PORPHYRINS 



263 



A given pyrrole ring always has one type a and one type b substituent, never two of the 

 same type. Four basic types of porphyrins would then appear to be possible: 



ir 



Fortunately type III porphyrins are by far the most common. Type I occurs rarely and 

 the other two types never in nature. A Roman numeral following the name of a porphyrin 

 refers it to one of these types (e.g. "coprophorphyrin Iir'). 



Chlorophyll derivatives have an additional 5-membered ring derived from a propionic 

 acid side-chain on position 6. It is numbered as shown: 



The functional forms of natural porphyrins all seem to have a metal bound as a chelate 

 complex in the center of the molecule replacing the two hydrogens otherwise present. 

 Naturally-occurring porphyrins which do not contain a metal are precursors or degrada- 

 tion products of the functional compounds. Significant amounts of them are found only 

 under abnormal circumstances. 



In plants we are most concerned with iron-containing hematin pigments and magne- 

 sium-containing chlorophyll pigments. These are biogenetically closely related, but ex- 

 perimentally they are approached in quite different ways. The hematin pigments are 

 tightly bound to protein, difficult to purify and present in plants in very small concentra- 

 tions. Chlorophyll, conversely, is easily extractable in large quantities and more con- 



