ALBUMINS OR PROTEINS. 179 



In the same way we can conceive a combination of two alanine mole- 

 cules forming alanyl-alanine; from two leucine molecules we obtain leucyl- 

 leucine, etc. Emil Fischer has called this whole class of compounds 

 " peptides." Just as the carbohydrates are divided into mono-, di-, tri-, 

 or polysaccharides, so Emil Fischer has classified the peptides according to 

 the number of amino acids participating in the composition of the mole- 

 cule as mono-, di-, tri-, tetra-, pent a-, hexa-, etc. and poly-pep tides. He 

 characterizes them according to the amino acids entering into their com- 

 position. We can just as successfully unite two or more different amino 

 acids as we can two similar ones in producing peptides. Emil Fischer 

 and his students have already produced a very large number of such 

 chains. As examples of these we mention Dipeptides: glycyl-alanine, 

 alanyl-glycine, alanyl-leucine, leucyl-alanine, leucyl-glycine, glycyH 

 tyrosine, glycyl-phenyl-alanin, leucyl-proline, prolyl-leucine, seryl-serine, 

 lysyl-lysine, arginyl-arginine, histidyl-histidine; Tripeptides: leucyl-glycyl- 

 glycine, leucyl-alanyl-alanin; Tetrapeptides : dileucyl-glycyl-glycine, tetra- 

 glycine, dialanyl-cystine, dileucyl-cystine; Pe nta-pep tides : penta-glycine, 

 leu cyl-tetragly cine, etc. The number of combinations possible by link- 

 ing these amino acids together is necessarily very great. If we also take 

 into consideration the fact that all of the amino acids, excepting glycocoll, 

 contain an asymmetric carbon atom (isoleucine has two of them), the 

 possible number of isomeric combinations is still further increased. The 

 number of individual optical isomers, according to van 't HoflPs formula, 

 is represented by 2 n , in which n indicates the number of asymmetric 

 carbon atoms which in this case if we neglect glycine and isoglycine 

 is equal to the number of component amino acids. 



In order to give a more satisfactory idea of the syntheses of peptides, an 

 example of each important method will be briefly given. 



If glycocoll is converted into its ester, CH 2 . NH 2 . CO . O . C 2 H 5 , the 

 latter goes over into glycine anhydride, a diketopiperazine: 



X CH 2 . CO x 



\ cif\ r*Ti / 



\j\J \jtt-2, 



according to the following equation: 



2 NH 2 . CH 2 . CO . O . C 2 H 5 = 2 C 2 H 5 OH + NH^ X NH. 



Ethyl alcohol co - CH 2 



From this substance Emil Fischer 1 succeeded in producing the first and 

 simplest of the peptides, by the action of dilute alkali: 



X CH 2 .CO V 



NH ( , NH + H 2 = NH 2 . CH 2 . CO . NH .CH 2 COOH. 



^CO .CH 2 



Glycine anhydride Glycyl-glycine 





1 Cf. literature E. Fischer: Ber. 39, 530 (1906). 



