CONSTITUENTS OF THE BIOSPHERE 



55 



Although unstable in the free state, the ring is present as derivatives in 

 which the hydrogen on the nitrogen atom in position 9 is replaced by a 

 substituent group. The group of these compounds is called "the flavins". 

 One example is lumiflavin (a photoderivate of lactoflavin). 



H CH3 



C N N 



^ "- / \ ^ "^_ 

 CH.C C C CO 



CH3C C C NH 



C N^ C 



H O 



6,7-dimethyl-9-methyl-isoalIoxazine or lumiflavin 



(/) Pteridine Derivatives 



This heterocyclic ring system arises from the condensation of the pyrimi- 

 dine and the pyrazine rings. 



6 

 Pteridine 



The pteridines are sparingly soluble in water and insoluble in volatile 

 organic solvents. They have a bright fluorescence. 



1. Pteri?is 



The pteridine nucleus is present everywhere in the biosphere, in the 

 form of a number of derivatives : leucopterin, xanthopterin, erythropterin, 

 etc. Pterins are found, especially, in the wings of butterflies. The fluores- 

 cence of tissues is often due to their presence. 



2. Folic Acid 



Also among these derivatives is pteroyl - glutamic acid or folic acid, so 

 called because of its abundant distribution in the leaves of plants. Folic acid 

 is the universal coenzyme for the transfer of C^ fragments and hence is 

 indispensable for the synthesis of purines and nucleic acids. 



