16 Spirazines 



Lecithin 



CH2O— CO— E 



CHO— CO— E' 



CH3 CH3 CHoO— P— OH 



\l /\ 



N— CH2— CHo— O O 



/\ 

 CH3 OH 



By suitable chemical treatment the above men- 

 tioned amino acids may be condensed, with the 

 elimination of water, to form either chain struc- 

 tures known as polypeptides, or ring structures 

 known as diketopiperazines : 



3 NH.— CHE^CO— OH = 



NH2— CHE— CO— NH— CHE— CO— NH— CHE^CO— OH + 

 2 H2O, 



2 NH2— CHE— CO— OH = CHE— CO 



/ \ 



HN NH + 2 H.,0. 



\ / 



CO— CHE 



(E. Fischer, Untersuchungen liber Aminosauren, Polypeptide, und 



Proteine, 1899-1906.) 



Since proteins constitute the principal struc- 

 ture-building food for animals, and upon diges- 

 tion are decomposed into amino acids, in which 

 form they are assimilated by the tissues, it is 

 generally thought that growth involves condensa- 

 tion processes of a similar character, and that 

 amino acids exist in living tissues either as long 



