SIDE CHAINS OF VARIOUvS PORPHYRINS 49 



shown that traces of porphyrins are widespread in both the animal and plant 

 kingdoms, as well as in a variety of microorganism, such as yeast. The bio- 

 chemistry of porphyrins in animals and microorganisms is discussed in detail 

 in Chapters XII and XIII in connection with the catabolism and anabolism 

 of hemoglobin and other hemoproteins. Plants have been investigated less 

 systematically, but traces of coproporphyrin have been found by Fischer 

 and co-workers in several plants and plant products, such as flour [cf. 861, 

 pp. 398, 480). 



In Table I we summarize instances in which porphyrins are found in the 

 animal kingdom in unusually large amounts or under conditions under 

 which no direct connection with hemoglobin metabolism is evident. 



In some instances, e.g., in the hen's egg shell, porphyrin is present 

 as a calcium salt, in others, e.g., in the colored spots on the egg shells 

 of other birds as a protein compound. It is possible that porphyrins 

 may be frequently in more or less firm combination with proteins. 



2. STRUCTURE OF BIOLOGICALLY IMPORTANT 

 PORPHYRINS 



2.1. Definition and Side Chains of Various Porphyrins 



The porphyrins are derivatives of the cyclic ring system porphin(e),* 

 C20H14N4 (Fig. 1), which contains four pyrrole-like rings welded 

 together to a complicated ring system by four CH groups (methene 



CH 



Fig. 1. Porphin. 



groups). It contains a central sixteen-membered ring of twelve 



carbon and four nitrogen atoms condensed with four pyrrole rings. 



A fuller discussion of this structure will be found in Section 6. 



* Dr. Patterson, Chairman of the American Committee on Organic Nomenclature, 

 suggested using the name porphiiif because of its weakly l)asic properties, but to 

 continue using porphyrin without the final e. This suggestion has been adopted by 

 Rothemund, but not by H. Fischer. Etioporphyrin is, however, a base as well as 

 porphin. 



