The Chemical Basis of Life 19 



they are no longer identical with the substances 

 which exist in living cells. The vulnerability of 

 proteins to even the most gentle chemical treat- 

 ment is well illustrated by the treatment of hair 

 for a short time with dilute hot sodium carbonate 

 solution, which will reduce the cystine yield from 

 about three percent to almost nothing. If an indi- 

 vidual fiber of protein can be called a "molecule" 

 merely because it represents the smallest state of 

 subdivision in which that substance exists in 

 nature, then it appears that a single fiber of cotton 

 should also be called a "molecule." 



By suitable chemical treatment the above-men- 

 tioned amino acids can be condensed, with the 

 elimination of water, to form either chain struc- 

 tures known as polypeptides or ring structures 

 known as diketopiperazines : 



3 NH 2 — CHE^CO— OH = 



NH 2 — CHE^CO— NH— CHE— CO— NH— CHEr-CO— OH + 



2 H 2 0; 



2 NH 2 — CHEr— CO— OH = CHEr— CO 



/ \ 



NH NH + 2 H 2 0. 



\ / 



CO— CHE 



Numerous methods for preparing polypeptides 

 have been devised by Emil Fischer and his col- 



