CHEMICAL COMPOSITION OF PLANTS 53 



All of these amino acids are obtained from "proteins through 

 hydrolysis, apparently without any great changes in the structure 

 of the amino acid molecule. This leads to the belief that they 

 are linked to each other in such a way that the (acid) — COOH 

 group of one unites with the (basic) — NH2 group of another. 

 It must be remembered that amino acids are amphoteric com- 

 pounds, able to react as weak acids and as weak bases at the same 

 time. As a result of such unions, there accumulates in the 

 protein molecule a number of CO — NH — CH groups that are the 

 ones that give the biuret test. An amino acid by itself will not 

 give this reaction. 



Basing his work upon these suppositions, Emil Fischer (1906) 

 attempted to bring about a synthesis of the protein molecule. 

 Binding together amino acids, first by twos, then by threes, and 

 finally by as many as 18 or 19, he obtained complex compounds 

 named by him polypeptides, which gave the biuret reaction and, 

 in general, possessed many of the properties of proteins. The 

 structure of the polypeptide may be represented by the following 

 chain formula: 



— NH— CH— CO— NH— CH— CO— NH— CH— CO— 



Ri R2 R3 



where Ri, R2, etc., are parts of different amino acids, connected 

 with the link CH of the main chain to form lateral chains. 

 According to the theory of Emil Fischer, actual proteins repre- 

 sent nothing else than very complex polypeptides constructed 

 in the same manner as his artificial polypeptides. 



The investigations of Emil Fischer made an epoch in the study 

 of proteins, and the preceding scheme for the structure of a 

 protein molecule was for a long time generally accepted. Recent 

 researches, however, have shown that besides such peptide bonds 

 in the protein molecule, there also exist bonds that are not 

 peptide linkages, leading to the formation of ring or cyclic struc- 

 tures, not open-chain compounds. From the demonstration of 

 such cyclic compounds (Abderhalden and Karrer) it is assumed 

 that the protein molecule contains bonds of a diketopiperazine 

 character. Hence, the conclusion may be drawn that the protein 

 molecule is of a much more complicated structure than it seemed 

 directly after the brilliant work of Fischer. 



