INTRODUCTION 3 



There are three ways in which we can conceive that the amino 

 acids are combined together in the protein molecule : 



I. The carbon atoms can be linked together directly : 



J0_ 



I I 



Under these conditions the protein molecule would be a huge 

 branched carbon chain ; its decomposition into larger and smaller com- 

 plexes, especially by the action of ferments, is difficult to understand ; 

 and further, the decomposition of chains of carbon atoms even by the 

 action of the very active enzyme trypsin, has never been observed. 

 This mode of combination is therefore very unlikely. 



II. The carbon atoms can be linked together by an oxygen atom : 



An ether-like combination of the amino acids was suggested by 

 Nasse from the analogy between the hydrolysis of proteins by enzymes 

 and that of the carbohydrates and fats. On 'account of the small 

 number of hydroxyl groups in the molecules of the amino acids, which 

 is limited to those contained in tyrosine, serine and oxyproline, the 

 principal method of combination cannot be of this nature. 



For the same reasons an ester-like combination of the carboxyl 

 group of an amino acid with a hydroxyl group cannot occur, nor is the 

 acid anhydride method of combination possible. A further reason 

 against this method is the strongly basic character of such compounds, 

 as was first shown by Curtius in the case of glycine ester. 



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



C N C 

 I I 



This structure admits of several different arrangements, of which 

 the three following are the most likely : 



CH. NH CH. CH, NH CO CH^NH C(NH> 



(a) (*) (') 



A linking as in Scheme (a) is found, for example, in proline ; but 

 it cannot occur to any large extent, since if the union between amino 

 acids be thus effected the molecule would become strongly acid in 

 character, owing to the free carboxyl groups. 



A linking as in Scheme (c), which is that of guanidine, occurs in 

 arginine, and this is the only compound amongst the decomposition 

 products of proteins in which this arrangement of the atoms occurs : 



