POLYPEPTIDES 309 



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 



CHo— CO 

 NH2CH0COOH / " \ 



= NH NH + 2H0O 



COOHCH2NH2 \ / 



CO— CH2 



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„— CO CH2— COOH 



/ " \ / 



NH NH + H„0=NH 



\ / " \ 



CO— CHa COCH2NH2 



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 : — 



CH2ClCOCI + NH.jCH;jCONHCH2COOC.jH5=CH.jClCONHCH2CONHCH2COOC2H5 + HCI 



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

 ammonia, yields a tripeptide, 



CHaClCONHCH2CONHCH2COOH-<-NH3=CH2NH2CONHCH2CONHCHnCOOH + 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 RjCHNH^COCl. This 

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

 acid as follows : — 



R3 Ro 



RjCHNH.COCl + NHaCH-COOH = RiCHNHaCONH-CHCOOH + HCl 



The resulting polypeptide may, of course, be of considerable 

 complexity, according to the nature of Rj and R2. 



By these and similar methods, employing other combina- 



