CONSTITUENTS OF THE BIOSPHERE 47 



N CH N CH 



!l II 11 il 



HC CH HC CH 



\ / \ / 



N S 



H 



Imidazole Thiazole 



(e) Derivatives of Thiazole 



This five-membered ring, containing both nitrogen and sulphur, to- 

 gether with a pyrimidine ring, forms part of the structure of thiamine 

 pyrophosphate which is an important coenzyme. 



(/) Thioctic Acid 



Another important sulphur-containing heterocyclic ring is that of 

 6:8-dithio-«-octanoic acid otherwise known as thioctic or a-lipoic acid. 



S — CH2 

 CH2 



I Thioctic acid 



S— CH 



I 



(CH2)4 



COOH 



This substance, an important coenzyme for oxidative decarboxylation, 

 operates by opening and closing of its ring at the two sulphur atoms. 



I -) i_r 



,/^^l (CH.O.-COOH -* /\ (CH.0,-COOH 



s-s ^ZYu ^^ ^^ 



Oxidized thioctic acid Reduced thioctic acid 



{g) Derivatives of Six-membered Heterocylic Rings containing Oxygen 



The principal rings in this category are those of pyran, chroman, and 

 flavan. The pyran ring, as has already been stated (p. 19) is the basic 

 skeleton of the sugars in their stable forms. 



