Chapter 13 

 PORPHYRINS 



The porphyrin ring system is widely distributed in nature, occurring throughout the 

 plant and animal kingdoms. Porphyrins serve as part of the oxygen transport and storage 

 systems of vertebrates, in the respiratory chains of most cells, and as prosthetic groups 

 of certain enzymes in plants and animals. A different modification of the porphyrin nu- 

 cleus serves as a basis for the chlorophyll and related pigments of green plants. 



The parent nucleus of the natural porphyrins is the simplest cyclic tetrapyrrole, 

 porphin (here shown with the ring-numbering used for all porphyrins): 



Although fixed double bond positions are shown, it is more likely that the entire molecule 

 is a resonance hybrid of several possible double bond arrangements. The two central 

 hydrogen atoms may also be shared among the four nitrogens. These considerations also 

 apply to the more complex porphyrins, although certain combinations of substituent groups 

 may stabilize one resonance form more than another. 



All of the natural porphyrins carry alkyl substituents in the available positions of 

 the pyrrole rings. The variety and arrangement of substituents is quite limited so that 

 important generalizations may be drawn concerning them. The reasons for these regu- 

 larities in porphyrin structure become apparent on considering the pathways of biosyn- 

 thesis. The groups which occur may be divided into two categories: 



O 



a. 



-CH3, 



-C 

 \ 



and 



-CH2COOH 



H 



b. -CH2CH3, 



-CH=CH, 



and 



-CH2CH2COOH 



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