44 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



indigo, that considerable amounts of pyrrole bases are present in plants, 

 and that tryptophane contains a pyrrole ring. One particular class of pyr- 

 role derivatives, the porphyrins, are of great interest due to their wide 

 distribution both in the free and in the combined state. The porphyrins 

 crystallize readily, have a very high melting point and fluoresce brilliantly 

 in ultra-violet light. Their colouring power is extremely great and spectro- 

 scopic analysis allows very small quantities of porphyrins to be detected. 

 They are molecules containing four pyrrole rings and their structure may 

 be represented as based upon a hypothetical tetrapyrrole, porphin, the 

 structure of which in each particular case is modified by the addition of 

 various groups. The position of the double bonds can only be arbitrarily 

 fixed in a structure of this sort. 



The four pyrrole rings of porphin and the porphyrins are connected by 

 methyne bridges (= CH — ) which are denoted by the Greek letters a, j8, 

 y and 8. The porphyrins are derived from the parent porphin nucleus by 

 replacing the hydrogen atoms on carbons 1 to 8 by various radicals. 



The relative positions of the substituent groups can give rise to several 

 isomers. Let us consider, for instance, in one category of porphyrins, the 

 tetramethyl-tetrapropionyl-porphins, also called coproporphyrins. There 

 are four possible isomers corresponding to different positions of the four 

 methyl groups : 



1 : 1, 3, 5, 7- 



II : 1, 4, 5, 8- 



III : 1, 3, 5, 8- 



IV: 1, 4, 6, 7- 



These four isomers have the following structures : 



HOOC - H.C - HX H CH 



H,C 



HOOC - CH, - HjC 



CH, - CH, - COOH 



CH, H CHj - CHj - COOH 



