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 



CH2— CO 

 NH2CH2COOH / \ 



= NH NH + 2H,jO 



COOHCH2NH, \ / 



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. 



CHj— CO CH^— COOH 



/ \ / 



NH NH + HjO = NH 



\ / \ 



CO— CHo COCH.NHj 



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 : — 



CH2CICOCl + NHoCH.jCONHCH2COOC.jH5=CHoClCONHCH2CONHCH2COOC2H5 + HCI 



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

 ammonia, yields a tripeptide, 



CH2CICONHCH2CONHCH2COOH+NH3=CH3NH2CONHCH2CONHCH2COOH+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 R^CHNHc^COCl. This 

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

 acid as follows : — 



R2 Ra 



R1CHNH2COCI + NHjCH-COOH = R1CHNH2CONHCHCOOH + HCl 

 The resulting polypeptide may, of course, be of considerable 

 complexity, according to the nature of R^ and R.,. 



By these and similar methods, employing other combina- 



