4 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



such Unkings therefore cannot play a great part in the constitution of 

 the molecule. 



A large number of significant facts support Scheme () as the 

 most feasible method for the combination together of the amino 

 acids. 



(a) The products of hydrolysis. 



(i) A small proportion of the total nitrogen of the protein molecule 

 is liberated on hydrolysis as ammonia ; this points to the presence of 

 this linking in the form of acid amide, CO NH 2 , combinations. 



(ii) The greater portion of the total nitrogen about 90 per cent. 

 is present in the products of hydrolysis in the form of amino (NH 2 ) 

 groups. These groups are not present in the protein molecule as 

 such, since by the action of nitrous acid on the protein the amount of 

 nitrogen liberated is very small in amount, and in no way corresponds 

 to the amount obtainable if the greater part of the nitrogen be present 

 in the form of amino groups. 



It must therefore be assumed that the NH 2 groups of the end pro- 

 ducts exist in the protein molecule in the form of NH groups. 



(b) The biuret reaction. 



The biuret reaction, which is one of the chief characteristics of a 

 protein, is according to Schiff given by those substances which contain 

 two CO NH complexes, or two CS NH or C(NH) NH com- 

 plexes, and under certain conditions two CH 2 NH complexes, 

 combined together directly, or by a carbon atom, or by a nitrogen 

 atom, e.g., 



CO NH. CH, NH, 



I I ' 



CO NH, 



CO NH a 



cic 



:o NH, 



CO NH, 



CH, 



CO NH, 



Malonamide 



NH 



CO NH a 

 Biuret 



CH, NH(CH,) 

 CO NH, 



Glycine amide 



Sarcosine amide 



Oxamide 



and also 



CO NH, 

 CH.NH, 

 CH, 



CO NH, 

 Aspartic acid diamide 



All these compounds give very intense biuret reactions. The presence 

 of 



CH NH 

 CO NH 



