1 82 PHYSIOLOGICAL CHEMISTRY. 



view has not been generally accepted as a whole. Other in- 

 vestigators appear to have shown that what Siegfried took for 

 a single body of constant composition is really a mixture of 

 substances, yielding pretty constant results on analysis. In 

 this mixture the hexone bases, arginine, lysine and histidine, 

 are important components. These are all diamino acids with 

 six carbon atoms and, because of their constant occurrence in 

 digestive mixtures and other products of protein decomposi- 

 tion, they must be looked upon as essential factors in the pro- 

 tein structure. Leucine and tyrosine always seem to accom- 

 pany the hexones in these decompositions. 



Although by prolonged digestion products are reached 

 which do not give the biuret reaction, it is claimed by Fischer 

 and others in recent work that residues remain which are still 

 relatively complex. The name polypeptides has been given 

 by Fischer to such residues, and their relations to chemical 

 substances of definite composition pointed out. But even 

 these may be finally broken down into amino acids. 



Synthesis of Polypeptides. In the last few years a number of these 

 polypeptides have been produced by several synthetic processes. Among 

 such bodies described by Fischer the following may be cited as illustra- 

 tions : 



DIGLYCYLGLYCINE, NH 2 CH 2 CO NHCH 2 CO NHCH 2 COOH. This is a 

 tripe p tide and, as the formula shows, is formed by a condensation of three 

 groups of aminoacetic acid. 



ALANYLGLYCYLGLYCINE, CH 3 CHNH 2 CO NHCEUCO NHCH 2 COOH. In 

 this compound alanine, a-aminopropionic acid, is one of the groups brought 

 into the combination with glycine. 



PHENYLALANYLGLYCYLGLYCINE, C 6 H 5 CH 2 CHNH 2 CO NHCH 2 CO NH 

 CH 2 COOH. This body is of interest because of the occurrence of phenyl- 

 alanine among the commoner protein cleavage products, where reagents 

 are used. Residues containing this group appear to be much more re- 

 sistant toward tryptic fermentation. 



LEUCYLPROLINE. Proline o-pyrrolidine carboxylic acid. 



CH 3 \ /CH 2 CH 2 



>CH-CH 2 CH-CO-N< 

 CH 3 / | \CH CH 2 



NH 2 



COOH 



In this case the synthesis of leucine and the pyrrolidine carboxylic acid 

 has been made. In trypsin digestion residues containing the latter body 



