46 UNITY AND DIVERSITY IN BIOCHEMISTRY 



In the protoporphyrins, the eight carbon atoms of porphin carry two 

 vinyl groups, four methyl groups and two propionic acid groups. One of 

 the fifteen possible isomers of this structure is protoporphyrin IX or 

 porphin-l:3:5:8-tetramethyl-2:4-divinyl-6:7-dipropionic acid, commonly 

 called protoporphyrin. This is the porphyrin most widely distributed in 

 nature, it is a constituent of haemoglobin, catalase, peroxidase, of cyto- 

 chrome-6, etc. During the course of putrefaction protoporphyrin loses its 

 unsaturated side chains and is transformed into deuteroporphyrin III. 

 On the other hand, it can be derived by a series of operations from copro- 

 porphyrin III, another proof of its chemical kinship with the porphyrins 

 of Series III. 



[b) Derivatives of Pyrrolidine 



Pyrrolidine is the saturated ring compound corresponding to pyrrole. 



H2C CH2 



\ / 

 H 



N 



Pyrrolidine 



Its skeleton is found in molecules present everywhere in the biosphere, 

 as in proline, vitamin Bjg, nicotine and in other alkaloids, etc. 



(c) Indole Derivatives 



Indole is the compound whose ring is present in tryptophan. It is also 

 present in the natural auxin a-indolyl-acetic acid, a substance produced at 

 the level of the apical bud and of the young leaves in plants, and transported 

 to the region of the stem when growth takes place. 



H 



HC C CH ^^\ 



HC C CH ^^ C C-CH.COOH 



H H C N^ 



Indole H H 



a-indoyl-acetic acid 



(d) Imidazole Derivatives 



The five-membered imidazole ring containing two nitrogen atoms is 

 present in the naturally occurring amino acid, histidine. 



