2 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



against this mode of combination is the strongly basic character of such 

 compounds, which was first shown by Curtius in the case of glycine 

 ester. 



III. The carbon atoms can be linked together by a nitrogen atom : 



JU-c- 

 I I 



Several possibilities immediately occur for this mode of combination, of 

 which the three following are the most likely : 



CH 2 NH CH 2 CH., NH CO CH 2 NH C(NH) 



I. II. III. 



A linking as in Scheme I., which occurs for example in proline, 

 cannot occur to any large extent, since if two amino acids be thus 

 combined together the molecule would become strongly acid in char- 

 acter owing to the free carboxyl groups. 



A linking as in Scheme III., which is that of guanidine, occurs in 

 arginine. Only in this compound does such a complex occur in the 

 protein molecule, and therefore such linkings cannot be of the chief 

 importance for the constitution of the molecule. 



A large number of important facts support Scheme II. as being 

 the most important for the combination together of the amino acids. 



(a) The products of hydrolysis. 



A small proportion of the total nitrogen of the protein molecule is 

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

 acid amide, 



CO NH 2 , 

 form of combination. 



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, and the remainder in the form 'of imino (NH) groups, as in 

 arginine. 



The amino 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 



