18 CHEMICAL STATICS 



between them became one of paramount importance. Hof- 

 meister (30) has pointed out that it is possible to conceive of 

 several ways in which the acids may be linked together such as: 



A. Direct union of the carbon atoms, as: 



I I 



-C-C- 



I I 



under which condition the molecule would be an immense chain 

 of carbon atoms, and would not be readily hydrolysable, as the 

 proteins are, into its constituent amino-acids. 



B. Ether-like unions, as: 



I I 



-c-o-c- 



I I 



Such unions, however, would only be possible when one of the 

 two amino-acids contained a hydroxyl group. Only tyrosin, 

 serin and oxyprolin, among the amino-acids which occur in the 

 proteins contain such groups, however, therefore this mode of 

 union cannot be of general occurrence in the proteins. 



C. The carbon atoms may be united by a nitrogen atom, as: 



I I I 



-C-N-C- 

 I I 



Several varieties of this mode of union are possible, such as: 

 -CH2-NH-CH2- -CH2-NH-CO- -CH2-NH-C(NH) 

 I II III 



The syntheses accomplished by Emil Fischer, which I have 

 described above, the occurrence of peptids among the products 

 of the partial hydrolysis of proteins, and their digestibility by 

 proteolytic enzymes, demonstrate that type II is the most gen- 

 eral mode of union among the amino-acids which constitute 

 the protein molecule. The fact that the proteins give the biuret 

 reaction also supports this view. It has been shown by Schiff (61) 

 that only those substances which contain two — CO — NH — groups, 

 or two — CS — NH — , or two C(NH) — NH groups, or, under certain 

 conditions, two — CH — NH— groups yield the biuret test. That 

 such groups as: 



I 

 -COHN-C-COHN- 



