INTRODUCTION 1 1 



This structure represents : 



1. The guanidine group in arginine remaining free in the linking 

 together of amino acids. 



2. The second carboxyl group of the dibasic acid combined with 

 ammonia ; other amino acids might be attached before the final COOH 

 group is combined-with ammonia. 



3. Any of the units except proline taking a position in any part of 

 these chains ; proline, since it contains only a carboxyl group for link- 

 age, must stand at the end of a chain. 



4. A diamino acid continuing the acid amide formation when a 

 proline unit occurs. 



5. The possibility of rings being made by salt formation of the free 

 carboxyl group arising from a dibasic acid (either directly or indirectly 

 as under 2) with either the terminal proline or the terminal guanidine 

 group of arginine. 



6. A diketopiperazine or anhydride ring formed by the final union 

 of the terminal NH and CO groups. 



Such a structure will explain the difference in action between pep- 

 sin and trypsin. Trypsin will hydrolyse a chain of amino acids with 

 a terminal amino or carboxyl group. Pepsin will open the anhydride 

 ring at one or more junctions and will give several proteoses and pep- 

 tones with free terminal NH 2 and COOH groups capable of being 

 attacked by trypsin. Those proteins which are resistant to the action 

 of trypsin until they have been acted upon by pepsin will have all their 

 units contained in the anhydride ring. 



The presence of a diamino acid for the continuation of the linking 

 is not probable, because some proteins contain no lysine, and may at 

 the same time contain a large amount of proline. Lysine also appar- 

 ently stands at the end of a chain since it is not obtained by hydrolysis 

 after the action of nitrous acid, and since after the treatment of pro- 

 tein with alkali it is obtained in the optically active form (Dakin). 

 Kossel has also shown that the e-amino group of lysine takes no 

 part in the linking of the units in certain protamines. 



It is only the presence of proline as unit which insists upon the use 

 of the diamino acid to continue the chain. An anhydride ring need 

 not contain lysine for the attachment of a chain. This need only be 

 attached to a dibasic acid in the ring, and under these conditions the 

 end member of such a chain will have a free carboxyl group which can 

 be neutralised by salt formation with arginine or lysine or by forma- 

 tion of amide with ammonia. It is still very probable that proline is 



