carboxylic acid 

 groups of the side chains are esterified with methyl alcohol and with 

 phytol (an alcohol with a long hydrophobic chain) which gives the 

 chlorophylls the character of waxes. We know little as yet about 

 the combination of chlorophylls with proteins and, while such a 

 combination appears likely, it has not yet been proved that a specific 

 chlorophyll-protein exists in the chloroplasts. 



Figure 6 



The tetrapyrrole open chain compounds are known mainly as 

 metabolic products of the breakdown of hemoglobin in the animal 

 body. The bile pigments, e.g., bilirubin, biliverdin, and urobilins, 

 belong to this class. 



A far greater variety of more or less stable hydrogenation stages 

 exists in this class than is found in the tetrapyrrolic substances with 

 closed ring systems. This explains the variety of differently colored 

 (red, green, violet, yellow) bile pigments. Except as stages in the 

 breakdown of closed ring tetrapyrrolic metal complexes, the metal 

 complexes of these pyrrole pigments have not yet been found to 

 possess any physiologic significance. Lately it has been shown that 

 bile pigments occur in lower animal species, in which they cannot be 

 derived from hemoglobin breakdown. In some instances they form 

 ornamental pigments, e.g., in the egg shells of birds. The only com- 

 pounds of this class for which a functional importance has been 



