STRUCTURAL ISOMERISM 



55 



porphyrins, the number of possible isomerides is increased to fifteen. 

 The reader may work this out for himself or look up the handbook 

 of Fischer and Orth {861, p. 409). It suffices to say that protopor- 



Fig. 3. Isomerides of etioporphyrin. 



phyrin and the other porphyrins derived from hemoglobin and natural 

 hematin compounds are all of type IX (Fig. 4). The evidence for the 

 structure of the porphyrins is dealt with fully in Fischer's work (861) 



p- \y p 



Fig. 4. Protoporphyrin. 



and in a number of reviews, to which the reader is referred (603, 

 790-792, 79 If,88S, 1887, 2255, 3028). There are also a number of earlier 

 reviews by Willstatter, Kuster, Fischer, and Schumm (783,786,787, 

 789,882,1^0,1601,2506,3086,3091). 



