52 III. PORPHYRIN CHEMISTRY 



The porphyrins differ structurally from porphin and from one 

 another by various side chains which substitute the eight hydrogen 

 atoms in the /3-positions of the pyrrole rings in porphin. While 

 porphin does not occur in nature and has been synthesized com- 

 paratively recently, some of the porphyrins are found free in many 

 living species, while others are of importance as the metal-free com- 

 ponents of hematin compounds. 



Table II gives the names and formulas of the porphyrins which 

 are of importance for the subject of this book, together with the side 

 chains by which they are characterized and symbols which will be 

 used for these side chains in later formulas in order to save space. 



It will be seen that with the exception of uroporphyrin all other 

 porphyrins carry four methyl side chains. Etioporphyrin (or more 

 <;orrectly "mesoetioporphyrin," cf. Section 3.2.) carries in addition 

 four ethyl groups, but no acidic groups. The next four porphyrins 

 are all closely related to protoporphyrin, the porphyrin from which 

 hemoglobin and hemin are derived. They all carry two propionic 

 acid groups in addition to the four methyl groups. The remaining 

 two places are either unsubstituted (deuteroporphyrin) or substituted 

 by nonacidic alkyl groups (ethyl in mesoporphyrin, vinyl in proto- 

 porphyrin and a-hydroxyethyl in hematoporphyrin) . Coproporphyrin 

 contains four methyl and four propionic acid side chains, uropor- 

 phyrin (probably) four acetic acid and four propionic acid side chains. 

 Thus mesoporphyrin can be considered dicarboxylated, copro- 

 porphyrin tetracarboxylated, and uroporphyrin octacarboxylated 

 etioporphyrin, while the other blood porphyrins are closely related 

 to mesoporphyrin, from which they differ by having two side chains 

 different from the two ethyl groups of mesoporphyrin. 



The symbols for the side chains need little explanation. In the 

 German literature "Ae" (Aethyl) is used for ethyl and "S" ("Saure") 

 for the propionic acid side chain. AC will be used for the acetic acid 

 side chain in order to differentiate it from acetyl (Ac). 



The discussion of chlorophyll and the porphyrins derived from it 

 is beyond the scope of this book. The main differences between 

 chlorophyll derivatives and porphyrins, dihydroporphin system and 

 isocyclic ring, have been discussed in Chapter I. Some of the por- 

 phyrins derived from chlorophyll, e.g., pheoporphyrin as and phyl- 

 loerythrin still contain this isocyclic ring, others, such as phyllopor- 

 phyrin, a remnant of it in the form of a methyl group substituting 

 one of the methene groups between pyrroles, while others again, such 



