THE PROTEINS 363 



Constitution of the Proteins. 



The work of Emil Fischer and his pupils has shown that the 

 amino acid units are combined together in the form of acid amides, i.e. 

 the carboxyl group of one amino acid is combined with the amino 

 group of another amino acid, e.g. : 



Glycyl-glycine - CH C (NH ) . CO NH . CH a . COOH 

 Alanyl-leucine - CH^ . CH(NH 2 ) . CO NH . CH(C 4 H 9 ) . COOH 

 Leucyl alanine - C 4 H 9 . CH(NH 2 ) . CO NH . CH(CH 3 ) . COOH 



in which the units may either be the same or different, and combined 

 in any possible order. 



These combinations of amino acids have been termed by Emil 

 Fischer the polypeptides. The above compounds are dipeptides. In 

 the same way we may have : 



Tripeptides, e.g. 



Diglycyl-glycine or glycyl-glycyl-glycine, 



CH 2 (NH 2 ) . CO NH . CH 2 . CO NH . CH 2 . COOH. 



Glycyl-alanyl-tyrosine, 



CH 2 (NH 2 )CO NH . (CHCH 3 )CO NH . CH(CH 2 . C 6 H 4 OH) . COOH. 



Alanyl-glycyl-tyrosine, 



CH 3 . CH(NH 2 ) . CO NH ..CH 2 . CO NH . CH(CH 2 . C 6 H 4 OH) . COOH. 



Tetrapeptides, e.g. 



Glycyl-alanyl-glycyl-tyrosine, 

 CH 2 (N H 2 ) . CO NH . CH(CH a )CO NH ,CH 2 . CO NH . CH(CH 2 . C 6 H 4 OH) . COOH. 



Pentapeptides, hexapeptides, etc. 



The most complex polypeptide known is an octadecapeptide, 

 which is composed of eighteen units made up of three leucine and 

 fifteen glycine units. This compound, if it had been found in nature, 

 would undoubtedly have been regarded as a true protein. 



The synthesis of these polypeptides has been effected in three 

 ways. The simplest of the methods of combining two or more amino 

 acids together is by the action of the acid chloride derivative of one 

 unit upon the other unit, e.g. alanyl-leucine is formed by the action of 

 alanyl-chloride upon leucine : 



CH 3 . CH(NH 2 ) . CO . Cl + H 2 N . CH(C 4 H 9 ) . COOH = 

 Alanyl chloride. Leucine. 



HC1 + CH 3 . CH(NH,) . CO NH . CH(C 4 H 9 ) . COOH. 

 Alanyl-leucine. 



By the action of another amino acid chloride upon this compound 

 a tripeptide will be formed and the continuation of the process will 

 lead eventually to the most complex polypeptides. The new com- 

 pound (di- or tripeptide) is again an amino acid and can be con- 

 verted into its acid chloride. This acid chloride will react with 

 another amino acid, a dipeptide, tripeptide, etc., yielding a tetra-, a 

 penta-, a hexapeptide. 



