POLYPEPTIDES 319 



with natural proteins. To these synthetic substances he has 

 given the general name of Polypeptides. 



The simplest polypeptide known is glycylglycine ; this 

 substance was obtained as follows : 



Glycine, when kept for some time in aqueous solution, 

 loses water from two molecules, giving an anhydride 



CH 2 CO 

 NH 2 CH 2 COOH / \ 



=r NH NH + 2H a O 



COOHCH 2 NH 2 \ / 



CO CH, 



Glycine anhydride or 

 diketopiperazine 



This substance, when boiled with hydrochloric acid, is hydro- 

 lysed, the ring being opened with the formation of the di- 

 peptide glycylglycine. 



CH S CO CH 2 COOH 



/ \ / 



NH NH + H 2 = NH 



\ / \ 



CO CH 



To give anything like a complete account of the methods 

 employed in the synthesis of polypeptides is outside the 

 province of this book. It may, however, be mentioned that 

 a very fruitful method of synthesizing these substances con- 

 sists in acting on an amino acid or a polypeptide with chlor- 

 acetyl chloride, thus : 



CH 2 ClCOCl+NH 2 CH;jCONHCH 2 COOC 2 HB=CH 2 ClCONHCH 2 CONHCH 2 COOC 2 H s +HCl 



The latter, after conversion into the acid, and treatment with 

 ammonia, yields a tripeptide, 



CH 2 ClCONHCH3CONHCH 2 COOH+NH 3 =CHjNH 2 CONHCH 2 CpNHCH 2 COOH+HCl 



Diglycylglycine a Tripeptide 



Another valuable method consists in treating an amino 

 acid suspended in acetyl chloride with phosphorus pentachlo- 

 ride and so obtaining an acid chloride RjCHNHgCOCl. This 

 latter is then allowed to act upon the amino group of a second 

 acid as follows : 



R, R, 



RjCHNH-jCOCl + NH 2 CH-COOH = RjCHNH-jCONH-CHCOOH + HC1 



The resulting polypeptide may, of course, be of considerable 

 complexity, according to the nature of R x and R 2 . 



By these and similar methods, employing other combina- 



