54 III. PORPHYRIN CHEMISTRY 



indication of the nature and order of the side chains by replacing H 

 with the appropriate symbols. Since this book is concerned a good 

 deal with changes in the porphyrin nucleus, particularly the carbon 



HH 



Fig. 2. Symbols. 



atoms which unite the pyrrole rings, however, a different symbol will 

 be used which allows changes in the nucleus as well as in the side 

 chains to be depicted. The symbol B (Fig. 2) gives the central sixteen- 

 membered ring in a way not deviating very far from its true shape, 

 but omits the four pyrrole rings. The four corners of the cross point- 

 ing inward represent the four nitrogen atoms, the four corners 

 pointing outward the methene bridges between pyrrole rings. The 

 eight substituents in the jS-position of the pyrrole rings are connected 

 by dotted lines to the carbon atoms of the central ring, from which 

 they are actually separated by the /3-carbon atoms of the pyrrole 

 rings. The formulas of the different porphyrins are given by inserting 

 the symbols for the /8-side chains (cf. Table II) in the appropriate 

 positions. 



2.3. Structural Isomerism 



Of each of these porphyrins distinguished by the nature of their 

 side chains, again several isomerides are possible, since the relative 

 order of the side chains can also vary. The simplest case is that of 

 the etio-, copro-, and uroporphyrins, in which only two different 

 kinds of substituents occur, e.g., methyl and ethyl in etioporphyrin. 

 Here four isomerides are possible, which have been called I-IV by 

 H. Fischer (Fig. 3). 



Only compounds derived from I (alternating substitution) or III 

 (unsymmetrical substitution) have been found in nature and it will 

 be seen that those derived from III are far more important. 



If three different types of side chains are present, as in the blood 



